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Dallas Semiconductor DS1287 real-time clock manufactured in 1988
Types of hobbyist RTC modules commercially available from China

A real-time clock (RTC) is an electronic device (most often in the form of an integrated circuit) that measures the passage of time.

Although the term often refers to the devices in personal computers, servers and embedded systems, RTCs are present in almost any electronic device which needs to keep accurate time of day.

Terminology

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The term real-time clock is used to avoid confusion with ordinary hardware clocks which are only signals that govern digital electronics, and do not count time in human units. RTC should not be confused with real-time computing, which shares its three-letter acronym but does not directly relate to time of day.

Purpose

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Although keeping time can be done without an RTC,[1] using one has benefits:

  • Reliably maintains and provides current time through disruptive system states such as hangs, sleep, reboots, or if given sufficient backup power, full shutdown and hardware reassembly, without the need to have its time set again.
  • Low power consumption[2] (important when running from alternate power)
  • Frees the main system for time-critical tasks
  • Sometimes more accurate than other methods

A GPS receiver can shorten its startup time by comparing the current time, according to its RTC, with the time at which it last had a valid signal.[3] If it has been less than a few hours, then the previous ephemeris is still usable.

Some motherboards are made without RTCs. The RTC may be omitted out of desire to save money or reduce possible sources of hardware failure.

Power source

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Lithium battery inside the real-time clock IC

RTCs often have an alternate source of power, so they can continue to keep time while the primary source of power is off or unavailable. This alternate source of power is normally a lithium battery in older systems, but some newer systems use a supercapacitor,[4][5] because they are rechargeable and can be soldered. The alternate power source can also supply power to battery backed RAM.[6]

Timing

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Most RTCs use a crystal oscillator,[7][8] but some have the option of using the power line frequency.[9] The crystal frequency is usually 32.768 kHz,[7] the same frequency used in quartz clocks and watches. Being exactly 215 cycles per second, it is a convenient rate to use with simple binary counter circuits. The low frequency saves power, while remaining above human hearing range. The quartz tuning fork of these crystals does not change size much from temperature, so temperature does not change its frequency much.

Some RTCs use a micromechanical resonator on the silicon chip of the RTC. This reduces the size and cost of an RTC by reducing its parts count. Micromechanical resonators are much more sensitive to temperature than quartz resonators. So, these compensate for temperature changes using an electronic thermometer and electronic logic.[10]

Typical crystal RTC accuracy specifications are from ±100 to ±20 parts per million (8.6 to 1.7 seconds per day), but temperature-compensated RTC ICs are available accurate to less than 5 parts per million.[11][12] In practical terms, this is good enough to perform celestial navigation, the classic task of a chronometer. In 2011, chip-scale atomic clocks became available. Although vastly more expensive and power-hungry (120 mW vs. <1 μW), they keep time within 50 parts per trillion (5×10−11).[13]

Examples

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Dallas Semiconductor (DS1387) real-time clock from an older PC. This version also contains a battery-backed SRAM.
Dallas DS1307 RTC chip in DIP-8 package

Many integrated circuit manufacturers make RTCs, including Epson, Intersil, IDT, Maxim, NXP Semiconductors, Texas Instruments, STMicroelectronics and Ricoh. A common RTC used in single-board computers is the Maxim Integrated DS1307.

The RTC was introduced to PC compatibles by the IBM PC/AT in 1984, which used a Motorola MC146818 RTC.[14][15] Later, Dallas Semiconductor made compatible RTCs, which were often used in older personal computers, and are easily found on motherboards because of their distinctive black battery cap and silkscreened logo. A standard CMOS interface is available for the PC RTC.[16]

In newer computer systems, the RTC is integrated into the southbridge chip.[17][18]

Some microcontrollers have a real-time clock built in, generally only the ones with many other features and peripherals.

Radio-based RTCs

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Some modern computers receive clock information by digital radio and use it to promote time-standards. There are two common methods: Most cell phone protocols (e.g. LTE) directly provide the current local time. If an internet radio is available, a computer may use the network time protocol. Computers used as local time servers occasionally use GPS[19] or ultra-low frequency radio transmissions broadcast by a national standards organization (i.e. a radio clock[20]).

Software-based RTCs

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The following system is well-known to embedded systems programmers, who sometimes must construct RTCs in systems that lack them. Most computers have one or more hardware timers that use timing signals from quartz crystals or ceramic resonators. These have inaccurate absolute timing (more than 100 parts per million) that is yet very repeatable (often less than 1 ppm). Software can do the math to make these into accurate RTCs. The hardware timer can produce a periodic interrupt, e.g. 50 Hz, to mimic a historic RTC (see below). However, it uses math to adjust the timing chain for accuracy:

time = time + rate.

When the "time" variable exceeds a constant, usually a power of two, the nominal, calculated clock time (say, for 1/50 of a second) is subtracted from "time", and the clock's timing-chain software is invoked to count fractions of seconds, seconds, etc. With 32-bit variables for time and rate, the mathematical resolution of "rate" can exceed one part per billion. The clock remains accurate because it will occasionally skip a fraction of a second, or increment by two fractions. The tiny skip ("jitter") is imperceptible for almost all real uses of an RTC.

The complexity with this system is determining the instantaneous corrected value for the variable "rate". The simplest system tracks RTC time and reference time between two settings of the clock, and divides reference time by RTC time to find "rate". Internet time is often accurate to less than 20 milliseconds, so 8000 or more seconds (2.2 or more hours) of separation between settings can usually divide the forty milliseconds (or less) of error to less than 5 parts per million to get chronometer-like accuracy. The main complexity with this system is converting dates and times to counts of seconds, but methods are well known.[21]

If the RTC runs when a unit is off, usually the RTC will run at two rates, one when the unit is on and another when off. This is because the temperature and power-supply voltage in each state is consistent. To adjust for these states, the software calculates two rates. First, software records the RTC time, reference time, on seconds and off seconds for the two intervals between the last three times that the clock is set. Using this, it can measure the accuracy of the two intervals, with each interval having a different distribution of on and off seconds. The rate math solves two linear equations to calculate two rates, one for on and the other for off.

Another approach measures the temperature of the oscillator with an electronic thermometer, (e.g. a thermistor and analog-to-digital converter) and uses a polynomial to calculate "rate" about once per minute. These require a calibration that measures the frequency at several temperatures, and then a linear regression to find the equation of temperature. The most common quartz crystals in a system are SC-cut crystals, and their rates over temperature can be characterized with a 3rd-degree polynomial. So, to calibrate these, the frequency is measured at four temperatures. The common tuning-fork-style crystals used in watches and many RTC components have parabolic (2nd-degree) equations of temperature, and can be calibrated with only 3 measurements. MEMS oscillators vary, from 3rd degree to fifth degree polynomials, depending on their mechanical design, and so need from four to six calibration measurements. Something like this approach might be used in commercial RTC ICs, but the actual methods of efficient high-speed manufacturing are proprietary.

Historic RTCs

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Some computer designs such as smaller IBM System/360s,[22] PDP-8s[23] and Novas used a real-time clock that was accurate, simple and low cost. In Europe, North America and some other grids, the frequency of the AC mains is adjusted to the long-term frequency accuracy of the national standards. In those grids, clocks using AC mains can keep perfect time without adjustment. Such clocks are not practical in portable computers or grids (e.g. in South Asia) that do not regulate the frequency of AC mains.

These computers' power supplies use a transformer or resistor divider to produce a sine wave at logic voltages. This signal is conditioned by a zero crossing detector, either using a linear amplifier, or a schmitt trigger. The result is a square wave with single, fast edges at the mains frequency. This logic signal triggers an interrupt. The interrupt handler software usually counts cycles, seconds, etc. In this way, it can provide an entire clock and calendar. In the IBM 360, the interrupt updates a 64-bit count of microseconds utilized by standardized systems software. The clock's jitter error is half if the clock interrupts for each zero crossing, instead of each cycle.

The clock also usually formed the basis of computers' software timing chains; e.g. it was usually the timer used to switch tasks in an operating system. Counting timers used in modern computers provide similar features at lower precision, and may trace their requirements to this type of clock. (e.g. in the PDP-8, the mains-based clock, model DK8EA, came first, and was later followed by a crystal-based clock, DK8EC.)

A software-based clock must be set each time its computer is turned on. Originally this was done by computer operators. When the Internet became commonplace, network time protocols were used to automatically set clocks of this type.

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Real-time clock

View on Grokipedia
from Grokipedia
A real-time clock (RTC) is an electronic device, most commonly implemented as an integrated circuit, that continuously tracks and provides the current time of day, date, and related temporal data, maintaining accuracy even when the primary power supply is disconnected through the use of a dedicated backup battery or capacitor. RTCs operate using a low-frequency quartz crystal oscillator, typically at 32.768 kHz, which generates precise timing pulses divided down to one pulse per second to increment internal counters and registers representing seconds, minutes, hours, days, months, and years. These devices include a controller to manage timekeeping logic, a power-switching mechanism to seamlessly transition to backup power (often a coin cell battery with capacities supporting years of operation), and interfaces such as I²C or SPI for communication with host microprocessors or systems. Key features encompass calendar functions to handle leap years and month lengths, programmable alarms for timed interrupts, square-wave outputs for external synchronization, and low-power modes with consumption as low as 0.5 μA to minimize battery drain. Modern RTC modules often integrate the crystal, oscillator, and IC into a compact package for simplified design and enhanced stability, with temperature-compensated variants achieving accuracies of ±5 ppm over wide ranges like -40°C to +85°C. The development of RTCs traces back to the , with widespread adoption in computing beginning in the 1980s through chips like the MC146818, which was incorporated into the PC/AT in 1984 to provide persistent timekeeping independent of system power cycles. This evolution has positioned RTCs as indispensable components in diverse electronics, from personal computers and servers for boot-time synchronization and logging, to embedded systems and (IoT) devices for event scheduling and sensor timestamping. In automotive applications, they support battery management systems and ; in consumer electronics like wearables and cameras, they enable precise timestamps and low-power wake-ups; and in industrial settings, they facilitate automation, , and compliance with time-sensitive protocols. Overall, RTCs ensure reliable temporal reference in power-constrained environments, underpinning functionalities that range from basic clock displays to complex synchronized operations across modern digital ecosystems.

Fundamentals

Definition and Terminology

A real-time clock (RTC) is an electronic device, typically implemented as an integrated circuit, that maintains accurate timekeeping by counting seconds, minutes, hours, days, months, and years, even when the primary system power is disconnected. This functionality ensures continuous operation through a dedicated low-power supply, distinguishing it from the main system clock that ceases during power-off states. RTCs are commonly integrated into computing devices, embedded systems, and portable electronics to provide persistent time data. Key terminology associated with RTCs includes "time-of-day clock" (TOD), which refers to the RTC's role in tracking wall-clock time in a human-readable format, often synonymous with RTC in technical contexts. The term "battery-backed clock" describes the RTC's reliance on a secondary power source to sustain operation independently. In contrast, RTCs differ from general-purpose timers or counters, which are designed for measuring intervals, generating pulses, or handling events rather than maintaining absolute time-of-day records. At its core, an RTC operates on the principle of a low-power oscillator, usually a 32.768 kHz quartz , that generates periodic pulses to increment a set of counter registers representing the current time and date. These registers accumulate ticks from the oscillator to update time units sequentially, with logic to handle transitions such as carrying over from 59 seconds to the next minute. Time in RTCs is typically represented in either (BCD) or pure binary formats, where BCD encodes each decimal digit in four bits for easier human-readable conversion, while binary offers compact storage but requires arithmetic adjustments for display. Advanced RTCs incorporate handling by checking if a year is divisible by 4 (except for century years not divisible by 400), ensuring accurate progression. Century rollover issues, exemplified by the Y2K problem, arose in older RTCs that stored years as two digits (00-99) assuming a 1900-1999 range, leading to erroneous calculations and date errors post-1999 without software or hardware updates. Modern designs mitigate this with full four-digit year storage and robust century logic.

Purpose and Applications

Real-time clocks (RTCs) serve as critical components for maintaining accurate and continuous timekeeping in electronic systems, enabling essential functions such as scheduling tasks, logging events, and generating timestamps, particularly in scenarios involving low power consumption or offline operation. These devices provide a persistent time reference that supports system operations without relying on the main processor or external networks, ensuring reliability for time-sensitive processes like automated wake-ups or event sequencing. By operating independently, RTCs uphold system uptime, allowing applications to resume with precise temporal awareness even after interruptions. In personal computers, RTCs initialize the or with the current date and time during boot sequences, facilitating immediate system synchronization without . Embedded systems, including IoT devices, leverage RTCs for timestamping data logs, which is vital for monitoring environmental conditions or tracking operational metrics over extended periods. such as digital watches and household appliances utilize RTCs to deliver consistent time displays and control timed functions like alarms or cycles. In industrial controls, RTCs enable precise timestamping in systems, supporting event logging for diagnostics, compliance, and process optimization. The primary benefits of RTCs include safeguarding time continuity during power outages via integrated backup mechanisms, which allows devices to retain accurate timing post-restoration. This capability is particularly advantageous for supporting low-power sleep modes, where systems can enter energy-saving states while relying on the RTC for periodic awakenings, thereby extending battery life in portable applications. Additionally, RTCs facilitate across multi-device networks by offering a unified time base, essential for coordinated operations in distributed environments like arrays or control systems. RTCs address key challenges in time management, such as preventing drift in portable devices through stable oscillation, which ensures long-term accuracy without frequent recalibration. This reliability provides a dependable foundation for software operations, minimizing errors in time-dependent algorithms and enhancing overall system integrity in offline or intermittent power scenarios.

Design and Components

Power Sources

Real-time clocks (RTCs) primarily draw power from a main supply during normal operation but incorporate backup sources to maintain functionality during outages. Battery-backed systems are the most prevalent, utilizing coin cell lithium batteries such as the CR2032, which deliver a nominal 3 V output with high (typically 200-225 mAh capacity) and support lifespans of 5-10 years in low-drain scenarios. These batteries ensure reliable timekeeping by providing stable voltage over extended periods, often integrated directly into RTC modules for seamless operation. Supercapacitors offer an alternative for short-term backup, particularly in applications demanding rapid recharge and high cycle life (exceeding 100,000 cycles), though they exhibit higher self-discharge rates compared to batteries. For instance, a 0.47 F can sustain an RTC for up to 24 hours at a 330 nA load in a 3.3 V system. To optimize energy efficiency, RTCs employ integrated low-power designs that minimize static and dynamic power dissipation through techniques like reduced clock frequencies and leakage control. These designs achieve typical standby current draws of 0.5-5 μA, with advanced variants as low as 0.86 μA when configured for infrequent compensation cycles. Such low consumption enables years of autonomous operation on backup sources, critical for embedded systems where main power interruptions are common. This power profile underscores the role of RTCs in preserving timing integrity during brief or prolonged outages without external intervention. Backup switching mechanisms facilitate automatic between the main supply and source, often incorporating voltage to monitor rail integrity and avert . For example, devices like the TLV840 supervisor detect undervoltage on the primary rail (e.g., below 3.0 V) and activate a secondary battery rail via power switches or LDOs, with reset delays (e.g., 1 ms) to prevent transient shorts. These circuits ensure glitch-free transitions, safeguarding RTC registers and counters. Post-2020 developments have introduced solar-assisted and energy-harvesting RTCs tailored for ultra-low-power IoT deployments, where ambient light sources supplement traditional backups to enable battery-free or extended-lifespan operation. In such systems, photovoltaic cells harvest indoor illumination (as low as 5 ) to power autonomous nodes, maintaining RTC accuracy indefinitely without manual recharging. This approach enhances sustainability in remote or distributed IoT networks by reducing reliance on replaceable batteries.

Timing Mechanisms

Real-time clocks (RTCs) primarily rely on oscillators to generate a reference for timekeeping. The most common type is the crystal oscillator, particularly the 32.768 kHz tuning fork crystal, which offers high stability and low power consumption suitable for battery-operated devices. This is chosen because it is exactly 2^15 Hz, enabling efficient division to 1 Hz using a simple binary counter. For cost-sensitive applications, RC oscillators may be employed as simpler alternatives, though they typically require for adequate accuracy. Emerging MEMS-based oscillators provide compact, shock-resistant options that mimic while reducing size and improving reliability in harsh environments. The oscillator output feeds into a counter architecture that tracks time by incrementing at 1 Hz. This is achieved through a chain, often a 15-stage binary counter that divides the 32.768 kHz signal down to 1 Hz. Counters in RTCs are typically implemented in either binary or (BCD) format; BCD is preferred for direct compatibility with human-readable time displays, while binary offers simpler logic for computation. Timekeeping logic processes the 1 Hz pulses to maintain a full , progressing from seconds through minutes, hours, days, months, and years while accounting for variable month lengths and . This logic includes features like alarm interrupts, which trigger outputs when a programmed time matches the current count, and programmable timers for periodic events such as wake-up signals. Frequency stability in RTC oscillators is influenced by environmental factors, particularly , and is enhanced through compensation techniques. Quartz crystals with built-in temperature compensation can achieve ±20 ppm accuracy over 0-70°C, minimizing drift without external adjustments. These oscillators operate at ultra-low power levels, often in the nanowatt range, to support long-term battery life.

Accuracy and Calibration

The accuracy of a real-time clock (RTC) is primarily determined by the stability of its quartz crystal oscillator, with typical uncalibrated precision ranging from ±1.5 to ±2 minutes per month at room temperature, though this can degrade significantly under varying conditions. Factors such as temperature fluctuations, crystal aging, and mismatches in load capacitance directly influence this precision; for instance, a temperature-induced 20 ppm shift in a tuning-fork crystal can result in approximately 1 minute of drift per month, while a load capacitance mismatch—such as using a 12 pF crystal on a circuit designed for 6 pF—can cause the RTC to run 3 to 4 minutes fast per month. Drift in RTCs arises from multiple sources, including crystal aging, which typically manifests as a change of 1 to 3 ppm in the first year, decreasing to about 2 ppm per subsequent year due to internal mechanisms like , surface changes, and stress relief in the lattice. Environmental factors exacerbate this, with causing adsorption or desorption on the surface that alters its resonant , while variances—such as initial tolerances of ±20 ppm and inconsistencies in cut angle or vibration mode—introduce baseline errors that compound over time. To maintain or improve accuracy, calibration techniques include manual adjustments, such as software-based trimming of digital registers or hardware modifications via trimmer capacitors to fine-tune the load capacitance and pull the frequency within ±30 ppm. Automatic temperature compensation using a TCXO integrates a sensor and varactor to dynamically adjust the crystal frequency, achieving stabilities as low as 3.5 to 5 ppm over -40°C to 85°C without external intervention. Periodic syncing with external references, such as GPS-derived 1 PPS signals, corrects cumulative drift by resetting the RTC counter, often performed at intervals like every 8 hours to ensure sub-second accuracy. Standards like IEEE 1588 () enable RTC integration in networked systems for sub-microsecond , where the RTC serves as a base adjusted via PTP messages to align with a grandmaster clock. NIST provides guidelines for timekeeping accuracy, recommending RTC-like devices maintain within ±0.5 seconds between updates and offering calibration services traceable to UTC(NIST) with uncertainties as low as ±1 ns for 1 PPS signals.

Types and Implementations

Hardware RTCs

Hardware real-time clocks (RTCs) are dedicated integrated circuits designed to maintain accurate timekeeping independently of the main system processor, typically powered by a low-energy source to ensure continuity during power loss. These devices often incorporate oscillators for precise reference, achieving accuracies on the order of ±2 parts per million (ppm) over a wide range. Prominent examples of hardware RTC chips include the DS3231 from (now ), which features an integrated temperature-compensated (TCXO) for enhanced accuracy, supporting communication and operating from a 3.3V supply with battery backup. This chip maintains time down to seconds, minutes, hours, day, month, and year, with automatic compensation up to the year 2100. Another foundational example is the MC146818, originally developed by for IBM's PC/AT in 1984, which served as the standard RTC for early personal computers and included 64 bytes of RAM for system configuration storage alongside timekeeping functions. Hardware RTCs are commonly integrated directly into microcontrollers, such as the series from , where they share the die with the CPU to minimize footprint and power draw in embedded systems. Alternatively, they function as standalone modules connected via serial interfaces like or SPI, enabling easy addition to platforms without built-in timing hardware, such as Arduino-based projects or industrial controllers. For instance, the 's RTC peripheral uses an external 32.768 kHz crystal and supports sub-second precision through a programmable prescaler. Key features of hardware RTCs include programmable square wave outputs for generating clock signals at frequencies from 1 Hz to 32.768 kHz, useful for synchronizing external devices or driving displays. Many chips, like the DS3231, provide up to two time-of-day alarms with capabilities and include non-volatile storage, often backed by or battery-maintained SRAM, to preserve time registers and user data across power cycles. These alarms can trigger on specific seconds, dates, or weekdays, facilitating scheduled operations without CPU intervention. In practical applications, hardware RTCs are essential in servers for initializing during boot-up from a persistent source, ensuring reliable logging and scheduling even after outages, as seen in implementations using the MC146818 derivatives in x86 architectures. In wearable devices, such as smartwatches, compact RTCs like those in the ecosystem enable always-on time displays with minimal battery drain, supporting features like step counting tied to real-time timestamps.

Software-Based RTCs

Software-based real-time clocks (RTCs) emulate timekeeping functionality entirely through operating system software and general-purpose processor timers, without relying on dedicated hardware circuits. These implementations leverage the OS kernel's timekeeping subsystem to maintain a wall-clock time that approximates continuous real-time progression, often serving as a substitute for hardware RTCs in environments like virtual machines or resource-constrained systems. In Linux, the kernel implements the system clock using clock sources such as the Time Stamp Counter (TSC) on x86 processors, which provides a high-frequency, monotonic counter of CPU cycles. High-resolution timers (hrtimers) then trigger periodic updates to advance the clock, translating cycle counts into nanoseconds via scaling factors (mult/shift arithmetic) while handling wrap-arounds through bitmasks. For instance, the CLOCK_REALTIME POSIX clock, accessible via clock_gettime(), represents wall-clock time maintained by accumulating nanoseconds from these sources and applying user-space adjustments. Similarly, in Windows, the system time is kernel-managed, with QueryPerformanceCounter() providing high-resolution timestamps based on the performance counter, which derives from CPU cycles or synthetic sources for interval measurements exceeding 1 microsecond resolution. To maintain accuracy, software RTCs employ algorithms for periodic adjustments and drift compensation. Time advances by incrementing based on elapsed CPU cycles from the clock source, with the kernel's accumulating seconds and nanoseconds in a struct timekeeper. Drift—arising from variations in CPU frequency or temperature—is compensated through offsets similar to those in the Network Time Protocol (NTP), where the clock discipline algorithm calculates frequency adjustments using a (PLL) and frequency-locked loop (FLL). In NTPv4, offsets (θ) are derived from exchanges (θ = ½[(T2-T1) + (T3-T4)]), and the local clock is slewed by up to 200 ppm per adjustment, incorporating wander (RMS frequency differences) to stabilize against . These mechanisms ensure long-term , though software clocks typically exhibit higher drift rates (e.g., up to 500 ppm) compared to hardware RTCs' crystal-based precision of ±2 ppm. The advantages of software-based RTCs include low implementation cost, as no additional hardware is required, and high flexibility for integration with OS features like . However, they consume more power due to reliance on active CPU timers and are vulnerable to system interruptions, such as halts, suspends, or reboots, which can cause time loss unless persistently stored (e.g., in ). In virtualized environments, these limitations are mitigated through host-guest , but guest clocks may still drift if the does not compensate for stolen time. Representative examples illustrate these principles in practice. In systems, functions like clock_gettime(CLOCK_REALTIME) and gettimeofday() provide software-emulated RTC access, with the kernel applying NTP-derived corrections for drift. In hypervisors such as , the virtual RTC is fully emulated: the time-of-day (TOD) clock initializes from host UTC plus a configurable offset (e.g., via rtc.diffFromUTC), while periodic interrupts (e.g., 64 Hz) are delivered in apparent guest time, with VMware Tools periodically syncing to correct drifts exceeding 60 seconds and avoiding backward jumps.

Specialized RTCs

Specialized real-time clocks (RTCs) enhance accuracy by integrating external synchronization references, such as radio broadcasts, signals, or network protocols, to periodically the internal oscillator and maintain to (UTC). These implementations are essential in environments where standalone RTCs fall short, including consumer timing devices, navigation systems, and industrial networks, by compensating for drift through automated corrections. Radio-based RTCs receive long-wave time signals to achieve with national time standards. The station, operated by the National Institute of Standards and Technology (NIST) in the United States, broadcasts a 60 kHz carrier with digital time codes, enabling receivers to attain accuracy of approximately 30 milliseconds under typical conditions, with overall timekeeping within 1 second when periodically synchronized. Similarly, the transmitter in , managed by the (PTB), delivers a 77.5 kHz signal with time codes synchronized to UTC, enabling receivers to achieve high accuracy, typically 5-25 milliseconds under good conditions near the transmitter, degrading with distance but still suitable for many applications with appropriate antennas. These systems are widely used in home atomic clocks, where integrated receivers automatically decode the signals to adjust the display and compensate for propagation delays. GPS-based RTCs utilize the (GPS) for disciplined timing, deriving corrections from satellite-transmitted UTC signals to achieve sub-microsecond precision. The 1-pulse-per-second (PPS) output from a GPS receiver, when locked to multiple satellites, transfers the inherent stability of the GPS constellation to the local RTC, with typical timing errors below 100 nanoseconds under clear sky conditions. This approach is common in navigation devices like handheld GPS units and vehicle trackers, where the RTC maintains high accuracy even during brief signal interruptions by holding the disciplined oscillator. Network-synced RTCs leverage Ethernet-based protocols for distributed synchronization in automated systems. The (PTP, IEEE ) supports sub-microsecond accuracy by measuring and compensating for packet transit delays in local networks, making it ideal for industrial automation applications such as synchronized and power grid monitoring. In contrast, the Network Time Protocol (NTP) provides millisecond-level synchronization over Ethernet, suitable for broader automation setups where devices query a central server to align clocks and log events consistently. Cellular network-based RTCs draw timing from or base stations to enable synchronization in mobile and edge environments. In networks, base station signals offer microsecond-level precision, supporting low-latency integrations in for applications like industrial IoT and autonomous systems, with post-2020 deployments emphasizing enhanced phase synchronization via protocols like PTP over cellular backhaul.

Historical Development

Early Innovations

The development of real-time clocks (RTCs) drew inspiration from pre-electronic mechanical timekeeping devices that laid the groundwork for precise timing in computational systems. Mechanical clocks utilized gears, relays, and synchronous motors to maintain operational timing, influencing the shift toward more reliable electronic alternatives. These mechanical systems, while innovative for their era, suffered from bulkiness, frequent maintenance needs, and limited accuracy due to wear and environmental factors, prompting the transition to integrated electronic designs in the mid-20th century. In the 1970s, the advent of technology enabled early integrated digital clock chips, primarily for consumer s and calculators. A seminal example was the MM5314, introduced around 1974, which integrated timekeeping functions including hours, minutes, and seconds display driving in a single MOS IC. This chip represented an early by combining an oscillator, divider circuits, and output drivers on one die, allowing for compact, low-cost time display systems without discrete components. It operated on P-channel and was widely adopted in hobbyist kits and commercial products, marking the initial commercialization of electronic clock circuits beyond mainframe circuits. A key milestone in RTC evolution came with the integration of battery-backed for persistent time storage, addressing the need for uninterrupted operation during power loss. The MC146818, released in the early and first deployed in the PC/AT in 1984, pioneered this feature by combining a real-time clock with 64 bytes of non-volatile RAM. Powered by a small battery, it retained time and date information even when the system was off, using a 32.768 kHz quartz crystal for accuracy within seconds per month. This design, known as the "" (Motorola Timekeeping Element) concept, became a standard for personal computers, enabling automatic system configuration and boot-time synchronization. Early RTC implementations faced significant challenges, particularly in power efficiency and interface standardization. Pre- chips like the MM5314 consumed relatively high power (tens of milliamps) due to NMOS/PMOS architectures, limiting battery life in portable applications and requiring external regulators. The shift to in chips like the MC146818 reduced consumption to microwatts in standby, but initial designs still struggled with leakage currents and crystal stability under varying temperatures. Additionally, the lack of uniform interfaces—such as varying serial/parallel ports and addressing schemes—complicated integration across microprocessors from , , and others, hindering widespread adoption until de facto standards emerged in the PC era.

Modern Advancements

In the 1990s and 2000s, real-time clocks transitioned from parallel interfaces to serial protocols such as and SPI, enabling more efficient integration with microcontrollers and reducing pin counts in compact designs. , originally developed by in the early , saw widespread adoption in RTC applications during this period due to its multi-master capabilities and support for low-speed peripherals, with enhancements like fast-mode (400 kHz) standardized in 1992. Similarly, SPI interfaces gained prominence for their full-duplex operation, facilitating faster data transfer in embedded systems. This shift coincided with the proliferation of system-on-chips (SoCs), where RTCs were increasingly embedded directly into ARM-based processors for mobile devices, optimizing power and space in early smartphones and PDAs. ARM's PrimeCell RTC, an AMBA-compliant peripheral, exemplified this integration, providing standalone timekeeping within SoCs for battery-backed operation. From the 2010s onward, advancements focused on ultra-low-power designs tailored for (IoT) applications, achieving current consumption below 1 μA to extend battery life in always-on sensors. For instance, sub-threshold techniques enabled RTCs with power draws as low as 180 nA, as demonstrated in Maxim Integrated's 2019 chip, which supported years of operation on coin-cell batteries. EM Microelectronic's EM3028 RTC, released in 2018, further pushed boundaries with 50% longer battery life than competitors while maintaining ±2 ppm accuracy, ideal for wireless sensor networks. Security enhancements emerged alongside these, notably in the (TPM) 2.0 standard ratified in 2014, which introduced trusted time functions through a secure internal clock and non-volatile counter to prevent tampering and ensure monotonic time progression even during power cycles. This secure time mechanism, combining a hardware timer with cryptographic protection, supports attestation and audit logs in enterprise and IoT environments. Additionally, efforts toward quantum-resistant synchronization protocols began incorporating into time-stamping schemes, safeguarding RTC syncing against future threats in distributed systems. By the 2020s up to 2025, recent trends emphasized intelligent and networked RTC enhancements for precision and efficiency. AI-assisted drift prediction leveraged models to forecast clock inaccuracies caused by environmental factors like , improving long-term accuracy in GPS-denied scenarios; for example, neural networks trained on historical achieved sub-nanosecond corrections in clock applications. Synchronization with and networks enabled RTCs in IoT devices to derive precise time via protocols like (Network Identity and ), allowing sub-millisecond alignment without dedicated GPS hardware and supporting low-power wide-area deployments. Sustainability-driven designs incorporated energy-harvesting mechanisms, such as ' 2024 EFR32 Series 2, which powered RTCs from ambient sources like solar or RF energy, eliminating batteries and reducing e-waste in remote sensors. These models harvested micro-watts continuously, ensuring perpetual operation while aligning with green IoT standards. Looking ahead, integration of technology promises tamper-proof timestamps for RTCs, anchoring time data to immutable ledgers for verifiable event logging in supply chains and secure transactions. Blockchain-based schemes, such as those using hash-linked blocks for long-term time-stamping, ensure chronological integrity without relying on central authorities, with prototypes demonstrating resistance to retroactive alterations. This fusion could enable decentralized, auditable time sources in , building on RTCs' role in distributed systems.

References

  1. https://www.analog.com/en/resources/glossary/rtc.html
  2. https://ecsxtal.com/what-is-a-real-time-clock-rtc/
  3. https://www.epsondevice.com/crystal/en/techinfo/column/rtc/about-rtc.html
  4. https://ariya.io/2012/08/real-time-clock
  5. https://www.microchip.com/en-us/products/clock-and-timing/components/real-time-clocks
  6. https://www.nisshinbo-microdevices.co.jp/en/products/real-time-clock/introduction/
  7. https://www.ti.com/lit/gpn/BQ3285E
  8. https://www.st.com/resource/en/application_note/an4759-introduction-to-using-the-hardware-realtime-clock-rtc-and-the-tamper-management-unit-tamp-with-stm32-mcus-stmicroelectronics.pdf
  9. https://www.electronics-tutorials.ws/connectivity/real-time-clocks.html
  10. https://www.analog.com/en/resources/technical-articles/state-machine-logic-in-binarycoded-decimal-bcdformatted-realtime-clocks.html
  11. https://www.gresham.ac.uk/sites/default/files/2017-04-04-MartynThomas_Y2K-T.pdf
  12. https://www.ti.com/lit/pdf/SLAU403
  13. https://www.asus.com/support/faq/1043640/
  14. https://embeddedhash.in/real-time-clock-in-embedded-systems/
  15. https://www.electronicsforu.com/technology-trends/learn-electronics/real-time-clock-chips
  16. https://heqingele.com/blog/real-time-clocks-industrial-applications/
  17. https://www.epsondevice.com/crystal/en/products/rtc/offer/pdf/rtc-fundamentals.pdf
  18. https://www.digikey.com/en/articles/enabling-timekeeping-function-and-prolonging-battery-life-in-low-power-systems
  19. https://www.hilscher.com/service-support/glossary/real-time-clock-rtc
  20. https://www.wevolver.com/article/shrinking-time-the-role-of-rtc-modules-in-wearable-tech-and-miniaturized-devices
  21. https://ieeexplore.ieee.org/document/7417980
  22. https://www.arrow.com/en/research-and-events/articles/cr2032-batteries-keep-a-light-shining-in-the-window
  23. https://abracon.com/uploads/resources/Abracon-Application-Note-Using-Supercapacitors-as-RTC-Power-Backup.pdf
  24. https://www.nxp.com/products/analog-and-mixed-signal/real-time-clocks:MC_71246
  25. https://www.analog.com/en/resources/design-notes/power-considerations-for-accurate-realtime-clocks.html
  26. https://www.ti.com/lit/pdf/snva959
  27. https://www.mdpi.com/2079-9268/13/1/12
  28. https://ecsxtal.com/news-resources/why-are-32-768-khz-crystals-and-oscillators-used-in-real-time-clocks/
  29. https://www.sitime.com/products/32-khz-mpower-oscillators
  30. https://abracon.com/Precisiontiming/AB08X5-RTC.PDF
  31. https://www.renesas.com/en/document/mat/serial-io-real-time-clock-ic-upd4990a-users-manual
  32. https://www.analog.com/media/en/technical-documentation/data-sheets/ds1307.pdf
  33. https://www.nxp.com/docs/en/data-sheet/PCF2123.pdf
  34. https://ww1.microchip.com/downloads/en/DeviceDoc/00003051B.pdf
  35. https://www.st.com/resource/en/application_note/an934-how-to-use-the-digital-calibration-feature-in-timekeeper-and-serial--realtime-clock-rtc-products-stmicroelectronics.pdf
  36. https://www.analog.com/en/resources/design-notes/crystal-considerations-with-analog-devices-realtime-clocks-rtcs.html
  37. https://www.analog.com/en/resources/technical-articles/timekeeping-accuracy-automatic-and-affordable.html
  38. https://www.ti.com/lit/an/slua051/slua051.pdf?ts=1762701123927
  39. https://www.mouser.com/pdfDocs/rtcmoduletimingsolutionforyourproject.pdf?srsltid=AfmBOoqK5Mikwk_CcRM0syzxzltQEOQ2nLa_7sQB44poWz-98sYs3DdG
  40. https://www.renesas.com/jp/en/document/apn/an1400-isl12022-rtc-accuracy-optimization-calibration-procedure?srsltid=AfmBOoq-U_sSQTNxz_uQOmAyNxyygccqxY907_HW0AqENsYAEQFg5KYc
  41. https://www.edn.com/rtc-design-part-2-temperature-compensation-is-critical/
  42. https://thecavepearlproject.org/2024/10/22/setting-accurate-rtc-time-with-a-gps-the-ds3231-aging-offset-to-reduce-drift/
  43. https://tf.nist.gov/general/pdf/1424.pdf
  44. https://tf.nist.gov/general/pdf/2429.pdf
  45. https://www.nist.gov/pml/time-and-frequency-division/time-services/calibration-services-time-and-frequency-division
  46. https://docs.kernel.org/timers/timekeeping.html
  47. https://learn.microsoft.com/en-us/windows/win32/api/profileapi/nf-profileapi-queryperformancecounter
  48. https://www.baeldung.com/linux/timekeeping-clocks
  49. https://datatracker.ietf.org/doc/html/rfc5905#section-11.3
  50. https://datatracker.ietf.org/doc/html/rfc5905#section-8
  51. https://www.cse.psu.edu/~buu1/teaching/spring06/papers/vmware-timing.pdf
  52. https://www.nist.gov/pml/time-and-frequency-division/time-distribution/radio-station-wwvb
  53. https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77.html
  54. https://www.hopf.com/dcf77-gps_en.php
  55. https://tf.nist.gov/general/pdf/2294.pdf
  56. https://www.masterclock.com/ynchronizing-industrial-automation-systems-over-ethernet.html
  57. https://www.gsma.com/solutions-and-impact/technologies/networks/networks-blog-series/the-5g-era-how-5g-is-changing-the-world/
  58. http://www.elektormagazine.com/news/circuit-the-mos-clock-5314
  59. https://archive.org/stream/bitsavers_nationaldaMOSIntegratedCircuits_31346159/1974_National_MOS_Integrated_Circuits_djvu.txt
  60. http://www.os2museum.com/wp/troubled-time/
  61. https://www.embeddedrelated.com/showarticle/90.php
  62. https://www.ti.com/lit/pdf/sbaa565
  63. https://embeddedcomputing.com/technology/processing/chips-and-socs/the-soc-evolution-from-early-handsets-to-edge-intelligence
  64. https://developer.arm.com/documentation/dui0224/latest/programmer-s-reference/real-time-clock--rtc
  65. https://www.electronicsweekly.com/news/products/digital-integrated-circuits/smallest-lowest-power-rtc-2019-07/
  66. https://www.emmicroelectronic.com/news_and_pressrelease/em-microelectronic-enables-green-iot-revolutionary-ultra-low-power-precision
  67. https://trustedcomputinggroup.org/wp-content/uploads/TPM-Rev-2.0-Part-1-Architecture-01.07-2014-03-13.pdf
  68. https://www.mdpi.com/2072-4292/17/18/3177
  69. https://www.1ot.com/blog/time-synchronization-iot
  70. https://investor.silabs.com/news-releases/news-release-details/silicon-labs-streamlines-energy-harvesting-product-development
  71. https://www.prnewswire.com/news-releases/silicon-labs-streamlines-energy-harvesting-product-development-for-battery-free-iot-302122629.html
  72. https://eprint.iacr.org/2022/319.pdf
  73. https://arxiv.org/pdf/2008.06210
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