REAL-TIME CLOCKS
Application Note 632: Aug 01, 2001
Evaluating the Accuracy of Maxim Real-Time Clocks (RTCs)
This app note describes methods for measuring the accuracy of the Maxim Real-Time
Clocks (RTCs) using available accurate time sources and the Maxim evaluation kits.
Evaluating the accuracy of Maxim real-time clocks (RTCs) can be simplified using the modern
personal computer (PC), an Internet connection, and RTC evaluation software. Whether you
use a Maxim evaluation kit and its associated software to evaluate the accuracy of a Maxim
real-time clock, or use your own software and hardware configuration, you can take advantage
of the easy methods presented here for long-term accuracy testing of RTCs.
RTC Evaluation Software
Figure 1 shows the main screen of typical Maxim RTC evaluation software. The "offset 1 sec"
in the current-time display section of the screen is computed as offset = RTC time - PC time.
This is now an easy and convenient way to keep track of the difference between the RTC time
and PC time and thus allows for long-term accuracy testing of the RTC.
When reading the RTC, the timekeeping registers are latched during a burst read. Even though
the RTC evaluation software reads the timekeeping registers approximately four times per
second, the best guaranteed resolution is 1 second, as all four reads could happen inside of a 1second update. Therefore, an initial offset calculation of ±1 second is within the resolution of
the setup when testing for long-term RTC accuracy.
The only issue now is the accuracy of the standard that the RTC time is being compared
against, the PC clock.
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Figure 1: The main screen of typical Maxim RTC evaluation software
PC Time Accuracy
As most of us know, PC clocks are not particularly good at keeping accurate time. Simple
clocks like a wristwatch and most of the clocks in your home keep better time than a standard
PC clock.
Each PC contains two clocks. Although they are known by several different names, we will call
them the "hardware clock" and the "software clock." The software clock runs when the PC is
turned on and stops when the PC is turned off. The hardware clock uses a backup battery and
continues to run even when the PC is turned off.
An Intel 8254 timer-counter (or functionally equivalent device) generates the software clock.
This timer-counter generates an interrupt every 54.936 milliseconds, or about 18.2 times per
second. The PC BIOS (basic input output system) contains a software routine that counts the
interrupt requests and generates a time-of-day clock that can be read or set by other software
programs. For example, the operating system might use the time-of-day information from the
software clock to date and stamp files.
The software clock is a poor timekeeper. Any change in the interrupt-request rates causes the
clock to gain or lose time. If the PC is left on for long periods of time, the software clock can
be off by large amounts. A minute or more per day that the PC was left on is not an uncommon
error rate. It is also possible for an ill-behaved software program to use the timer-counter for
another purpose and change the interrupt rate. This could cause the software clock to rapidly
gain or lose time.
When the PC is turned off, the software clock stops running and loses all of its time-of-day
information. For this reason, a hardware clock is also necessary. The hardware clock is either a
separate real-time clock or a RTC function integrated into the PC's chipset. The hardware clock
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is updated once per second and does not resolve to fractions of a second. Its timing accuracy is
determined by the quality of the crystal oscillator it uses as its time base. A typical crystal
usually is less than $1 in single quantities, with an initial frequency error of greater than
±20ppm, which will translate to greater than ±1.7 seconds per day. In actual operation, with
temperature effects included, most hardware clocks gain or lose 5 to 15 seconds per day.
When the PC is turned off, the hardware clock runs from a battery. When the computer is
turned back on, the software clock starts running again and sets itself (within 1 second) to the
hardware clock. Although the hardware and software clocks are synchronized at power-up, they
run at different rates and will gain or lose time relative to each other while the computer is
running.
As you can see, neither the software nor the hardware PC clock is suitable as an accurate
timekeeping reference. Fortunately, there are easy ways to solve this PC timekeeping accuracy
problem. One of the easiest is to synchronize the PC clock to an Internet service.
Internet Time-Setting Services
If your PC is connected to the Internet, you can synchronize its clock to an Internet time server.
The process requires an active Internet connection and client software.
Internet time servers use several standard timing protocols. The major three are Time Protocol,
Daytime Protocol, and Network Time Protocol (NTP). The time servers are continually
listening for timing requests sent using any of these protocols. When the server receives a
request, it sends time to your computer in the appropriate format. The protocol depends upon
the type of client software used. Most client software requests that the time be sent using either
the Daytime Protocol or NTP. Client software that uses the Simple Network Time Protocol
(SNTP) makes the same timing request as an NTP client, but does less processing and provides
less accuracy. Table 1 summarizes the protocols and their port assignments.
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Table 1. Internet Time Protocols
Protocol
Time
Protocol
NIST
Document
Format
Port
Assignments
RFC-868
Unformatted 32-bit binary number contains time
Port 37
in UTC seconds since January 1, 1900.
tcp/ip, udp/ip
Daytime
Protocol
RFC-867
Exact format not specified in standard. The only
Port 13
requirement is that time code is sent as standard
tcp/ip, udp/ip
ASCII characters.
Network
Time
Protocol
(NTP)
The server provides a data packet that includes a
64-bit time stamp containing the time in UTC
seconds since January 1, 1900, with a resolution
RFC-1305 of 200 picoseconds. NTP provides accuracy of 1
to 50 milliseconds. NTP client software
normally runs continuously and gets periodic
updates from the server.
Simple
Network
Time
Protocol
(SNTP)
RFC-1769
The data packet sent by the server is the same as
NTP, but the client software does less
processing and provides less accuracy.
-
Port 123
udp/ip
NIST (National Institute of Standards and Technology) operates an Internet Time Service from
Boulder, Colorado, using multiple servers distributed around the country. The NIST servers
distribute time using the Time, Daytime, and NTP formats.
For a current list of IP addresses for the NIST servers and sample client software (Daytime
Protocol) that can be downloaded see the following:
http://www.boulder.nist.gov/timefreq/service/its.htm.
NIST also maintains a listing of other client software packages for a variety of computers at:
http://www.boulder.nist.gov/timefreq/general/softwarelist.htm.
UTC and NIST Time
Coordinated Universal Time (UTC) is a stable reference frequency used for counting seconds.
The frequency, or rate, of UTC is computed by the International Bureau of Weights and
Measures (BIPM), based near Paris, France. The BIPM uses a weighted average from about
250 atomic clocks located in about 50 national laboratories to construct a time scale called
International Atomic Time (TAI). Once TAI is corrected for leap seconds, it becomes UTC, or
the official world time scale. NIST distributes a real-time version of UTC called UTC(NIST), a
time scale referenced to atomic oscillators located in Boulder, Colorado, to the public through
their time and frequency services. At its source, UTC(NIST) is kept in as close agreement as
possible with other national and international standards (typically within a few nanoseconds).
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Other Accurate Time Alternatives
If your computer is not connected to the Internet, NIST also provides Dial-Up Time Setting
Service through a standard telephone line and analog modem. This service is called Automated
Computer Time Service (ACTS). With calibration, ACTS can set a computer clock with an
uncertainty of less than 10 milliseconds.
For more information about ACTS and to obtain software, see the following:
http://www.boulder.nist.gov/timefreq/service/acts.htm.
Alternately, you can get accurate time, all the time, by using a radio clock. Some are standalone devices with a digital time display. These can often be interfaced to the PC with a serial
connection. For a list of radio clock manufacturers, see the following:
http://www.boulder.nist.gov/timefreq/general/receiverlist.htm.
A last alternative for calibrating your PC clock, and the least desirable in terms of ease of use,
is to use a regular voice phone line service to check the current accurate time. Such a service is
provided by the Time Service Department, U.S. Naval Observatory, Washington, D.C. The
Land Line Voice Time, without delay, is 900-410-TIME (50 cents the first minute, 45 cents a
minute thereafter). NIST time can be accessed over the voice phone line at (303) 499-7111.
NISTIME 32: 32-Bit TCP Time Client
The main screen for the NIST TCP Time Client is shown in Figure 2. Selecting the Now option
in the Query Server drop-down menu will cause the current NIST time to be obtained and the
difference between the PC clock and NIST time computed, as shown in Figure 3. Clicking on
OK performs an adjustment of the PC clock to match NIST time. A verification of adjustment
then appears, as in Figure 4. Clicking on OK will remove this screen and return control to the
main screen for the NISTIME 32. If you are performing several cycles of calibration to reduce
the PC clock error in comparison to the NIST standard, you will need to wait between
calibrations until the message in the main screen changes from "No server selected" to a line
typically like "Server 2 selected using tcp port 13." At that time, another calibration can be
performed.
Figure 2. NISTIME 32 main screen
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Figure 3. NIST time vs. PC time comparison screen
Figure 4. Verification of PC clock adjustment screen
Accuracy Evaluation Example: MAX6901
Equipment and Conditions
Evaluation System:
MAX6901EVSYS
•
RTC hardware evaluation board
•
MAXSMBUS module (PC parallel port to 3-wire serial conversion)
•
RTC evaluation software
PC Clock Calibration Software:
NISTIME 32
Frequency Counter:
Tektronix CMC251
•
Typical frequency is 32.768kHz, 0V to +5V swing, 40% duty cycle
•
Channel-A period measurement
•
Pre-scale = IN
•
Gate = X1000
•
Typical display reads 30.51758µs
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•
Accuracy of reading is calculated to be ±0.418PPM
Lab Ambient:
76°F to 78°F
Initial Conditions
The MAX6901 evaluation board was run using the default of VCC = +5V. The initial frequency
measurement was taken using the CMC251. The measured time period of the MAX6901
timekeeping oscillator was 30.51748µs.
Calculate how long it takes to reach a 1-second error due to initial crystal tolerance (assume
temperature effects are negligible):
finitial = 32768 - (1/Tinitial)
finitial = 32768 - (1/30.51748µs) = -0.105361Hz
tinitial = {[1/[(f + finitial)/32768]] - 1s}/1s
tinitial = {[1/[(32768Hz - 0.105361Hz)/32768]] - 1}/1s = +3.215µs/s
t1sec_error = 1/ tinitial
t1sec_error = 1/3.215µs = 311041.99s = 86.4hr = 3.6days
Therefore, the RTC will take about 3.6 days to reach an error of +1 second. Because the RTC
software measures RTC error against the PC clock's error, and the PC clock is usually wrong,
the PC clock must be corrected using an Internet time-setting service before comparing the
RTC against the PC clock. The procedure is outlined below.
Procedure
1. Install MAX6901 evaluation software.
2. Install NISTIME 32 software.
3. Connect the MAX6901EVKIT to a +12V supply; leave supply off.
4. Connect a parallel PC cable to the MAXSMBUS board.
5. Plug the MAX6901EVKIT into the MAXSMBUS board.
6. Turn on the +12V supply.
7. Start the MAX6901 evaluation software, and stop cyclic burst reads.
8. Start NISTIME 32 software, and update the PC clock until the error between the PC
clock and NIST time is less than 0.4 seconds. This may take more than one iteration to
accomplish.
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9. In the MAX6901 evaluation software main window, click on "Set from computer's
clock."
10. Confirm that the offset is no more than 1 second.
11. Each and every time the accuracy of the RTC is to be checked, you must repeat Step 8.
If the MAX6901 evaluation software is stopped and the parallel-port cable
disconnected, then when the RTC time is checked you must reconnect the parallel-port
cable and restart the MAX6901 evaluation software. Do not click on "Set from
computer's clock" on this restart, or you will need to start the accuracy test from the
beginning.
12. Note: When checking the RTC time accuracy, it is recommended that all other
Windows programs be exited so as to minimize errors in the PC clock after calibration.
Results
Table 2 shows the results of the test run on the MAX6901 for long-term accuracy. From
calculations in the Initial Conditions section, we expected that the first measurable error of 1
second on the RTC would occur after 86.4 hours. The actual tests conducted yielded about 96
hours before the 1-second error occurred. Temperature effects were not logged accurately
enough to be taken into account. Using an Internet time server from NIST and simple RTC
evaluation software from Maxim, it was found that easy and accurate long-term testing can be
conducted on real-time clocks.
Table 2. MAX6901 RTC Accuracy Data
Date
Time
Offset
Comments
4-19-01
5:10PM
+1 second
-
4-20-01
9:00AM
+1 second
After PC clock
calibration with NIST
4-23-01
5:10PM
+2 second
After PC clock
calibration with NIST
______________________
References
Lombardi, Michael (Time and Frequency Division, National Institute of Standards and
Technology), Computer Time Synchronization
NIST Time and Frequency FAQ (Precise Time)
MORE INFORMATION
MAX6900: QuickView -- Full (PDF) Data Sheet (584k) -- Free Sample
MAX6901: QuickView -- Full (PDF) Data Sheet (232k) -- Free Sample
MAX6902: QuickView -- Full (PDF) Data Sheet (216k) -- Free Sample
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