AN632 Datasheet, PDF (3/8 Pages) Maxim Integrated Products – Evaluating the Accuracy of Maxim Real-Time Clocks (RTCs)

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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