AN2678

M41T82-83-93 series RTCs

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1.3

Measuring FT and the timing error

For the analog calibration, a frequency counter is recommended. This will provide the

highest accuracy. The next consideration is using the microprocessor’s timer. This can be

used to adjust the analog calibration and will be used for the digital portion.

Microprocessor timers can come in a variety of configurations with many different speeds

and input options, so this document can only address them in a general sense. Figure 1

shows the basic setup between the RTC and the microprocessor. The processor's timer is

used to measure the frequency of the RTC's frequency test signal (FT) and then adjust the

Figure 1.

Basic setup of microprocessor and RTC

Fundamentally, the processor needs to measure the period of the timer and calculate the

error or frequency deviation of the FT signal, but the measurement process is itself prone to

error. However, it should be possible to minimize that error to an acceptable level.

The key concept is that the microprocessor's timer runs off a clock signal derived from the

microprocessor's oscillator with AT cut crystal. Since this oscillator has low drift over

temperature, the timer’s clock signal will be low drift, too. Thus, using this timer, the RTC can

be calibrated to approach the accuracy of this timing chain, thus reducing the timekeeping

errors associated with watch crystals due to temperature drift.

In Figure 2, the microprocessor's clock is called TCLK, and is used to sample the 512 Hz

frequency test signal, FT. Each sample includes some uncertainty. But the uncertainty

does not change as more samples are added. Thus, with many samples, the uncertainty

will be very small in comparison to the measured period.

For example, in Figure 2, the timer can detect that a transition occurred on the FT signal

between two successive samples, but it cannot determine exactly where between those

RTC

TIMER

OSC

OSC

AT cut

crystal

Watch

crystal

FT = 512Hz

2

uP

ACAL

DCAL

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