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ACCURATE TIMEBASE FOR LOW-COST ST7 APPLICATIONS WITH INTERNAL RC...

1 SOFTWARE SOLUTION

1.1 MEASUREMENT PRINCIPLE

To measure the frequency of the internal RC, software uses the input capture pin of 16-bit

TIMER A. A calibrated source is applied with 50Hz on the capture pin. Every falling edge on

this pin generates an interrupt (at 20ms time duration).

Software has taken the internal RC frequency of typically 4 MHz as per the datasheet. The

TIMER clock is selected as fcpu/2 (1 MHz). Counter overflow is generated after 65.532 ms

(0xFFFF). This internal RC frequency should be chosen between the datasheet. max. and

min. RC frequency values. Capture1 and Capture2 are stored as shown in Figure 1.

Figure 1. Timer Input Capture to measure the Reference frequency.

Here two possibilities can occur during the calculation of the capture elapsed time. If Capture1

and Capture2 are on the single counter (as shown in Figure 1), time is calculated simply by

Capture Counter = Capture2 - Capture1

Second possibility is that counter overflow generates between the measurement of the

Capture1 and Capture2. This leads to a more complex calculation given by

Capture Counter= ((0xffff - capture1) + capture2)

This Measured Capture Counter is compared with the Ideal Capture Counter. Ideal Capture

Counter is calculated manually and here for the fcpu = 2 MHz, it gives 0x4E20. With this infor-

mation Internal RC deviation is found.

FFFFh

0000h

Calibrated source for

5 Vpp, 50 Hz

Free running

counter of TIMER A

Capture1

Capture2

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