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RM0312

Auto-wakeup (AWU)

Example 1

•

fLS = 38 kHz

•

Target time interval = 16 ms

The appropriate interval range is: 13.47 ms - 26.95 ms

so the AWUTB[3:0] value is 0x5.

The APRDIV value is:

16 ms = 24 x APRDIV / fLS => APRDIV = (16*10-3 x fLS ) / 24 = 38

so the APR[5:0] value is 38 (0x26)

Example 2

•

fLS = 38 kHz

•

Target time interval = 20 ms

The appropriate interval range is: 13.47 ms - 26.95 ms

So the AWUTB[3:0] value is 0x5.

The APRDIV value is:

20 ms = 24 x APRDIV / fLS => APRDIV = (20*10-3 x fLS) / 24 = 47.5

So the AWUTB[3:0] can be either 47 or 48 which gives a time base of 19.79 ms or 20.21 ms

respectively. This is not exactly 20 ms.

12.3.3

LSI clock frequency measurement

Due to the oscillator frequency dispersion, to obtain a precise AWU time interval or beeper

output, the exact LSI frequency has to be measured.

Use the following procedure:

Set the MSR bit in the Control/status register (AWU_CSR) to connect the LSI clock

internally to a timer input capture.

Measure the frequency of the LSI clock using the Timer input capture interrupt.

Write the appropriate value in the APR [5:0] bits in the Asynchronous prescaler register

(AWU_APR) to adjust the AWU time interval to the desired length. The AWUTB[3:0]

bits can be modified to select different time intervals.

LSI clock frequency measurement can also be used to calibrate the beeper frequency (see

Section 13.2.2).

RM0312 Datasheet(PDF) 87 Page - STMicroelectronics