AN2604 - Application note

RTC calibration basics

9/14

As described above, the STM32F10xxx RTC clock calibration circuit subtracts cycles only

from crystal clocks. And based on the fact that the RTC prescaler value is set by default to

32 768, faster crystal frequencies (> 32 768 Hz) can be calibrated whereas slower crystal

frequencies (< 32 768 Hz) cannot be compensated for. So only crystal frequencies in the

range [32 772, 32 768] can be calibrated.

Since the crystal frequency may vary about 32.768 kHz, a solution may be considered that

consists in setting the RTC prescaler to 32 766 (instead of 32 768). The crystal frequency is

thus compared to 32 766 instead of 32 768. In this way, a crystal frequency in the range [32

770, 32 766] can be compensated.

Throughout the rest of the document, the considered RTC prescaler value will be 32 766.

62

59

153

126

120

311

63

60

156

127

121

314

Table 1.

Calibration table: compensation values in ppm and seconds per month

(30 days) (continued)

Calibration

value

Value in ppm

rounded to

the nearest

ppm

Value in seconds

per month (30

days) rounded to

the nearest

second

Calibration

value

Value in ppm

rounded to

the nearest

ppm

Value in seconds

per month (30

days) rounded to

the nearest

second