RTC calibration basics

AN2604 - Application note

6/14

1.2

Methodology

The RTC of STM32F10xxx products is driven by a quartz crystal-controlled oscillator with a

nominal frequency of 32.768 kHz. The crystal oscillator is one of the most accurate circuits

to provide a fixed frequency. There are two causes of clock error:

1.

temperature variation

2.

crystal variation

As mentioned previously, most clock chips compensate for crystal frequency and

temperature variations by using cumbersome trim capacitors. The STM32F10xxx design

employs periodic counter corrections. The digital calibration circuit removes 0 to 127 cycles

upon the value that has been loaded into the seven least significant bits of the BKP’s RTC

clock calibration register. Since the RTC clock calibration register is in the backup domain,

the calibration value is not lost even if the device is powered off provided that a battery is

Figure 2.

RTC calibration clock output

1.

The clock output on the ANTI_TAMP pin is the RTC clock before calibration, so its value is not changed by

the calibration.

in the calibration register. As a result, the oscillator clock can be slowed down from 0 to

121 ppm.

Table 1 on page 7 shows how many ppm and seconds per month (30 days) each bit

represents in real time.

Oscillator

Div64

LSE

LSI

HSE/128

ANTI_TAMP

512 Hz output

for frequency test

Enabled by CCO bit in

BKP_RTCCR register

Clock

calibration

CAL[6:0]bits in

BKP_RTCCR register

RTC

RTCCLK

32 768 Hz

ai14626