RM0090: Reference manual - STMicroelectronics

Real-time clock (RTC) RM0090
23.6.7 RTC calibration register (RTC_CALIBR)
Address offset: 0x18
Power-on reset value: 0x0000 0000
System reset: not affected
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Reserved
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Reserved
Bits 31:8 Reserved
Note: This register can be written in initialization mode only (RTC_ISR/INITF = ‘1’).
This register is write protected. The write access procedure is described in RTC register
write protection on page 625.
645/1416 Doc ID 018909 Rev 3
DCS
DC[4:0]
Reserved
rw rw rw rw rw rw
Bit 7 DCS: Digital calibration sign
0: Positive calibration: calendar update frequency is increased
1: Negative calibration: calendar update frequency is decreased
Bits 6:5 Reserved, must be kept at reset value.
Bits 4:0 DC[4:0]: Digital calibration
DCS = 0 (positive calibration)
00000: + 0 ppm
00001: + 4 ppm (rounded value)
00010: + 8 ppm (rounded value)
..
11111: + 126 ppm (rounded value)
DCS = 1 (negative calibration)
00000: −0 ppm
00001: −2 ppm (rounded value)
00010: −4 ppm (rounded value)
..
11111: −63 ppm (rounded value)
Refer to Case of RTCCLK=32.768 kHz and PREDIV_A+1=128 on page 630 for the exact
step value.