ST STM32F301 6 Series Reference Manual page 644

Real-time clock (RTC)
24.6.18 RTC alarm B sub second register (RTC_ALRMBSSR)
This register can be written only when ALRBE is reset in RTC_CR register, or in initialization
mode.
This register is write protected.The write access procedure is described in
register write
Address offset: 0x48
Backup domain reset value: 0x0000 0000
System reset: not affected
31 30 29
Res. Res. Res. Res.
15 14 13
Res.
rw rw
Bits 31:28 Reserved, must be kept at reset value.
Bits 27:24 MASKSS[3:0]: Mask the most-significant bits starting at this bit
Bits 23:15
Bits 14:0 SS[14:0]: Sub seconds value
644/874
protection.
28 27 26 25
MASKSS[3:0]
rw rw rw
12 11 10 9
rw rw rw rw
0x0: No comparison on sub seconds for Alarm B. The alarm is set when the seconds unit is
incremented (assuming that the rest of the fields match).
0x1: SS[14:1] are don't care in Alarm B comparison. Only SS[0] is compared.
0x2: SS[14:2] are don't care in Alarm B comparison. Only SS[1:0] are compared.
0x3: SS[14:3] are don't care in Alarm B comparison. Only SS[2:0] are compared.
...
0xC: SS[14:12] are don't care in Alarm B comparison. SS[11:0] are compared.
0xD: SS[14:13] are don't care in Alarm B comparison. SS[12:0] are compared.
0xE: SS[14] is don't care in Alarm B comparison. SS[13:0] are compared.
0xF: All 15 SS bits are compared and must match to activate alarm.
The overflow bits of the synchronous counter (bits 15) is never compared. This bit can be
different from 0 only after a shift operation.
Reserved, must be kept at reset value.
This value is compared with the contents of the synchronous prescaler counter to determine
if Alarm B is to be activated. Only bits 0 up to MASKSS-1 are compared.
24 23 22
Res. Res.
rw
8 7 6
SS[14:0]
rw rw rw
RM0366 Rev 5
Section : RTC
21 20 19 18
Res. Res. Res. Res.
5 4 3 2
rw rw rw w
RM0366
17 16
Res. Res.
1 0
rw rw