RM0440 Rev 4 1553/2126
RM0440 Real-time clock (RTC)
1592
Verifying the RTC calibration
RTC precision is ensured by measuring the precise frequency of RTCCLK and calculating
the correct CALM value and CALP values. An optional 1 Hz output is provided to allow
applications to measure and verify the RTC precision.
Measuring the precise frequency of the RTC over a limited interval can result in a
measurement error of up to 2 RTCCLK clock cycles over the measurement period,
depending on how the digital calibration cycle is aligned with the measurement period.
However, this measurement error can be eliminated if the measurement period is the same
length as the calibration cycle period. In this case, the only error observed is the error due to
the resolution of the digital calibration.
• By default, the calibration cycle period is 32 seconds.
Using this mode and measuring the accuracy of the 1 Hz output over exactly 32 seconds
guarantees that the measure is within 0.477 ppm (0.5 RTCCLK cycles over 32 seconds,
due to the limitation of the calibration resolution).
• CALW16 bit of the RTC_CALR register can be set to 1 to force a 16- second calibration
cycle period.
In this case, the RTC precision can be measured during 16 seconds with a maximum error
of 0.954 ppm (0.5 RTCCLK cycles over 16 seconds). However, since the calibration
resolution is reduced, the long term RTC precision is also reduced to 0.954 ppm: CALM[0]
bit is stuck at 0 when CALW16 is set to 1.
• CALW8 bit of the RTC_CALR register can be set to 1 to force a 8-second calibration
cycle period.
In this case, the RTC precision can be measured during 8 seconds with a maximum error of
1.907 ppm (0.5 RTCCLK cycles over 8 s). The long term RTC precision is also reduced to
1.907 ppm: CALM[1:0] bits are stuck at 00 when CALW8 is set to 1.
Re-calibration on-the-fly
The calibration register (RTC_CALR) can be updated on-the-fly while RTC_ICSR/INITF = 0,
by using the follow process:
1. Poll the RTC_ICSR/RECALPF (re-calibration pending flag).
2. If it is set to 0, write a new value to RTC_CALR, if necessary. RECALPF is then
automatically set to 1
3. Within three ck_apre cycles after the write operation to RTC_CALR, the new calibration
settings take effect.
35.3.14 Timestamp function
Timestamp is enabled by setting the TSE or ITSE bits of RTC_CR register to 1.
When TSE is set:
The calendar is saved in the timestamp registers (RTC_TSSSR, RTC_TSTR, RTC_TSDR)
when a timestamp event is detected on the RTC_TS pin.
When TAMPTS is set:
The calendar is saved in the timestamp registers (RTC_TSSSR, RTC_TSTR, RTC_TSDR)
when a tamper event is detected on the TAMP_INx pinx.
When ITSE is set:
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