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STM32F334 RTC Prescaler, Bug in Datasheet ?

Question asked by Manu Abraham on Apr 17, 2017
Latest reply on Apr 21, 2017 by Imen D

Hi,

 

While trying to calibrate the RTC on the F334, tripped on this rather possible bug..

 

I guess this is a bug in the Application Note AN3371

Doc ID 018624 Rev 5

 

Figure 11. Smooth calibration block, the RTCCLK is assumed to be 32768,

as well as in

Figure 18. RTC_CALIB clock sources

 

Just above Fig 18, it also states:

Setting 1 Hz as the output signal
1. Select LSE “32768 Hz” as the RTC clock source.
2. Set the asynchronous prescaler to the default value “128“.
3. Set the synchronous prescaler to the default value “256“.
4. Enable the output calibration by setting “COE” to ‘1’.
5. Select 1 Hz as the calibration output by setting CALSEL to ‘1’.

 

 

But, according to the datasheet RM0364, DocID025177 Rev 2

RTC prescaler register (RTC_PRER)

Bits 22:16 PREDIV_A[6:0]: Asynchronous prescaler factor
This is the asynchronous division factor:
ck_apre frequency = RTCCLK frequency/(PREDIV_A+1)
Bit 15 Reserved, must be kept at reset value.
Bits 14:0 PREDIV_S[14:0]: Synchronous prescaler factor
This is the synchronous division factor:
ck_spre frequency = ck_apre frequency/(PREDIV_S+1)

 

Asynchronous prescaler is 7 bits wide. Setting "128" will overflow the bitfield, but setting 127 (0x7f) does seem correct.

But, 32768/128 = 256Hz, but the figure states 512Hz.

 

Isn't this a bug in the document ? Is there an already update to the Application Note already available ?

 

Thanks.

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