Reading RTC-registers stops DMA transfer on STM32L051

[oleg_kochurov]

Posted on March 03, 2017 at 07:00

I am using ADC with DMA on STM32L051. Conversion time is 50us (20 000 conv. per second). When I read RTC-registers (RTC_SSR, RTC_TR, RTC_DR) sometimes the CPU freezes for approx. 113 us causing ADC overrun and loss of one conversion result (113 us > 50 us * 2).

I have found the only way to solve this problem:

1) Put RTC read commands inside of interrupt handler. 

2) Read only one RTC-register at a time.

I am completely unsatisfied by this solution. Is there any better way to read RTC-registers without ADC result loss?

Conditions:

1) CPU: 

STM32L051R8T6

2) System clock is 1.048 MHz (MSI).

3) AHB and APB2 clocks are the same, APB1 clock is 

1.048/4 MHz.

4) RTC clock source is external 32.768 kHz crystal.

5) ADC sequence (2 conversions) started by TIM6, DMA-interrupt after 100 conversions is used.

Thanks for any help!

[Jan Waclawek]

Posted on March 03, 2017 at 08:44

You mean that one sample is 'skipped', i.e. literally missing from the DMA buffer?

Or that the processor doesn't succeed to read from a circularly-filled DMA buffer fast enough and the buffer gets overwritten by DMA while you read?

JW

[oleg_kochurov]

Posted on March 03, 2017 at 09:22

I mean that one sample is skipped. ADC overrun bit is set by hardware. This skip matches reading RTC-registers.

[waclawek.jan]

Posted on March 06, 2017 at 14:47

How exactly are you reading the RTC? Post code.

JW

[oleg_kochurov]

Posted on March 07, 2017 at 07:04

int RTC_SSR, RTC_TR, RTC_DR;

...

...

RTC_SSR = RTC -> SSR;

RTC_TR = RTC -> TR;RTC_DR = RTC -> DR;

[Tomas DRESLER]

Posted on May 05, 2017 at 21:20

Seems like you may saturate the AHB bus. The DMA accessing ADC requires ~10 cycles to complete one transfer, which means 10 us, reading one RTC register takes 8-12 clocks (8-12 us) each, entry and exit from interrupt takes 32 cycles with full bus load (32 us). Does the overrun happen at the time of some interrupt or immediately/randomly? Can you test with IRQs disabled?

I'd try two things primarily:

RTC: BYPSHAD=0

APB1 prescaler=1 or 2, to increase throughput to RTC

[Uwe Bonnes]

Posted on May 06, 2017 at 13:22

Another easy thing to test would be to increase MSI speed.

[Uwe Bonnes]

Posted on May 06, 2017 at 13:42

@Edison: Yous state number of cycles of the DMA transfer. Are these number public documented or only insider knowledge?

[waclawek.jan]

Posted on May 06, 2017 at 20:09

Uwe,

while L0 is not listed there, I believe AN2548 pertains to L0's DMA too.

JW