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damh
damh
Associate III
June 28, 2013
Question

RTC F207

  • June 28, 2013
  • 1 reply
  • 459 views
Posted on June 28, 2013 at 10:10

MCU: STM32 F207 VG

We try to upgrade a device from STM32 F107 to F207.

We are glad to use the ''old'' RTC and to have a simple 32bit counter with a free settable prescaler (our RTC works at 1024Hz).

Now the ''new'' RTC is a BCD-coded version, normally working at 1Hz.

Is there a simple way, to:

- get it back into a simple counter (synchronize to LSE) and

- to work without VCC (power over VBAT only)?

To take a 32bit counter like TIM2 with external trigger over external pin connection from RTC calibration out seems to be a possibility but is there a smarter way?

The reference manual speaks about a possiblity to get the WUKF out to a pin (Output selection in RTC->CR to ''Wakeup output enabled'').

Have anyone verified this feature yet? What (unmentioned) precondition must be met to get it working?

Or is it a feature with notice ''implemented but not wired''? ;)

Thanks a lot!
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    1 reply

    damh
    damhAuthor
    Associate III
    June 28, 2013
    Posted on June 28, 2013 at 13:54

    Ok, ALARMOUTTYPE of RTC->TAFCR has to be set to get WUKF on PC13.

    WUKF has to be cleared by software. So this function cannot generate a square wave signal and it could be programmed in an interrupt service routine. This cannot be done in this case, because interrupts are often disabled for a longer time than the interrupt period time.

    RTC_CALIB has an undocumented feature, too:

    The RTC_CALIB out function seems to be defined as (PCNT is the acual prescaler counter value (running cyclic from 0 to PREDIV_A)):

    if (PCNT<0x20)

        RTC_CALIB = LOW;

    else if (PCNT<0x40)

        RTC_CALIB = HIGH;

    else if (PCNT<0x60)

        RTC_CALIB = LOW;

    else

        RTC_CALIB = HIGH;

    This has the effect, that the signal at PC13 looks curios, if PREDIV_A is not 0x3F or 0x7F!

    If PREDIV_A is less than 0x20 there is a constant LOW signal at PC13 ;)
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