RTC F207

damh · ‎2013-06-28

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!

damh · ‎2013-06-28

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 😉