MKM34 WDOG behaviour after power on reset

[raul10]

Dear all,

I have an own pcb with the MKM34 on it. My software is running well so now I want to implement the WDOG. However, I found out that after a power on reset the WDOG behaves different to what I expected with the default settings described in the manual.

According to the reset values the WDOG is enabled with the clock source from LPO and a timeout value of 0x4C4B40 which should be a timeout of 83 minutes.

But with this small piece of code which is executed immedeately after reset without any other code in between I found out that the WDOG generates a reset 200 µs after power on reset.

void Startup(void)
{
   // configure Port B3 as output
   SIM->SCGC5 |=  SIM_SCGC5_PORTB_MASK;
   PORTB->PCR[3] = 0x100;
   GPIOB->PDDR |= 0x8;

   // set Port B3
   GPIOB->PSOR |= 0x8;

   while (TRUE) ;
}

I confirmed that it is really a WDOG by setting port B3 only if RCM_SSRS0_SWDOG_MASK was set.

Does anybody have an explanation for this?

[MarMi]

Hello,

after device power up the WDOG_STCTRLH[CLKSRC] is set to 1 meaning that WDOG is sourced from alternate clock source (32.768 kHz RTC oscillator) after power on reset. I will ask our documentation team to change WDOG_STCTRLH[CLKSRC] reset value from 0 to 1 in next revision of the reference manual. 

Kind regards,

Martin M.

[raul10]

Hi Martin,

Thank you for the information. But this should give a timeout of 152 seconds after power on reset whereas I observed 200 µs. Do you know the reason for this?

[MarMi]

Hi Raul,

I don't have any quick explanation for such short timeout.

Have you populated external 32.768kHz crystal on your PCB - if not please refer to: Watchdog configuration in MKMxxZxxACxx5RM.

You can also run wdog_test example from Kinetis-M bare metal drivers on your board to reveal potential issue in code.

If none of suggestions work for you please share your watchdog configuration example with me so I can run it on my TWR board.

Kind regards,

Martin M.

[raul10]

Hi Martin,

My board has an external 32.768 kHz crystal.

I haven't made any WDOG configuration since I wanted to see the WDOG behavior after power on. So the piece of code in my first post is really all what is running on my board. The Startup() routine is called directly by the reset vector.

[MarMi · Accepted solution]

Hi Raul,

You must unlock the registers within WCT { 256 bus clock cycles } after system reset, failing which the WDOG issues a reset to the system... For more details, refer to MKM34Z128 reference manual, section Watchdog configuration time.

The WCT of 256 bus clock cycles for 1MHz bus clock equals to 256 us timeout. 

Kind regards,

Martin M.

[raul10]

Hi Martin,

It seems that I have missed that point. Thank you for your explanation, that's the reason for the behaviour.

Kind regards,

Raul