External Watchgod Monitor S32K148

[vanketnguyen]

I have some concerns regarding FS32K148 EWM Module:

1. Is it ok to use the internal redundant watchdog EWM to generate the reset signal for MCU instead of an external HW watchdog.
2. Please confirm if we require to pull down with external resistor the EWM_OUT_b signal when EWM is used. ( See Reference below)
3. Will the EWM_Out_b pin will be in high impedance when MCU is reset ( till next EWM configuration)  or  will it be kept low without external pull down

[Reference:  In RM there is a mentioning about having a pull down for EWM_Out_b signal ( Page 475).

“EWM_OUT_b pad must be in pull down state when EWM functionality is used and when EWM is under Reset.”

“The EWM_OUT_b is asserted after any reset by the virtue of the external pull-down mechanism on the EWM_OUT_b signal. Then, to deassert the EWM_OUT_b signal, set EWMEN bit in the CTRL register to enable the EWM” ]

[sebastian_s]

Hello there,

sorry for digging up the past but this became of interest for me recently. I'd like to clarify point 1 a bit. The EWM does not reset the MCU by default but would it be possible to use this output to trigger the MCU reset? If my understanding is correct the EWM does monitor the MCU functionality and if an error is detected, this could pull low the MCU RESET pin. Once the MCU enters reset state, the pin reverts to being HighZ and the MCU boots up again. The only question would be, is it a push/pull or open-drain output (can it simply be connected in parallel with a power-good pin of the power supply)?
According to the reference manual "For safety, a redundant watchdog system, External Watchdog Monitor (EWM), is
designed to monitor external circuits, as well as the MCU software flow. This provides a
back-up mechanism to the internal watchdog that resets the MCU's CPU and peripherals."

[danielmartynek]

Hi vanketnguyen, 

1.

The redundant watchdog EWM does not reset the MCU.

To reset the MCU, you need either the internal WDOG module or an external one like an SBC.

2. 

Adding an external pull-down resistor is the only way to ensure the EWM_OUT_b pin is LOW during MCU reset.

3. 

Because the EWM_OUT_b is an alternative function of some pins like PTA2.

Please have a look at the S32K148 IO_Signal_Description_Input_Multiplexing.xlsx file that is attached to the RM.

By default (out of reset), PTA2 is connected to ADC1 module, channel 0.
Therefore, whenever the MCU reset, the PTA2 pin becomes an ADC input until it is changed by the SW (routed to EWM, EWM enabled).

Regards,

Daniel