TCAN2451-Q1: nRST pin stuck to high

Hi Shiyu,

If the nRST is high then the device won't restart when the nRST pin is asserted low. 

The device only drives nRST low when the device is in restart mode - so if devices downstream rely on the output low signal from this pin during restart mode they won't see the signal. 

Ultimately - if the nRST is stuck high -the restart functionality cannot be accessed through the nRST pin but most other device operation should be okay if only nRST pin is impacted. 

Best,

Parker Dodson 

Hi,

Q1: If the MOSFET is open circuited between its source and drain connections - it can still accept an input if the pull-up resistor to VCC1 is okay (when I saw you said stuck high I thought you meant the 30k had a short or something similar - but if the MOSFET isn't shorted to ground the pin will not output the indication signal that device has been restarted) - it won't be able to generate any output signal. So if the MOSFET is broken with an open circuit between S/D if if gate voltage is being applied the device will not output a low signal - the actual SBC should still work but if the application requires the SBC to alert its controller of a restart that functionality will be gone. The tINITWD timer will also not start because it is looking for the low to high transition after restart mode - which depending on device configuration may cause issues that could send the device to sleep mode or FSM if enabled. 

Q2: The device will automatically enter standby mode from restart mode regardless if the FET on the nRST pin is working or not. The device doesn't stay in restart mode for long - it is a transitory state that will always transition into standby if the restart counter hasn't been exceeded - the device does not directly transition from restart mode to normal mode. 

Q3: LIMP pin and nRST pins are not directly related in operation:

"The LIMP pin is for the limp home function and is an open-drain, active low, output. It is used for a limp home
mode if the watchdog has timed out causing a reset. The pin should be pulled up with an external resistor
connected to the battery supply, VSUP. For the LIMP pin to be turned off, the watchdog error counter must reach
zero from correct input triggers. If programmed, any event that triggers the fail-safe mode also turns on the LIMP
pin. LIMP pin can be configured to be active in Sleep mode in case of VCC1SC, OVCC1, Watchdog error or
thermal shutdown faults using the register bit LIMP_SLP_FLT_EN (8'h1A[7]). This feature enables LIMP to be
activated on entering Sleep mode due to any faults in case fail-safe mode is disabled.
LIMP pin can also be programmed to be used as a low-side switch. The low-side switch can be configured using
the register bit LIMP_LSS_SEL. To use the pin as low-side switch, LIMP functionality needs to be disabled using
the register bit LIMP_DIS. Otherwise, the device will ignore the low-side switch setting. The low-side switch can
be used as either always On or always Off or be configured to use PWM1, PWM2 or Timer1 or Timer2 using the
register bit LIMP_LSS_CNTL."

I.e. the state of the LIMP pin is not directly to the state of the nRST pin 

Essentially if the MOSFET is broken the SBC loses the ability to signal a connected device to the nRST pin when the device enters restart mode. It also can potentially impact WD functionality.  If the 30k pull-up is still working as normal the nRST pin will still act as an input assuming -only the MOSFET was damaged for whatever reason. 

Best,

Parker Dodson