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Behaviour of MC33908 RST_b with VAUX in fault condition

Question asked by Andrew Smith on Mar 7, 2017
Latest reply on Mar 10, 2017 by Tomas Vaverka

I'm trying to find the right solution to allow the MC33908 to power up a MPC5744P when the VAUX supply is held in a fault state.

VAUX would typically be used to power external sensors in an automotive environment, so there is a possibility it may be held at 0V in a fault situation.

 

In this case:

VAUX has been configured as a tracker supply to VCCA

VAUX is defined as non-safety-critical

The VAUX_FS bits in the INIT_SUPERVISOR2 register are cleared so that undervolts or overvolts on this regulator should have no impact on RST_b and FS0_b.

 

If I create a short circuit with the processor running code, the PSU self-protects after 50ms (specified in SPI comms), and code execution continues as normal.

I can command the processor to power down under software control, this switches off the V_CORE and VPRE, but note that the SBC itself still has a 12V battery connection, so it's state machine does not reset.

 

If I then command a wake-up, the VPRE, V_CORE, VCCA and VCAN regulators all start. VAUX is held down by the short-circuit to ground, and RST_b is held low by the SBC. This obviously prevents software running.

Obviously, I need it such that the processor is able to start in that condition - software could then do a periodic retry on the supply to bring things up once the fault condition is removed.

Is there something obvious I've missed in the setup of the SBC which would prevent it from allowing the processor to start?

Thanks in advance for any replies.

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