PMIC_STBY_REQ holding system in standby on power up

[xray]

We have a board with an imx6Q CPU that is powered by the MMPF0100 PMIC. 

In normal operation when you power the PMIC (apply a voltage on VIN) VSNVS comes up. PMIC_STBY_REQ (generated by the imx6) comes up at the same time as VSNVS, then after 20 milliseconds PMIC_STBY_REQ drops and the PMIC brings up the rest of it's supplies and the imx6 boots.

Occasionally we've observed the PMIC failing to start.  We traced the behavior to PMIC_STBY_REQ not droping after 20 milliseconds.  Instead it stays high and holds the PMIC in a standby state. 

What could cause PMIC_STBY_REQ to stay high before the CPU boots?  The reference manual shows that PMIC_STBY_REQ is generated by the Clock Controller Module (CCM) but  I don't understand what the CCM does on initial power up (just VSNVS power) and what needs to happen before it drives PMIC_STBY_REQ low.

[igorpadykov]

Hi j b

this may be caused by incorrect power up sequence

(so some modules not reset properly), for example repetitive

on-off-on-off.. without sufficient pause, so processor

voltages do not fall zero before next power-up.

Best regards

igor

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