picandies
Posts: 156
Joined: Wed Nov 26, 2014 5:13 pm

cm3 module release from reset

Mon Oct 22, 2018 11:55 pm

What are the detailed "startup" conditions for the module to leave reset & begin running?...By this, I mean I know applying +3.3V (along with some other voltages) starts it running. Is there some threshold it first crosses, then it waits xx ms before running? Is there something holding it in reset until a certain condition is reached, then reset is released and the show begins? Are there any adjustments available to these parameters? After software shutdown my unit seems to be trying to restart (don't want that), but I'm not sure what the trigger mechanism is. Ten secs after software shutdown the supply voltage is falling (since it gets shut off seconds later)

aBUGSworstnightmare
Posts: 1096
Joined: Tue Jun 30, 2015 1:35 pm

Re: cm3 module release from reset

Tue Oct 23, 2018 5:16 am

picandies wrote: What are the detailed "startup" conditions for the module to leave reset & begin running?...By this, I mean I know applying +3.3V (along with some other voltages) starts it running. Is there some threshold it first crosses, then it waits xx ms before running? Is there something holding it in reset until a certain condition is reached, then reset is released and the show begins? Are there any adjustments available to these parameters? After software shutdown my unit seems to be trying to restart (don't want that), but I'm not sure what the trigger mechanism is. Ten secs after software shutdown the supply voltage is falling (since it gets shut off seconds later)
Quote from page 17 of the Compute manual data sheet:
7.1 Supply Sequencing
Supplies should be staggered so that the highest voltage comes up first, then the remaining voltages
in descending order. This is to avoid forward biasing internal (on-chip) diodes between supplies, and
causing latch-up. Alternatively supplies can be synchronised to come up at exactly the same time as
long as at no point a lower voltage supply rail voltage exceeds a higher voltage supply rail voltage.
Is this condition achieved by your design? Are you using RUN signal?
How do you see 'your unit tries to restart'?

picandies
Posts: 156
Joined: Wed Nov 26, 2014 5:13 pm

Re: cm3 module release from reset

Tue Oct 23, 2018 6:56 am

No, the run is floating, so it should be pulled up by the internal 10k. since it is high the whole time, doesn't seem like that would cause a restart, especially if power is dropping 10 sec after the shutdown command.
When initially powered on, or after the RUN pin has been held low and then released, the BCM2837
will try to access the primary SD/eMMC interfac
Maybe just tie the run line low...since it would see the initial power on when power is later restored?

aBUGSworstnightmare
Posts: 1096
Joined: Tue Jun 30, 2015 1:35 pm

Re: cm3 module release from reset

Tue Oct 23, 2018 8:07 am

If you don't use RUN , the data sheet recommends to leave it open. As there is an internal pull-up it is not floating.

Any feedback regarding power sequencing?

picandies
Posts: 156
Joined: Wed Nov 26, 2014 5:13 pm

Re: cm3 module release from reset

Tue Oct 23, 2018 8:19 am

Powerup-wise the sequencing is fine...not so sure what happens when the power starts to fade out after the shutdown...a 5V (main) regulator with holdup drives the other regulators (3.3V, 1.8V, etc)...but it seems like the module is trying to restart (though gives up when the power completely fades).

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