Re: [Gumstix-users] Protecting UART ports while SYSEN is low

[sturnfie <stu...@gm...>]

Ewald Kantner wrote
> 
> Hello,
> 
> I have read this item in the board design requirements:
> 
> Use the SYSEN line to protect any IO pins to the OMAP CPU. SYSEN is 
> brought high when the Overo is ready to communicate; driving GPIOs 
> before this point can damage the processor.
> 
> Can this problem be avoided by simply adding a 330R series resistor on 
> the RX line of the Overo?
> 
> Best regards,
> Ewald
> 
> 

Hi Ewald,

Ahh yes that's a fun one. The issue seems to be with internal voltage domain
biasing of the OMAP. If the GPIOs are not initialized and a voltage is
present on the inputs, the internal voltage rails of the OMAP are driven
higher than intended and bad things happen during initialization. I had a
design that drove the UART1/3 lines immediately on power-on...one board
lasted 2 months,  another lasted 7, and a couple more are still running
(we're working on getting them returned for replacement with the new
revision..)

Limiting the current with a series resistor certainly is better than leaving
a direct line, but I'd suggest not driving the lines at all until the PMIC
indicates that the system is ready (via the SYSEN line). I now use SYSEN to
gate a voltage regulator on the low side of level shifters (I usually have
to step voltages down to the Overo's +1.8V drive and this was a convenient
way to get all the GPIO lines at once). 

IIRC, I think Steve Sakoman was the guy who tracked down the issue. You
might find more precise info on this forum somewhere in one of his posts.. 

Lucas

-----
-- 
Lucas Sturnfield
stu...@gm...

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