Vsys oscillation vs 3.3V rail

Werner Almesberger werner at openmoko.org
Wed Jul 2 06:40:56 CEST 2008


Andy Green wrote:
> Again I tested bringing external 3V3 to IO_3V3 and it causes the device
> to start OK without battery.

Hmm, not here :-(

io_3v3.png shows VB_SYS (yellow) and IO_3V3 (blue), VB_SYS bypassed to
GND with 10uF, nothing else happening.

io_3v3-zoom.png shows the same scenario, but zoomed into the event.

Then I added an external 3.3V through a 1 Ohm resistor.
io_3v3-powered.png shows what IO_3V3 looks like.

To check that I really got IO_3V3, I added the usual external 100uF
bypass to VB_SYS, removed the power injection, and let the system
boot my 0.5 Hz LED blinker. This is shown in io_3v3-normal.png

I tried varying the resistor, but that only allowed me to vary between
io_3v3.png and io_3v3-powered.png, but didn't help:

R	    IO_3V3
(Ohm)	min(V)	max(V)
  1	 3.00	3.60
 47	 2.00	3.12
100	 1.20	2.52
150	 0.72 	2.08
open	-0.08	1.48

With the 1 Ohm resistor, the PMU powers off after a minute or so.
Guess it doesn't like to share ;-)

So it seems that stabilizing IO_3V3 alone does not solve the issue.

- Werner
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