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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