GSM modem doesn't power off when Neo does

Werner Almesberger werner at
Thu Feb 21 05:42:48 CET 2008

Kyle Bassett wrote:
> Out of curiosity, could you provide some information on the physical changes
> to the board?

If you want to insert a switch, B1701 may be a good place to do this.
B1701 is the KLB0603K100SA choke that separates VB from VBAT. VB powers
the system (and it's the positive battery terminal), while VBAT powers
only the GSM modem.

B1701 is placed near the battery connector, see attachment.

If you don't mind soldering a wire to a QFN package, you could control
the switch from the PMU, using any of the GPOs. Using the PMU instead
of the MCU has the advantage that the line is held in a defined state
while there is power. (On the MCU, any sneak current floating through
the MCU and out of the GPIOs could cause problems.)

Regarding sending the AT at POFF, how about using a wrapper around gsmd ?
When starting, it would make sure the modem is powered on, then
system(3)s gsmd. When gsmd terminates, the wrapper would power down the
modem, and then exit itself.

Not perfect, but it should be pretty stable and easy to do.

- Werner
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