How does one return the GSM modem to a known state?

Joachim Steiger roh at
Thu Nov 1 04:04:40 CET 2007

Mike (mwester) wrote:
> I'm looking for a way to have a function within the kernel issue the
> required sequence to power off the GSM modem on the GTA01 when Linux shuts
> down.
> How can one return the GSM modem on the GTA01 to a known state from which it
> can be issued the AT at POFF command to power itself off?  The reset GPIO is
> not connnected on the GTA01, and the power GPIO only turns it on, does not
> turn off the modem -- so as far as I know, one must be able to issue the AT
> command to power off -- otherwise the device is powered off, but the phone
> remains on and fully functional (a clear violation in many situations, such
> as blast zones, or aircraft).

you can pull down the power_on gpio line and up again.
the modem resets itself as far as i know, or atleast the at command
interpreter reinits itself on rising flank.

> We need to assume that the modem can be in *any* state when the shutdown
> needs to be issued, hence the need to return it to a mode where it will
> accept and act on an AT command immediately.
> Is there a magic sequence for this?  Or can I just assume that issuing
> something like \r\rAT at POFF\r will *always* work?

the procedure which should work in any case is:
* trying if at at poff does kill the at-interpreter and pull down the
power_on gpio afterwards. (that should not be left on high)
* if the at-interpreter still lives, wiggle the power_on line up again
and try again.(that should succeed everytime, even when the interpreter
was alive at first, but the command did nothing)
but i must say that i _never_ have seen the modem crash that hard yet,
so this is more how to behave and still have the theoretical worst case
just make sure that power_on is also pulled low and does not stay high.
i've seen the modem send lowlevel debug still when not, but never when
the above is covered.



More information about the gsmd-devel mailing list