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Re: [lwip-users] hayes commands popping up in ppp stream

From: Sylvain Rochet
Subject: Re: [lwip-users] hayes commands popping up in ppp stream
Date: Fri, 14 Nov 2014 21:32:22 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

Hi chrysn,

On Fri, Nov 14, 2014 at 02:12:16PM +0100, chrysn wrote:
> (i'd be very careful with that kind of oob mechanisms from a
> synchronization point of view, but then again, i'd much rather have a
> modem on a clean spi interface with cs and interrupt than all that
> asynchronous serial stuff, but i digress.)

I agree, GSM/GPRS modems are buffered devices (dialup are not) so 
nothing prevent them to use a SPI bus + IRQ.

> from what i can tell, there won't be a 0x7e ('~') or 0x7d ('}') in the
> ascii output. thanks for the pointer to hdlc, i wouldn't have noticed
> that there are two separate layers involved when i looked over the lwip
> ppp code.

This is actually only a HDLC-like framing, HDLC is much more complicated 
than that. PPP only use the HDLC framing way, which is simple and 

> i'm curious, though: if what the sim900d does is not The Right Way, how
> would a process communicating with a gsm modem know when, for example,
> the cell signal strength changed, a call comes in or the modem connects
> to another cell? (granted, there's a dedicated ring pin, but there are
> many conditions, and assigning a gpio per feature is not how i want to
> run my pcbs.)

Packet muxer (requiring an input and output driver for this added layer) 
or by using the DTR pin to switch modem mode:

DTR low   (data -> command)
send AT+CSQ
recv AT+CSQ: 10,0
DTR high (command -> data)

Note that DTR pin should be previously configured with AT&D1:

"AT&D1 - Dropping the DTR signal puts the modem into Command Mode, 
without disconnecting the call. The computer may disconnect the call 
with the ATH command, or return to the call with ATO. This mode is 
useful if the computer wishes to change settings on the modem during the 
call (such as activating test modes)."[1]



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