On 07/26/2013 05:05 PM, Kevin Redon wrote:
Excerpts from Tom Schouten's message of 2013-07-26
22:55:32 +0200:
Hi folks,
Seems not all phones use 3V signaling if the PHONE-side VCC pin is tied
to 3V.
This one insists on 1.8V : BLU SAMBA JR PLUS
how does the phone behave?
if the at91 doesn't respond, the phone goes through the following
power/reset sequence, about 1 second apart:
1.8V
3V
3V
1.8V
the at91 seems to mostly interpret the 1.8V serial line correctly, so
the phone just starts talking at 1.8V until this eventually goes wrong.
Is this trick of tying VCC to 3V standard
behavior?
no, it is not standard. It just works (most of the time).
And it should not damage the SIM nor the phone.
For future version of the board, it might be
interesting to tie the VCC
pin to an ADC input, to detect different voltage levels so the firmware
can wait until phone decides to use 3V.
this will be done in the next board
good
( next to some GPIOs maybe ;)