February 16, 2013
about 2 years ago
A lot of members on this forum suffers all kind of strange behavior. Mayby I do have a solution. My cellular shield (which was placed on a Netduino Plsus 2) did not to operate reliably. The serial communication refuses completely or stops after a few commands, never reaching the +Sind:4. The SIM card was not recognized or marked as invalid.
It appeared to be caused by randomly and unintended resets during the initialization fase. The reason for this is obvious. The 10K pull-up resistor on the reset-line of the Netduiro is connected to the 3v3 instead of 5V in case of the Arduino. Due to the load of the SM5100b the voltage on the reset line drops to below 2v while according to specification this must be more than 2.2V. (page 20/21 of the datasheet. The sink current is also specified there (2ma.) The Netduiro is keep function but the SM5100B fails a lot of times.
The simple solution is to add a 4k7 resistor between RST and the 3V3. (in parallel to the 10k) A small SMD part fits nice between the two successive pins) Then the voltage rises to approximately 2.7V which is neatly within specification. Hope this help you.
I am not sure of this problem is restricted to Netduiro/SM5100b combinations because even with 5V the Reset pull-up is too weak and still very ctitical.
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