I see.. we are using the same GSM module
I have interfacing problem with this GSM module before when using PIC16F877A with VDD = +5v.
In my experiment, I used 3 different voltage translation circuits for testing.
1. Direct connect GSM_TX to MCU_RX and voltage divider circuit for MCU_TX to GSM_RX
2. Using NPN transistor fro GSM_TX to MCU_RX and signal diode for MCU_TX to GSM_RX
3. Using buffer chip
All configurations i tested with the PC + Hyperterminal and all work fine.
Now, when i tested to a PIC16F877A, (1) doesn't work reliably. I run the PIC + GSM module setup with a debugger (eICD2), and i can see 1 or 2 received characters that are wrong, i.e. instead of +CMTI="SM",1 in the buffer i get +CMTh = "SM"&1.
I tried (3) and it worked perfectly. I used a HC125 buffer chip and it worked. Pero hirap hanapin ang ganito na IC.
So I tried (2) also and it doesn't worked sometimes, with wrong characters received but not as often or as unreliable as (1). In my setup right now (2) worked just fine, but testing it on several GSM modules (10 GSM modules to be precise, during a seminar workshop last week in e-gizmo
) it still not as reliable.