I'm having a problem getting this GSM modem to work with an FPGA (Spartan 3). I've already implemented UART transmission on the FPGA side,the MAX232 to convert TTL->RS232, and 3.3V->5V logic level shifting to interface the FPGA with the MAX232.
I can see the messages sent by the FPGA on my PC (using the Arduino Serial Monitor) with this FTDI Breakout. I can also communicate with the GSM Modem by using the Arduino Serial Monitor to enter the AT command mode and send text messages.
When I send the same message from the FPGA to the modem,however, I receive an empty text bubble on my phone. I've double checked the baud rate settings, the string structure (including a Carriage Return at the end of every text message) and the TX(FPGA)->RX(GSM) wiring order.
I'm still uncertain of why the arduino terminal can communicate with the modem, but the same message sent from the FPGA doesn't transmit to my phone.
UART Settings: 115200 baud, 8 data bits, 1 stop bit, no parity, no flow control.The VHDL UART transmission and 115200Hz clock frequency generator component is shown below for reference.
EDIT: How the modem works:
The modem requires the use of AT commands to it either through SMS or Serial to configure it. I need it to simply send text messages so to do this I have to set the baud rate that the FPGA will use i.e. the default one of 115200Hz and the work mode i.e. SMS. Then exit AT mode and resume transparent mode. See pages 5 and 7 for a description of transparent mode and an illustration of the transmission process
This is done externally to the FPGA with the software that comes with modem to run on a PC. Once the parameters are configured, whenever the modem starts up it will start in transparent mode. So all that has to be done is send your text message and terminate with a carriage return character to signify end of text.
So the modem is in transparent mode and waiting for the text message data when it's connected to the FPGA. The modem does not transmit any data to the host (FPGA/Serial Monitor) when in transparent mode. It only looks out for the text message data in i.e. RX(GSM)
clk_115200Hz : process(clk,reset,clock3_tmp) variable a3: integer range 0 to 217; begin if(reset='1') then a3:=1; clock3_tmp<='0'; elsif rising_edge(clk) then a3 :=a3+1; if (a3 = 217) then clock3_tmp <= not clock3_tmp; a3 := 1; end if; end if; clock3 <= clock3_tmp; end process;
uart_transmit: process (clk,reset) variable bit_count: integer range 0 to 9:=0; begin if(reset='1') then tx_line<=idle; bit_count:=0; TX_DONE<='0'; elsif rising_edge(clk)then if(enable= '1')then if(TX_READY = '1')then if(bit_count=9)then TX_DONE<='1'; tx_line<=idle; bit_count:=0; else tx_line<=TX_MESSAGE(bit_count); bit_count:= bit_count+1; TX_DONE<='0'; end if; elsif(TX_READY = '0')then tx_line<=idle; end if; end if; end if; end process;