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interfacing with gprs modem

Rijurekha Sen
Intellectual 645 points
Other Parts Discussed in Thread: MAX3232

Hi,

I have designed a pcb with MAX3232 and DB9 male connector to use the UART pins of the C5505 usb stick. I have tested the UART  by connecting the interfacing pcb to the serial port of a laptop with a cross cable. All three uart examples, using DMA, interrupt and polling, given in the CSL, run perfectly with hyperterminal.

Now I need to send AT commands from the stick to a SIM300 GPRS modem. I have separately tested the GPRS modem, sending commands to it from the hyperterminal. It responds perfectly, can send sms and data.

I have attached a jpg image to show how I interface the usb stick with the SIM300 GPRS modem with a straight cable. I have also attached a .txt (changed extension from .c to .txt to enable file upload in forum). This .c file is a modification of the polling example c file of the CSL.

If I test this program with hyperterminal, it prints "AT" on the hyperterminal and when I type OK, displays that on CCS console. But when used with modem, this program  sometimes gives response and sometimes doesn't. In the latter case, the output is as follows

UART_init Successful
UART_reset Successful
UART_setup Successful
AT
command sent to modem
Before reading

And then it hangs. The behavior is similar if I use UART_fgetc instead of UART_read. Can somebody help?

Thanks in advance,
Riju
  • 0 in reply to Rijurekha Sen
    Intellectual 645 points

     

    I ran the program 18 times consecutively. I have given the output below. 
    I try to read 2 characters that should output 
    (Before reading, ' O' read from HyperTerminal, 
    Before reading, ' K' read from HyperTerminal) 
    in response to the AT command I am sending. I am giving the outputs below. 
    Sometimes I read nothing (1,7, 8,9), 
    sometimes I read 1 character (2,3,4,6,10,11,1,3,14,15,16,17,18) 
    and two characters (5,12). 
    1)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading

2)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading
  A read from HyperTerminal
  Before reading

3)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading

   read from HyperTerminal
  Before reading

   read from HyperTerminal

4)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading
  K read from HyperTerminal
  Before reading

5)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading

   read from HyperTerminal
  Before reading
  A read from HyperTerminal

6)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading
  T read from HyperTerminal
  Before reading

7)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading

8)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading

9)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading

    10)UART_init Successful
  UART_reset Successful
  UART_setup Successfu
  AT
  command sent to modem
  Before reading
  A read from HyperTerminal
  Before reading

11)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading
  T read from HyperTerminal
  Before reading

12)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading

   read from HyperTerminal
  Before reading

   read from HyperTerminal

13)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading

   read from HyperTerminal
  Before reading

14)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading
  A read from HyperTerminal
  Before reading

15)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading

   read from HyperTerminal
  Before reading

16)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading

   read from HyperTerminal
  Before reading

17)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading

   read from HyperTerminal
  Before reading

18)UART_init Successful
  UART_reset Successful
  UART_setup Successful
  AT
  command sent to modem
  Before reading
  K read from HyperTerminal
  Before reading
    


    

    I understand that I am getting the echo characters back (AT\r\n), but I do not see any pattern. 
    

    Enabling FIFO also doesn't seem to help? 
    

    Should I include a delay before I read each character? 
    

    Anyone who has used  a GPRS modem before, can you help?
    


    

    Thanks and regards,
    

    Riju

     

  • 0 in reply to Rijurekha Sen
    Intellectual 265 points

    Hi Riju,

     

    I was trying to interface with the modem using the C5505 before so I have some suggestions that you might want to consider:

    1. UART  clock: make sure modem & 5505 UART is configured at the same frequency which I think it is because you got response from modem sometimes. Try to run at lower clock (9600 bps as an example)

    2. Don't use the UART polling mode example in the CSL. The UART_read will keep waiting for the Data Ready & it will freeze your app. Use the interrupt example. Make sure to modify it  (ex. don't echo character in uart_txisr)

    3. Get data quickly in UART_rxisr & exit otherwise there will be an overrun error because RSR will be overwritten by coming data from modem. Don't use functions like printf() in rxisr. It's better to transfer data in UART's FIFO into a buffer & print that buffer when you get full response from modem.

     

    HTH

    Joe