AT commands in embedded systems - c

Question

I am using embedded C and trying to make application for GPRS terminal. My main problem is working with AT commands. I send AT command using serial line, but if it is some network oriented command its response could take time and because of that I have a lot of waiting, while processor don`t do anything. Idea is to make this waiting to be done in same way parallel like in different thread. Does anyone have idea how to do that because my system does not support threads?
I had idea to use some timers, because we have interrupt which is called every 5ms, but I don't know ho many seconds I have to wait for response, and if I compare strings in interrupt to check if all message is received it could be very inefficient, right?

Answer 1

you could either use interrupts, configure the serial interface to interrupt when data is available, or use an RTOS something, like FreeRTOS, to run two threads, one for the main code and the other to block and wait for the serial data.
Update: based on your comments, you say you don't know the size of the data, that's fine, in the interrupt handler check for the byte that terminates the data, this is a simple and generic example you should check the examples for your MCU:
void on_serial_char()
{
//disable interrupts
disable_interrupts();
//read byte
byte = serial.read();
//check if it's the terminating byte
if (byte == END) {
//set the flag here
MESSAGE_COMPLETE = 1;
}
//add byte to buffer
buf[length++] = byte;
//enable interrupts
enable_interrupts();
}
And check for that flag in your main loop:
...
if (MESSAGE_COMPLETE) {
//process data
...
//you may want to clear the flag here
MESSAGE_COMPLETE = 0;
//send next command
...
}

Answer 2

You can simply call a packetHandler in each mainLoopCycle.
This handler checks if new characters are available from the serial port.
The packetHandler will build the response message bit for bit, if the message is complete (CR LF found) then it calls a messageReceive function, else it simply returns to the mainLoop.
int main()
{
init();
for (;;)
{
packetHandler();
}
}
char msgBuffer[80];
int pos=0;
void packetHandler()
{
char ch;
while ( isCharAvailable() )
{
ch=getChar();
msgBuffer[pos++] = ch;
if ( ch == '\n' )
{
messageReceived(msgBuffer);
pos=0;
}
}
}

Answer 3

It sounds like you are rather close to the hardware drivers. If so, the best way is to use DMA, if the MCU supports it, then use the flag from the DMA hardware to determine when to start parse out the received data.
The second best option is to use rx interrupts, store every received byte in a simple FIFO, such as a circular buffer, then set some flag once you have received them. One buffer for incoming data and one for the latest valid data received may be necessary.