Electronic – How to use UART rx data from ISR to main loop

interruptspicuart

I am using PIC18F and currently trying to interface it with SIM900. I am using UART Rx interrupt. ISR is working correctly but I wonder how to get data in main loop. Lets say :

#pragma interrupt hi_prioriint
void hi_prioriint(void)
{
   if(PIR1bits.RCIF==1)
   {
      Rx = ReadUSART();  //store the received byte in Rx
   }
 }

Now I want to use Rx in main loop. For example, if I send AT command then I need to check if I received OK or ERROR from SIM900. (For the time being I am just testing it with a single byte and will convert it later into receiving strings because response from the SIM900 is in string form)

Example:

 putrsUSART("AT\r");        //sending AT command
 delay(500)                 //delay of 500ms
 if(strcmp(Rx,"OK")==0)     //checking the response
 {
    //if received OK then proceed further
 }

So how to use RX data of ISR in main loop?

Best Answer

The conventional way to do this is to use a circular buffer (sometimes also referred to as a circular queue). The Rx interrupt routine inserts received data bytes into the queue using its input pointer/index. The mainline code uses the output pointer/index to extract the data from the queue.

The big advantage with this procedure is to allow the Rx data to arrive in spurts that may may come faster than they can be consumed by the mainline loop due to latency in other tasks. As long as the queue depth is enough and the mainline code is able to consume the Rx data at least as fast as the long term data flow rate this is the way to go.

Note that there is a plethora of information online regarding the design, coding and operation of circular queues. One hint is that there can be a performance improvement in the use of circular queues if the depth of the queue is kept to an even power of two bytes. When this is done the queue index wrap from top to bottom of the queue can become a simple mask of the index with a value of (queue depth - 1).

Related Solutions

Electronic – Implementing an I2C buffer in C

I have no experience with PIC, but the problem seems generic enough. I would create a simple array with two independent pointers into the array: one read pointer and one write pointer. Whenever you receive a byte, you increment the write pointer and write at the new position; in your main loop you could then check if the read pointer and the write pointer are the same. If not, you simply read and process from the buffer and increase the read pointer for every byte until they are.

You could then either reset the pointers to the beginning of the array, or let them "flow over" to the beginning making essentially a circular buffer. This is easiest if the size of the array is a factor of 2 as you can then simply bitmask both pointers after their increments.

Some example (pseudo)code:

volatile unsigned int readPointer= 0;
volatile unsigned int writePointer=0;
volatile char theBuffer[32];
...
//in your ISR
writePointer = (writePointer+1) & 0x1F;
theBuffer[writePointer] = ReadI2C(); // assuming this is the easiest way to do it
                                     // I would probably just read the register directly
...
//in main
while (readPointer != writePointer) {
  readPointer = (readPointer+1) & 0x1F;
  nextByte = theBuffer[readPointer];
  // do whatever necessary with nextByte
}

Spi interrupt code

With SPI the master generates the clock so there's not really a concept of being able to receive multiple bytes back from a slave automatically, you have to send additional dummy bytes to generate the clock. Say if you had a sensor and you wanted command 0x01 to send back the current temperature the pseudo-code for the master may look like:

spi_write(0x01);
t_lsb = spi_write(0x00); // These dummy bytes are just needed to generate a clock
t_msb = spi_write(0x00); // So that the slave gets a chance to send the results back

On and the slave it might look like:

cmd = spi_read();
if (cmd == 0x01) {
  spi_write(t_lsb);
  spi_write(t_msb);
}

You can technically send back a byte from the slave for each byte sent from the master, but for most protocols there probably isn't anything useful to send back until the slave has received and decoded the command. Hopefully you can understand that pseudo-code and the general concept OK, it's been a while since I've programmed PIC SPI at the register level but it sounds like you're on top of that side of things.

Best Answer

Related Solutions

Electronic – Implementing an I2C buffer in C

Spi interrupt code

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