TSOP IR Detector

The first step of decoding is already done by the TSOP which converts IR signals to digital signals. The next step is for you to convert it signal to meaningful signal codes.

References

• TSOP1236 Datasheet
• Very Useful Tututorial @ Ladyada.net

LPC2148 Hints

You can either setup an interrupt on falling edge and capture the times or you can also setup auto-capture using CAPTURE input of your controller.

Setup

• Hook up TSOP signal to an External Interrupt Pin (EINT). Configure for falling edge for Active-Low signal.
• Create a Helper task, such as "DecoderTask"
  • Create a FreeRTOS Queue to send time-stamps from ISR to DecoderTask
  • Queue item size is sizeof(int) and depth as 20+ should work
• Have the ISR Task time-stamp the falling edges, and let the decoder task decode the signals
  • Note: You do not want to spend the entire signal length (many tens of milliseconds) in ISR
• Start Timer1 (FreeRTOS project already does this) @ resolution of 1 tick per microsecond.

ISR Task

Psuedocode:

   // Enter ISR
       unsigned int timer1Value = T1TC; // Timer 1 Counter Value
       xQueueSendFromIsr(&timer1Value, &taskWoken);
   // Exit ISR

Decoder Task

Psuedocode:

    while(1)
    {
        unsigned int startTime = 0;
        // Wait forever for the 1st signal time
        if(xQueueReceive(qId, &startTime, portMAX_DELAY))
        {
            // Got 1st signal, so get all signals within 10ms timeout
            unsigned int signalTimes[128];
            int sigNum = 0;
            while(sigNum < 128 && xQueueReceive(qId, &signalTimes[sigNum], 10))
            {
                signalTimes[sigNum] -= startTime;
                sigNum++;
            }

            // Calculate differences of falling edges
            for(int i = 0; i < sigNum-1; i++) {
                signalTimes[i] = signalTimes[i+1] - signalTimes[i];
            }
            for(int i = 0; i < sigNum; i++) {
                rprintf("Timestamp #u : %X\n", (i+1), signalTimes[i]);
            }
            // Now your array may look like: 10,10,20,10,10,20 etc, 
            // so turn these numbers to binary bits of an integer and you're done!
        }
    }