# xmpl_trx_rx.c - Receiving Frames

This example shows, how frames are received and their CRCR16 is checked in software.

Code Example

    /* $Id$ */
    /* Example for receiving frames */

    #include "board.h"
    #include "transceiver.h"
    #include "ioutil.h"
    #include  <util/crc16.h>
    #include "xmpl.h"

    static uint8_t rxfrm[MAX_FRAME_SIZE];
    static volatile uint8_t rxcnt;
    int main(void)
    {
    trx_regval_t rval;

        /* This will stop the application before initializing the radio transceiver
         * (ISP issue with MISO pin, see FAQ)
         */
        trap_if_key_pressed();

        /* Step 0: init MCU peripherals */
        LED_INIT();
        trx_io_init(SPI_RATE_1_2);
        LED_SET_VALUE(LED_MAX_VALUE);
        LED_SET_VALUE(0);

        /* Step 1: initialize the transceiver */
        TRX_RESET_LOW();
        TRX_SLPTR_LOW();
        DELAY_US(TRX_RESET_TIME_US);
        TRX_RESET_HIGH();
        trx_reg_write(RG_TRX_STATE,CMD_TRX_OFF);
        DELAY_US(TRX_INIT_TIME_US);
        rval = trx_bit_read(SR_TRX_STATUS);
        ERR_CHECK(TRX_OFF!=rval);
        LED_SET_VALUE(1);

        /* Step 2: setup transmitter
         * - configure radio channel
         * - go into RX state,
         * - enable "receive end" IRQ
         */
        trx_bit_write(SR_CHANNEL,CHANNEL);
        trx_reg_write(RG_TRX_STATE,CMD_RX_ON);
    #if defined(TRX_IRQ_TRX_END)
        trx_reg_write(RG_IRQ_MASK,TRX_IRQ_TRX_END);
    #elif defined(TRX_IRQ_RX_END)
        trx_reg_write(RG_IRQ_MASK,TRX_IRQ_RX_END);
    #else
    #  error "Unknown IRQ bits"
    #endif
        sei();
        LED_SET_VALUE(2);

        /* Step 3: Going to receive frames */
        rxcnt = 0;

        LED_SET_VALUE(0);
        while(1);
    }

    #if defined(TRX_IF_RFA1)
    ISR(TRX24_RX_END_vect)
    {
    uint8_t flen, *pfrm, tmp;
    uint16_t crc;

        /* upload frame and check for CRC16 validity */
        pfrm = rxfrm;
        flen = trx_frame_read(pfrm, sizeof(rxfrm), NULL);
        crc = 0;
        do
        {
            crc = _crc_ccitt_update(crc, *pfrm++);
        }
        while(flen--);
        /* if crc is correct, update RX frame counter */
        if (crc == 0)
        {
            rxcnt ++;
        }
        /* display current rx statistics
         * LED[0] toggles with every received frame
         * LED[1:n] display the count of frames received with valid CRC
         */
        tmp = (rxcnt<<1) | ((LED_GET_VALUE()&1)^1);
        LED_SET_VALUE(tmp);
    }
    #else  /* !RFA1 */
    ISR(TRX_IRQ_vect)
    {
    static volatile trx_regval_t irq_cause;
    uint8_t flen, *pfrm, tmp;
    uint16_t crc;

        irq_cause = trx_reg_read(RG_IRQ_STATUS);
        if (irq_cause & TRX_IRQ_TRX_END)
        {
            /* upload frame and check for CRC16 validity */
            pfrm = rxfrm;
            flen = trx_frame_read(pfrm, sizeof(rxfrm), NULL);
            crc = 0;
            do
            {
                crc = _crc_ccitt_update(crc, *pfrm++);
            }
            while(flen--);
            /* if crc is correct, update RX frame counter */
            if (crc == 0)
            {
                rxcnt ++;
            }
            /* display current rx statistics
             * LED[0] toggles with every received frame
             * LED[1:n] display the count of frames received with valid CRC
             */
            tmp = (rxcnt<<1) | ((LED_GET_VALUE()&1)^1);
            LED_SET_VALUE(tmp);
        }
    }
    #endif  /* RFA1 */

    /* EOF */