Implement the CAD mode as documented in the reference manual of the SX1272 (page 40).

The datasheet claim to be able to detect the packet preamble in ~2 symbols time.

So far two methods are documented to know if there is an ongoing communication.

• Using MODEMSTAT

uint8_t status;
spi_read_register(__sx1272_dev.spi, REG_LR_MODEMSTAT, &status, 1);
if (status & (0x04 | 0x01)) {
  sx1272_rx_internal_set(&__sx1272_dev, sx1272_rx_receiving);
}

• Using interrupt on DIO3

if (sx1272_read_register(SX1272_DEV.spi, REG_LR_IRQFLAGS) & RFLR_IRQFLAGS_VALIDHEADER) {
  SX1272_DEV.receiv_timestamp = RTIMER_NOW(); // - US_TO_RTIMERTICKS(439)
  sx1272_write_register(SX1272_DEV.spi, REG_LR_IRQFLAGS, RFLR_IRQFLAGS_VALIDHEADER);
  sx1272_rx_internal_set(&SX1272_DEV, sx1272_rx_receiving);
  LOG_DBG("Receiving packet\n");
}

I only managed to make the second one work. I should try to implement the first one and study which one detect an incoming packet faster to reduce the time the radio must spend listening for incoming messages.

Some MAC protocol available in Contiki will use the packetbuf directly in continuous listening mode, instead of passing the buffer as argument of sx1272_read_packet.

Busywaiting for an interrupt on the pin DI0,DI1,DI2,DI3 introduce some delay in the software. As we can see in the next picture, the time between the interrupt and the acknowledgement from the software can almost reach 1ms.

If we assign some pins from the RE-MOTE to handle the interrupts we could process the pending packet more quickly.

So far only two pins are used by the driver.

• DI0 acknowledge a completed transmition on TX side and acknowledge a pending packet on RX side
• DI3 acknowledge an ongoing packet reception

The following code could be used to process the interrupt on those two pins.

static void sx1272_interrupt_dio0() { 
  if (__sx1272_dev.mode == sx1272_mode_receiver 
      || __sx1272_dev.mode == sx1272_mode_receiver_single) {
    SX1272_DEV.rx_timestamp = RTIMER_NOW();
    sx1272_rx_internal_set(&__sx1272_dev, sx1272_rx_received);
    sx1272_write_register(__sx1272_dev.spi, REG_LR_IRQFLAGS, RFLR_IRQFLAGS_RXDONE);
  } else if (__sx1272_dev.mode == sx1272_mode_transmitter) {
    sx1272_write_register(__sx1272_dev.spi, REG_LR_IRQFLAGS, RFLR_IRQFLAGS_TXDONE);
    sx127x_set_opmode(&SX1272_DEV, sx1272_mode_standby);
  }
}
static void sx1272_interrupt_dio1() { 
  sx1272_write_register(__sx1272_dev.spi, REG_LR_IRQFLAGS, RFLR_IRQFLAGS_RXTIMEOUT);
}
static void sx1272_interrupt_dio3() { 
  if (__sx1272_dev.mode == sx1272_mode_receiver 
      || __sx1272_dev.mode == sx1272_mode_receiver_single) {
    SX1272_DEV.receiv_timestamp = RTIMER_NOW();
    sx1272_rx_internal_set(&__sx1272_dev, sx1272_rx_receiving);
    sx1272_write_register(__sx1272_dev.spi, REG_LR_IRQFLAGS, RFLR_IRQFLAGS_VALIDHEADER);
  }
}

TODO

• Assign pins that will handle the interrupts
• Setup interrupt in Contiki
• Do the timestamping inside the interrupts
• Handle both with or without interrupt usage of the driver.

• In which mode should the SX127X be running to change the channel ?
• In which mode should we go after completed transmission (currently standby

  sx127x/src/sx1272.c

  Line 207 in b74c17f

  sx127x_set_opmode(&SX1272_DEV, sx1272_mode_standby);

  ) ?
• Is the module settled to a default mode after TX completion ?
• Measure time to switch from sleep mode to standby and standby to sleep mode.
• Can we go in sleep mode instead of standby when turning off the radio.

SX1276,SX1277,SX1278,SX1279

• Version is 0x12
• Different rssi calculation method
• low datarate optimization is different
• set_bandwidth is different (different register mask)
• set_coding_rate is different (different register mask)
• set_crc is different (different register mask)
• fixed header len is different (different register mask)
• set_tx_power is different (different register mask)
• set_tx_power different max power

RFM 95

• Version is 0x11

During the transmition of a packet instead of just checking for the interrupt when it's done also use a timer to avoid infinite loop.