Contiki UDP packet transmission duration with CC2538

Question

Could someone explain me what is going on within the Contiki-OS when it transmits an UDP packet?

Here is the current consumption of my device in details running with the CC2538 chip:

My question is: why it takes so long to transmit an UDP broadcast packet (about 250ms) knowing that theoretically at 250kbps the packet of 408 bits length should be transmitted in approximately 2ms? I'd understand if the transmission last lets say ten milliseconds but here the difference is huge.

I use the example in contiki/examples/ipv6/simple-udp-rpl/broadcast-example.c

Does anyone have an idea?

Answer 1

I found the problem : the radio is not turned off properly after the transmission of a packet.

At the end of the function transmit() in the file cpu/cc2538/dev/cc2538-rf.c the radio is turned off only if it was previously off.

if(rf_flags & WAS_OFF) {
    rf_flags &= ~WAS_OFF;
    off();
}

But actually the program never goes in this condition and the radio is not turned off immediately after the transmission of a packet.

The problem arises because the function channel_clear() (called at the beginning of the transmit() function) clears this flag first. Thus the function transmit() doesn't know anymore that the radio was off before its execution and therefore the radio is kept on.

To fix the problem I put a local variable inside the channel_clear() which turn off the radio and clear the flag only if it is turned on inside the function itself.

static int
channel_clear(void)
{
  int cca;
  /* Fix: local variable */
  uint8_t intern_onoff;

  intern_onoff = 0;

  PRINTF("RF: CCA\n");

  /* If we are off, turn on first */
  if((REG(RFCORE_XREG_FSMSTAT0) & RFCORE_XREG_FSMSTAT0_FSM_FFCTRL_STATE) == 0) {
    rf_flags |= WAS_OFF;
    on();
    intern_onoff = 1;
  }

  /* Wait on RSSI_VALID */
  while((REG(RFCORE_XREG_RSSISTAT) & RFCORE_XREG_RSSISTAT_RSSI_VALID) == 0);

  if(REG(RFCORE_XREG_FSMSTAT1) & RFCORE_XREG_FSMSTAT1_CCA) {
    cca = CC2538_RF_CCA_CLEAR;
  } else {
    cca = CC2538_RF_CCA_BUSY;
  }

  /* If we were off, turn back off */
  if((rf_flags & WAS_OFF) == WAS_OFF && intern_onoff) {
    rf_flags &= ~WAS_OFF;
    off();
    intern_onoff = 0;
  }

  return cca;
}

The current consumption during a packet transmission looks like now to:

Note: the strobe time was reduced intentionally to 10ms with:

#define STROBE_TIME                        RTIMER_ARCH_SECOND / 100

This explain why there is only three strobes of transmission for the broadcast message.

The duration of a strobe is 3ms. Which means that the data rate is ~140kbps (?).