nRF24L01+ power consumption footprint

I hooked my Dirt Cheap Low Power Wireless Sensor Node (DCLPWSN 🙂 ) to the scope to see what is going on during transmissions. I used this library for the test, and this code:

/**
 * Pins:
 * Hardware SPI:
 * MISO -> 12
 * MOSI -> 11
 * SCK -> 13
 *
 * Configurable:
 * CE -> 8
 * CSN -> 7
 *
 */
#include <avr/sleep.h>
#include <avr/power.h>

typedef enum { wdt_16ms = 0, wdt_32ms, wdt_64ms, wdt_128ms, wdt_250ms, wdt_500ms, wdt_1s, wdt_2s, wdt_4s, wdt_8s } wdt_prescalar_e;
const short sleep_cycles_per_transmission = 1;
volatile short sleep_cycles_remaining = sleep_cycles_per_transmission;

#include <SPI.h>
#include <Mirf.h>  //https://github.com/aaronds/arduino-nrf24l01/tree/master/Mirf
#include <nRF24L01.h>
#include <MirfHardwareSpiDriver.h>

void setup(){  
   // disable ADC
  ADCSRA = 0;  

  // turn off various modules
  //PRR = 0xFF; 

    // turn off brown-out enable in software
  MCUCR = _BV (BODS) | _BV (BODSE);
  MCUCR = _BV (BODS); 

  //
  // Prepare sleep parameters
  //

    setup_watchdog(wdt_1s);

  /*
   * Setup pins / SPI.
   */

  /* To change CE / CSN Pins:
   * 
   */

  Mirf.cePin = 8;
  Mirf.csnPin = 7;

  Mirf.spi = &MirfHardwareSpi;
  Mirf.init();

  /*
   * Configure reciving address.
   */

  Mirf.setRADDR((byte *)"clie1");

  /*
   * Set the payload length to sizeof(unsigned long) the
   * return type of millis().
   *
   * NB: payload on client and server must be the same.
   */

  Mirf.payload = sizeof(unsigned long);

  /*
   * Write channel and payload config then power up reciver.
   */

   Mirf.channel = 7;

  /*
   * To change channel:
   * 
   * Mirf.channel = 10;
   *
   * NB: Make sure channel is legal in your area.
   */

  Mirf.config();
}

void loop(){
  //pinMode(9,OUTPUT);
  //digitalWrite(9,HIGH);
  unsigned long time = millis();
  Mirf.send((byte *)&time);
  while(Mirf.isSending()){
  }
  Mirf.powerDown();
    // Sleep the MCU.  The watchdog timer will awaken in a short while, and
    // continue execution here.
//  digitalWrite(9,LOW);

    while( sleep_cycles_remaining )
      do_sleep();

    sleep_cycles_remaining = sleep_cycles_per_transmission;

} 

  //
// Sleep helpers
//

// 0=16ms, 1=32ms,2=64ms,3=125ms,4=250ms,5=500ms
// 6=1 sec,7=2 sec, 8=4 sec, 9= 8sec

void setup_watchdog(uint8_t prescalar)
{
  prescalar = min(9,prescalar);
  uint8_t wdtcsr = prescalar & 7;
  if ( prescalar & 8 )
    wdtcsr |= _BV(WDP3);

  MCUSR &= ~_BV(WDRF);
  WDTCSR = _BV(WDCE) | _BV(WDE);
  WDTCSR = _BV(WDCE) | wdtcsr | _BV(WDIE);
}

ISR(WDT_vect)
{
  --sleep_cycles_remaining;
}

void do_sleep(void)
{
  set_sleep_mode(SLEEP_MODE_PWR_DOWN); // sleep mode is set here
  sleep_enable();
  sleep_mode();                        // System sleeps here
  sleep_disable();                     // System continues execution here when watchdog timed out
}

The output is as follows:

The consumption peaks to 27.8mA (but averaging that over the transmission period will give you roughlty 12mA as the datasheet states). The 4 bytes payload (unsigned long) take 2.88ms to transmit, that includes the overhead. I will still have to experiment with slower/higher data rates and how that affects range.