Arduino sketch for duinode reporting voltage and temperature

Duinode.ino

#include <SPI.h>
#include <avr/sleep.h>
#include "nRF24L01.h"
#include "RF24.h"
#include "printf.h"

RF24 radio(8, 7);

int led     = 3;
int temp    = A2;
int voltage = A3;
int node_id = 1;

uint64_t addresses[3] = { 0xF0F0F0F0D2LL, 0xF0F0F0F0E1LL, 0xF0F0F0F0A2LL };

const short sleep_cycles_per_transmission = 35;
volatile short sleep_cycles_remaining = sleep_cycles_per_transmission;

float readMCP9700(const int pin)
{
  // http://ww1.microchip.com/downloads/en/AppNotes/01001a.pdf
  int   v   = analogRead(pin);
  float t   = ( (v * 1.1 / 1024.0 ) - 0.5 ) * 100;
  float t_e = 244e-6 * (125.0 - t) * (t - -40.0) + 2e-12 * (t - -40.0) - 2.0;
  return t - t_e;
}

float readBatteryVoltage(const int pin)
{
  int v = analogRead(pin);
  return (v * 3.3 / 1024.0);
}

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_enable();

  sleep_mode();

  sleep_disable();
}

void setup() {
  analogReference(INTERNAL);
  Serial.begin(57600);

  radio.begin();
  radio.setRetries(15, 15);
  radio.setPALevel(RF24_PA_MAX);
  radio.setDataRate(RF24_1MBPS);
  radio.setChannel(76);
  radio.setPayloadSize(32);
  radio.openWritingPipe(addresses[node_id]);

  setup_watchdog(9);
}

void loop() {
  double v, t;
  char buf[32];

  radio.powerUp();

  v = readBatteryVoltage(voltage);
  t = readMCP9700(temp);

  snprintf(buf, 32, "N=%d;V=%d;T=%d", node_id, (int) (v * 100), (int) (t * 100));
  Serial.println(buf);
  radio.write(buf, 32);
  radio.powerDown();

  while (sleep_cycles_remaining) {
    do_sleep();
  }

  sleep_cycles_remaining = sleep_cycles_per_transmission;
}