/**
\defgroup quickref Function quick reference
@{

\section sensors Sensor functions

Each sensor has a set of functions for controlling it and query it for
its state. Some sensors also generate an events when the sensors
change. A sensor must be activated before it generates events or
relevant values.

- SENSORS_ACTIVATE(sensor) - activate the sensor
- SENSORS_DEACTIVATE(sensor) - deactivate the sensor
- sensor.value(0) - query the sensor for its last value

- \ref sensors_event - event sent when a sensor has changed (the data
  argument will referer to the actual sensor)

Example for querying the button:

- \ref SENSORS_ACTIVATE(button_sensor) - activate the button sensor
- \ref button_sensor.value(0) - the button has been pressed or not

\subsection quickref-sensors Sensor on the ESB platform

- \ref battery_sensor - query the battery voltage level
- \ref button_sensor - query the on-board button
- \ref pir_sensor - query the passive IR sensor (motion detector)
- \ref radio_sensor - query the radio signal strength
- \ref sound_sensor - query the microphone
- \ref temperature_sensor - query the temperature sensor
- \ref vib_sensor - query the vibration sensor

\section quickref-leds LED functions

- leds_on() - turn LEDs on
- leds_off() - turn LEDs off
- leds_invert() - invert LEDs
- leds_blink() - blink all LEDs

\section quickref-beeper Beeper functions

- beep() - click the beeper
- beep_beep() - beep
- beep_down() - pitchbend down beep
- beep_quick() - a number of quick beeps
- beep_spinup() - pitchbend up beep

\section quickref-timers Timer functions

Contiki provides a set of timer libraries. Event timers generates an
event when the timer expires and callback timers call a function when
the timer expires. The simple timers on the other hand have to be
actively queried to check when they have expired.

\subsection quickref-etimer Event timers

- etimer_expired() - check if an event timer has expired
- etimer_set() - set an event timer
- etimer_reset() - set an expired event timer to the next interval
- etimer_restart() - restart an event timer from the current point in time

\subsection quickref-ctimer Callback timers

- ctimer_expired() - check if a callback timer has expired
- ctimer_set() - set a callback timer
- ctimer_reset() - set an expired callback timer to the next interval
- ctimer_restart() - restart a callback timer from the current point in time

\subsection quickref-timer Simple timers

- timer_expired() - check if a simple timer has expired
- timer_set() - set a simple timer
- timer_reset() - set a simple event timer to the next interval
- timer_restart() - restart a simple timer from the current point in time


*//** @{ */