USB inhibits deep sleep

If it is important that your application knows whether or not deep sleep happened, then you can alternatively execute this code:

ISleepManager sleepManager = Spot.getInstance().getSleepManager();

SleepManager.ensureDeepSleep(5000);

This code will either deep sleep for the specified time of 5,000 milliseconds, or throw an UnableToDeepSleepException to explain why deep sleep was not possible. In this case no sleeping will happen, deep or shallow. The section Preconditions for deep sleeping explains the possible reasons for a failure to deep sleep. Note that this feature is only available if you are executing in the master isolate.

USB hosts require that any device that is plugged into a USB port is able to answer requests on the USB bus. This requirement prevents the Sun SPOT from deep sleeping when it is attached to a USB host. A Sun SPOT on USB uses shallow sleep instead. Connecting a simple external battery via the USB socket will not inhibit deep sleep.

Preconditions for deep sleeping

A number of preconditions must be met in order for the Java thread scheduler to decide to deep sleep:

•Deep sleep mode must not have been deactivated using the SleepManager (as shown above).

•All threads must be blocked either on synchronization primitives or on a timed wait with a time to wake up at least as long as the minimum deep sleep time (including the wake time allowance)

•At least one thread must be on a timed wait (that is, the Sun SPOT will not deep sleep if all threads are blocked on synchronization primitives).

•All device drivers must store any necessary persistent state in RAM (to protect against the associated hardware being switched off) and agree to deep sleep. A driver may veto deep sleep if switching off its associated hardware would cause problems. In this case all drivers are set up again and a shallow sleep is performed instead.

Deep sleep behaviour of the standard drivers

Device	Condition to permit deep sleep
CC2420	Radio receiver must be off
Timer-counter	The counter must be stopped
PIO	All pins claimed must have been released
AIC	All interrupts must be disabled
PowerManager	All peripheral clocks must be off

Note that the appropriate way for an application to turn off the radio receiver is to give your connections the WEAK_ON or OFF policy.

The deep sleep/wake up sequence

The device drivers for the Sun SPOT are written in Java, allowing them to interact with the SleepManager when it is determining if a deep sleep should be performed. The full sequence of activities follows.

1. The Java thread scheduler discovers all threads are blocked, with at least one performing a

Thread.sleep().