© 2014 Digi International Inc. 48
XBee/XBee-PRO
®
DigiMesh 2.4 User Manual
XBee Asynchronous Sleep Operation
DigiMesh Wake Timer
In cyclic sleep mode (SM=4 or SM=5), if serial or RF data is received, the module starts a sleep
timer (time until sleep). Any data received serially or by RF link resets the timer. The timer
duration can be set using the ST command. The module returns to sleep when the sleep timer
expires.
DigiMesh Sleeping Routers
The Sleeping Router feature of DigiMesh makes it possible for all nodes in the network to
synchronize their sleep and wake times. All synchronized cyclic sleep nodes enter and exit a low
power state at the same time. This forms a cyclic sleeping network. Nodes synchronize by
receiving a special RF packet called a sync message which is sent by a node acting as a sleep
coordinator. A node in the network can become a sleep coordinator through a process called
nomination. The sleep coordinator sends one sync message at the beginning of each wake
period. The sync message is sent as a broadcast and repeated by every node in the network.
The sleep and wake times for the entire network can be changed by locally changing the settings
on an individual node. The network will use the most recently set sleep settings.
Coordinating Sleep Modes in the DigiMesh Network
One node in a sleeping network acts as the sleeping coordinator. The process by which a node
becomes a sleep coordinator is described later in this document. During normal operations, at
the beginning of a wake cycle the sleep coordinator will send a sync message as a broadcast to
all nodes in the network. This message contains synchronization information and the wake and
sleep times for the current cycle. All cyclic sleep nodes receiving a sync message will remain
awake for the wake time and then sleep for the sleep period specified.
The sleep coordinator will send one sync message at the beginning of each cycle with the
currently configured wake and sleep times. All router nodes which receive this sync message will
relay the message to the rest of the network. If the sleep coordinator does not hear a re-
broadcast of the sync message by one of its immediate neighbors then it will re-send the
message one additional time. It should be noted that if SP or ST are changed, the network will
not apply the new settings until the beginning of the next wake time. See the Changing Sleep
Parameters section below for more information.
A sleeping router network is robust enough that an individual node can go several cycles without
receiving a sync message (due to RF interference, for example). As a node misses sync
messages, the time available for transmitting messages in the wake time is reduced to maintain
synchronization accuracy. By default, a module will also reduce its active sleep time progressively
as sync messages are missed.
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