© 2014 Digi International Inc. 53
XBee/XBee-PRO
®
DigiMesh 2.4 User Manual
care should be taken so that the values are not made too small.
2 Calculate the Sync Message Propagation Time (SMPT): This is the maximum amount
of time it takes for a sleep synchronization message to propagate to every node in the
network. This number can be estimated with the following formula: SMPT = NN * NH * (MT +
1) 18ms
3 Select desired duty cycle: The ratio of sleep time to wake time is the factor that has the
greatest effect on the RF module’s power consumption. Battery life can be estimated based
on the following factors: sleep period, wake time, sleep current, RX current, TX current, and
battery capacity.
4 Choose sleep period and wake time: The wake time needs to be long enough to transmit
the desired data as well as the sync message. The ST parameter will automatically adjust
upwards to its minimum value when other AT commands are changed that will affect it (SP,
NN, and NH). Use a value larger than this minimum. If a module misses successive sync
messages, it reduces its available transmit time to compensate for possible clock drift. Budget
a large enough ST time to allow for a few sync messages to be missed and still have time for
normal data transmissions.
Starting a Sleeping DigiMesh Network
By default, all new nodes operate in normal (non-sleep) mode. To start a sleeping network, fol-
low these steps:
1 Enable the preferred sleep coordinator option on one of the nodes, and set its SM to a sleep
compatible mode (7 or 8) with its SP and ST set to a quick cycle time. The purpose of a quick
cycle time is to allow commands to be sent quickly through the network during
commissioning.
2 Next, power on the new nodes within range of the sleep coordinator. The nodes will quickly
receive a sync message and synchronize themselves to the short cycle SP and ST.
3 Configure the new nodes in their desired sleep mode as cyclic sleeping nodes or sleep support
nodes.
4 Set the SP and ST values on the sleep coordinator to the desired values for the deployed
network.
5 Wait a cycle for the sleeping nodes to sync themselves to the new SP and ST values.
6 Disable the preferred sleep coordinator option bit on the sleep coordinator (unless a preferred
sleep coordinator is desired).
7 Deploy the nodes to their positions.
Alternatively, nodes can be set up with their sleep pre-configured and written to flash (using the
WR command) prior to deployment. If this is the case, the commissioning button and associate
LED can be used to aid in deployment:
1 If a preferred sleep coordinator is going to be used in the network, deploy it first. If there will
be no preferred sleep coordinator, select a node for deployment, power it on and press the
commissioning button twice. This will cause the node to begin emitting sync messages.
2 Verify that the first node is emitting sync messages by watching its associate LED. A slow
blink indicates that the node is acting as a sleep coordinator.
3 Next, power on nodes in range of the sleep coordinator or other nodes which have
synchronized with the network. If the synchronized node is asleep, it can be woken by
pressing the commissioning button once.
4 Wait a cycle for the new node to sync itself.
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