XBee®/XBee‐PRO® ZB SMT RF Modules
This firmware includes an adaptive polling enhancement where, if an end device receives RF data from its
parent, it sends another poll after a very short delay to check for more data. The end device continues to poll at
a faster rate as long as it receives data from its parent. This feature greatly improves data throughput to end
devices. When the end device no longer receives data from its parent, it resumes polling every 100ms.
I/O Sampling
End devices can be configured to send one or more I/O samples when they wake from sleep. To enable I/O
sampling on an end device, the IR command must be set to a non-zero value, and at least one analog or digital
I/O pin must be enabled for sampling (D0 - D9, P0-P4 commands). If I/O sampling is enabled, an end device
sends an I/O sample when it wakes and starts the ST timer. It will continue sampling at the IR rate until the
sleep timer (ST) has expired. See chapter 8 for details.
Waking End Devices with the Commissioning Pushbutton
If the commissioning pushbutton functionality is enabled (D0 command), a high-to-low transition on the AD0/
DIO0 pin (pin 33) will cause an end device to wake for 30 seconds. See the Commissioning Pushbutton section
in chapter 7 for details.
Parent Verification
Since an end device relies on its parent to maintain connectivity with other devices in the network, XBee end
devices include provisions to verify its connection with its parent. End devices monitor their link with their
parent when sending poll messages and after a power cycle or reset event as described below.
When an end device wakes from sleep, it sends a poll request to its parent. In cyclic sleep, if RF or serial data is
not received and the sleep timer is not started, the end device polls one time and returns to sleep for another
sleep period. Otherwise, the end device continues polling every 100ms. If the parent does not send an
acknowledgment response to three consecutive poll request transmissions, the end device assumes the parent
is out of range, and attempts to find a new parent.
After a power-up or reset event, the end device does an orphan scan to locate its parent. If the parent does not
send a response to the orphan scan, the end device attempts to find a new parent.
Rejoining
Once all devices have joined a ZigBee network, the permit-joining attribute should be disabled such that new
devices are no longer allowed to join the network. Permit-joining can be enabled later as needed for short
times. This provides some protection in preventing other devices from joining a live network.
If an end device cannot communicate with its parent, the end device must be able to join a new parent to
maintain network connectivity. However, if permit-joining is disabled in the network, the end device will not find
a device that is allowing new joins.
To overcome this problem, ZigBee supports rejoining, where an end device can obtain a new parent in the same
network even if joining is not enabled. When an end device joins using rejoining, it performs a PAN ID scan to
discover nearby networks. If a network is discovered that has the same 64-bit PAN ID as the end device, it will
join the network by sending a rejoin request to one of the discovered devices. The device that receives the
rejoin request will send a rejoin response if it can allow the device to join the network (i.e. child table not full).
The rejoin mechanism can be used to allow a device to join the same network even if permit-joining is disabled.
To enable rejoining, NJ should be set less than 0xFF on the device that will join. If NJ < 0xFF, the device
assumes the network is not allowing joining and first tries to join a network using rejoining. If multiple rejoining
attempts fail, or if NJ=0xFF, the device will attempt to join using association.
XBee Router/Coordinator Configuration
XBee routers and coordinators may require some configuration to ensure the following are set correctly:
• RF packet buffering timeout
• Child poll timeout
• Transmission timeout.
© 2010 Digi International, Inc.
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DCD [ DIGITAL CORE DESIGN ]
