User's Manual

XBee®/XBee‐PRO®ZBRFModules
©2009DigiInternational,Inc. 82
The XBee ZB 2x6x 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.
IO Sampling
End devices can be configured to send one or more IO samples when they wake from sleep. To
enable IO sampling on an end device, the IR command must be set to a non-zero value, and at
least one analog or digital IO pin must be enabled for sampling (D0 - D9, P0-P2 commands). If IO
sampling is enabled, an end device sends an IO 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 20) 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.