Specifications
XBee®/XBee‐PRO®SERFModules
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asserted (low). In cyclic sleep mode, the end device will only poll once before returning to sleep
unless the sleep timer (ST) is started (serial or RF data is received). If the sleep timer is started,
the end device will continue to poll every 100ms until the sleep timer expires.
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 at the regular rate.
Waking End Devices with the Commissioning Pushbutton
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. 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
(NJ0) so 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.
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.
The value of these timeouts depends on the sleep time used by the end devices. Each of these
timeouts are discussed below.