User Manual

XBee®/XBee‐PRO®ZBRFModules
©2011DigiInternational,Inc. 27
TInternalDataFlowDiagram
Serial Receive Buffer
When serial data enters the RF module through the DIN Pin (pin 3), the data is stored in the serial receive
buffer until it can be processed. Under certain conditions, the module may not be able to process data in
the serial receive buffer immediately. If large amounts of serial data are sent to the module, CTS
flow
control may be required to avoid overflowing the serial receive buffer.
Cases in which the serial receive buffer may become full and possibly overflow:
1. If the module is receiving a continuous stream of RF data, the data in the serial receive buffer
will not be transmitted until the module is no longer receiving RF data.
2. If the module is transmitting an RF data packet, the module may need to discover the desti-
nation address or establish a route to the destination. After transmitting the data, the module may
need to retransmit the data if an acknowledgment is not received, or if the transmission is a broad-
cast. These issues could delay the processing of data in the serial receive buffer.
Serial Transmit Buffer
When RF data is received, the data is moved into the serial transmit buffer and sent out the UART. If the
serial transmit buffer becomes full enough such that all data in a received RF packet won’t fit in the serial
transmit buffer, the entire RF data packet is dropped.
Cases in which the serial transmit buffer may become full resulting in dropped RF packets
1. If the RF data rate is set higher than the interface data rate of the module, the module could
receive data faster than it can send the data to the host.
2. If the host does not allow the module to transmit data out from the serial transmit buffer
because of being held off by hardware flow control.
Serial Flow Control
The RTS and CTS module pins can be used to provide RTS and/or CTS flow control. CTS flow control provides an
indication to the host to stop sending serial data to the module. RTS flow control allows the host to signal the
module to not send data in the serial transmit buffer out the uart. RTS
and CTS flow control are enabled using
the D6 and D7 commands.
CTS Flow Control
If CTS flow control is enabled (D7 command), when the serial receive buffer is 17 bytes away from being
full, the module de-asserts CTS
(sets it high) to signal to the host device to stop sending serial data. CTS is
re-asserted after the serial receive buffer has 34 bytes of space.
RTS Flow Control
If RTS flow control is enabled (D6 command), data in the serial transmit buffer will not be sent out the
DOUT pin as long as RTS
is de-asserted (set high). The host device should not de-assert RTS for long
Serial
Receiver
Buffer
RF TX
Buffer
Transmitter
RF Switch
Antenna
Port
Receiver
Serial Transmit
Buffer
RF RX
Buffer
Processor
DIN
DOUT
CTS
RTS