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Serial communication
If both ports are disabled on reset, the device uses the UART in spite of the wrong configuration so
that at least one serial port is operational.
Serial receive buffer
When serial data enters the device through the DIN pin (or the MOSI pin), it stores the data in the
serial receive buffer until the device can process it. Under certain conditions, the device may not be
able to process data in the serial receive buffer immediately. If large amounts of serial data are sent
to the device such that the serial receive buffer would overflow, then it discards new data. If the UART
is in use, you can avoid this by the host side honoring CTS flow control.
If the SPI is the serial port, no hardware flow control is available. It is your responsibility to ensure
that the receive buffer does not overflow. One reliable strategy is to wait for a TX_STATUS response
after each frame sent to ensure that the device has had time to process it.
Serial transmit buffer
When the device receives RF data, it moves the data into the serial transmit buffer and sends it out
the UART or SPI port. If the serial transmit buffer becomes full and the system buffers are also full,
then it drops the entire RF data packet. Whenever the device receives data faster than it can process
and transmit the data out the serial port, there is a potential of dropping data.
UART flow control
You can use the RTS and CTS pins to provide RTS and/or CTS flow control. CTS flow control provides an
indication to the host to stop sending serial data to the device. RTS flow control allows the host to
signal the device to not send data in the serial transmit buffer out the UART. To enable RTS/CTS flow
control, use the D6 and D7 commands.
Note
Serial port flow control is not possible when using the SPI port.
CTS flow control
If you enable CTS flow control (D7 command), when the serial receive buffer is 17 bytes away from
being full, the device de-asserts CTS (sets it high) to signal to the host device to stop sending serial
data. The device reasserts CTS after the serial receive buffer has 34 bytes of space. See
Control Threshold)
RTS flow control
If you send the D6 command to enable RTS flow control, the device does not send data in the serial
transmit buffer out the DOUT pin as long as RTS is de-asserted (set high). Do not de-assert RTS for
long periods of time or the serial transmit buffer will fill. If the device receives an RF data packet and
the serial transmit buffer does not have enough space for all of the data bytes, it discards the entire
RF data packet.
The UART Data Present Indicator is a useful feature when using RTS flow control. When enabled, the
DIO19 line asserts (low asserted) when UART data is queued to be transmitted from the device. For
more information, see
If the device sends data out the UART when RTS is de-asserted (set high) the device could send up to
five characters out the UART port after RTS is de-asserted.
XBee®/XBee-PRO SX RF Module User Guide
for the buffer size.
P9
(DIO19/SPI_ATTN).
UART flow control
FT (Flow
63
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