Operation UART flow control
XBee® SX 868 RF Module User Guide
57
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 FT (Flow
Control Threshold) for the buffer size.
In either case, CTS is not re-asserted until the serial receive buffer has FT-17 or less bytes in use.
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 P9 (DIO19/SPI_ATTN).
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.
Serial interface protocols
The XBee SX 868 RF Module supports both Transparent and Application Programming Interface (API)
serial interfaces.
Transparent operating mode
When operating in Transparent mode, the devices act as a serial line replacement. The device queues
up all UART data received through the DIN pin for RF transmission. When RF data is received, the
device sends the data out through the serial port. Use the Command mode interface to configure the
device configuration parameters.
Note Transparent operation is not provided when using SPI.
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