Serial Buffers; Serial Receive Buffer; Serial Transmit Buffer; Uart Flow Control - Digi XBee SX 868 User Manual

Operation

Serial buffers

To enable the UART port, DIN and DOUT must be configured as peripherals. To enable the SPI port,
SPI_MISO, SPI_MOSI, SPI_SSEL, and SPI_CLK must be enabled as peripherals. If both ports are
enabled, output goes to the UART until the first input on SPI. This is the default configuration.
When input occurs on either port, that port is selected as the active port and no input or output is
allowed on the other port until the next reset of the module.
If you change the configuration to configure only one port, that port is the only one enabled or used. If
the parameters are written with only one port enabled, the port that is not enabled is not used even
temporarily after the next reset.
If both ports are disabled on reset, the device uses the UART regardless of the incorrect configuration
to ensure 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
XBee® SX 868 RF Module User Guide
for the buffer size.
Serial buffers
FT (Flow
37