Serial communication UART flow control
Digi XBee® 3 Zigbee® RF Module
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Serial transmit buffer
When the device receives RF data, it moves the data into the serial transmit buffer and sends it out
the UART. 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.
In situations where 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 device, the device may
receive data faster than it can send the data to the host. Even occasional transmissions from a
large number of devices can quickly accumulate and overflow the transmit buffer.
2. If the host does not allow the device to transmit data out from the serial transmit buffer due to
being held off by hardware flow control.
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.
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.
RTS flow control
If you set D6 (DIO6/RTS) 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.
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.
Break control
If a serial break (DIN held low) signal is sent for over five seconds, the device resets, and it boots into
Command mode with default baud settings (9600 baud). If either P3 or P4 are not enabled, this break
function is disabled.
I2C
I2C master operation is supported using MicroPython.
See the Class I2C: two-wire serial protocol section in the Digi MicroPython Programming Guide for
details.
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