Serial Port Selection; Serial Buffers; Serial Receive Buffer - Digi XBee User Manual

Operation
The SPI slave controller must guarantee that there is time to move new transmit data from the
transmit buffer into the hardware serializer. To provide sufficient time, the SPI slave controller inserts
a byte of padding at the start of every new string of transmit data. Whenever the transmit buffer is
empty and the device places data into the transmit buffer, the SPI hardware inserts a byte of padding
onto the front of the transmission as if this byte were placed there by software.

Serial port selection

In the default configuration both the UART and SPI ports are configured for serial port operation. In
this case, serial data goes out the UART until the host device asserts the SPI_SSEL signal. Thereafter
all serial communications operate only on the SPI interface until a reset occurs.
If you enable only the UART, the XBee device uses only the UART, and ignores the SPI_SSEL.
If you enable only the SPI, the XBee device uses only the SPI, and ignores UART communications. If you
hold DOUT low during boot, then the XBee device only uses the SPI.
Once SPI is in use, do not attempt to apply changes (AC) which change the UART or SPI settings.
Instead, use the following:
0x09 frames to reconfigure UART/SPI/other settings.
n
WR to save the settings.
n
FR to reset the XBee/XBee-PRO ZigBee RF Module and use the new configuration settings.
n
If neither serial port is enabled, then UART remains enabled, the device uses only the UART, and it
ignores SPI_SSEL.

Serial buffers

The XBee/XBee-PRO ZigBee RF Module maintains internal buffers to collect serial and RF data that it
receives. The serial receive buffer collects incoming serial characters and holds them until the device
can process them. The serial transmit buffer collects the data it receives via the RF link until it
transmits that data out the serial port. The following figure shows the process of device buffers
collecting received serial data.

Serial receive buffer

When serial data enters the XBee/XBee-PRO ZigBee RF Module through the serial port, the device
stores the data in the serial receive buffer until it can be processed. Under certain conditions, the
device may receive data when the serial receive buffer is already full. In that case, the device discards
the data.
XBee/XBee-PRO® S2C ZigBee® RF Module
Serial buffers
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