Force Uart Operation; Data Format - Digi XBee 3 Cellular LTE-M User Manual

SPI operation
on SPI_SSEL causes the SPI_MISO line to be tri-stated such that another slave device can drive it, if so
desired.
If the output buffer is empty, the SPI serializer transmits the last valid bit repeatedly, which may be
either high or low. Otherwise, the device formats all output in API mode 1 format, as described in
Operate in API mode. The attached host is expected to ignore all data that is not part of a formatted
API frame.

Force UART operation

If you configure a device with only the SPI enabled and no SPI master is available to access the SPI
slave port, you can recover the device to UART operation by holding DIN / CONFIG low at reset time.
DIN/CONFIG forces a default configuration on the UART at 9600 baud and brings up the device in
Command mode on the UART port. You can then send the appropriate commands to the device to
configure it for UART operation. If you write those parameters, the device comes up with the UART
enabled on the next reset.

Data format

SPI only operates in API mode 1. The XBee Smart Modem does not support Transparent mode or API
mode 2 (which escapes control characters). This means that the AP configuration only applies to the
UART, and the device ignores it while using SPI. The reason for this operation choice is that SPI is full
duplex. If data flows in one direction, it flows in the other. Since it is not always possible to have valid
data flowing in both directions at the same time, the receiver must have a way to parse out the valid
data and to ignore the invalid data.
The XBee Smart Modem sends 0xFF when there is no data to send to the host.
Digi XBee® 3 Cellular LTE-M/NB-IoT Global Smart Modem User Guide
Force UART operation
153