XBee‐PRO®900HP/XBee‐PRO®XSCRFModules
©2014DigiInternationalInc. 26
If only the UART is enabled, then only the UART will be used, and SPI_SSEL will be ignored. If only the SPI is
enabled, then only the SPI will be used.
If neither serial port is enabled, the module will not support serial operations and all communications must
occur over the air. All data that would normally go to the serial port is discarded.
Forcing UART Operation
In the rare case that a module has been configured with only the SPI enabled and no SPI master is available
to access the SPI slave port, the module may be recovered to UART operation by holding DIN / CONFIG
low at
reset time. As always, DIN/CONFIG
forces a default configuration on the UART at 9600 baud and it will bring
up the module in command mode on the UART port. Appropriate commands can then be sent to the module to
configure it for UART operation. If those parameters are written, then the module will come up with the UART
enabled, as desired on the next reset.
SPI Port Selection
SPI mode can be forced by holding DOUT/DIO13 (pin 2) low while resetting the module until SPI_nATTN
asserts. By this means, the XBee module will disable the UART and go straight into SPI communication mode.
Once configuration is completed, a modem status frame is queued by the module to the SPI port which will
cause the SPI_nATTN line to assert. The host can use this to determine that the SPI port has been configured
properly. This method internally forces the configuration to provide full SPI support for the following
parameters:
•D1 (note this parameter will only be changed if it is at a default of zero when method is invoked)
•D2
•D3
•D4
•P2.
As long as a WR command is not issued, these configuration values will revert back to previous values after a
power on reset. If a WR command is issued while in SPI mode, these same parameters will be written to flash.
After a reset, parameters that were forced and then written to flash become the mode of operation. If the
UART is disabled and the SPI is enabled in the written configuration, then the module will come up in SPI
mode without forcing it by holding DOUT low. If both the UART and the SPI are enabled at the time of reset,
then output will go to the UART until the host sends the first input. If that first input comes on the SPI port,
then all subsequent output will go to the SPI port and the UART will be disabled. If the first input comes on the
UART, then all subsequent output will go to the UART and the SPI will be disabled.
When the slave select (SPI_nSSEL) signal is asserted by the master, SPI transmit data is driven to the output
pin SPI_MISO, and SPI data is received from the input pin SPI_MOSI. The SPI_nSSEL pin has to be asserted
to enable the transmit serializer to drive data to the output signal SPI_MISO. A rising edge on SPI_nSSEL
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 module formats all output in API mode 1 format, as described in chapter 7. The
attached host is expected to ignore all data that is not part of a formatted API frame.
XBee-PRO 900HP Data Format
The SPI will only operate in API mode 1. Neither transparent mode nor API mode 2 (which escapes control
characters) will be supported. This means that the AP configuration only applies to the UART and will be
ignored while using the SPI.
XBee-PRO 900HP SPI Parameters
Most host processors with SPI hardware allow the bit order, clock phase and polarity to be set. For
communication with all XBee radios the host processor must set these options as follows:
•Bit Order - send MSB first
•Clock Phase (CPHA) - sample data on first (leading) edge
•Clock Polarity (CPOL) - first (leading) edge rises
This is SPI Mode 0 and MSB first for all XBee radios. Mode 0 means that data is sampled on the leading edge
and that the leading edge rises. MSB first means that bit 7 is the first bit of a byte sent over the interface.
To Next Page
Loading...
Need help?
General
