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Operation SPI communications
XBee/XBee-PRO® S2C ZigBee® RF Module 41
SPI communications
The XBee/XBee-PRO ZigBee RF Module supports SPI communications in slave mode. Slave mode
receives the clock signal and data from the master and returns data to the master. The following
table shows the signals that the SPI port uses on the device.
Signal Function
SPI_MOSI (Master Out, Slave In) Inputs serial data from the master
SPI_MISO (Master In, Slave Out)
Outputs serial data to the master
SPI_SCLK (Serial Clock)
Clocks data transfers on MOSI and MISO
SPI_SSEL (Slave Select)
Enables serial communication with the slave
The preceding four pins are standard for SPI. This module also supports an additional pin, which may
be configured to alert the SPI master when it has data to send. This pin is called SPI_ATTN. If the
master monitors this pin (through polling or interrupts), it can know when it needs to receive data
from the module. SPI_ATTN asserts whenever it has data to send, and it remains asserted until all
available data has been shifted out to the SPImaster.
In this mode:
n
Data/clock rates of up to 5 Mb/s are possible.
n
Data is most significant bit (MSB) first.
n
Frame Format mode 0 is used. This means CPOL= 0 (idle clock is low) and CPHA = 0 (data is
sampled on the clock’s leading edge).
The following diagram shows the frame format mode 0 for SPI communications.
SPI operation
When the master asserts the slave select (SPI_SSEL), the device drives SPI transmit data to the
output pin (SPI_MISO), and receives SPI data from the input pin SPI_MOSI. The SPI_SSEL pin must be
asserted to enable the transmit serializer to drive data to the output signal SPI_MISO. A rising edge
on SPI_SSEL resets the SPI slave shift registers.
If the SPI_SCLK is present, the SPI_MISO line is always driven whether with or without the SPI_SSEL
line driven.
If the input buffer is empty, the SPI serializer transmits a busy token (0xFF). Otherwise, all
transactions on the SPI port use API operation. See
API Operation for more information.