2. RF Module Operation
Serial Communications
XBee RF Modules interface to a host device through a serial port. Through its serial port, the module can
communicate with any logic and voltage compatible UART, through a level translator to any serial device (for
example, through a RS-232 or USB interface board), or through a Serial Peripheral Interface, which is a synchronous
interface to be described later.
Two Wire serial Interface (TWI) is also available, but not supported by Digi. For information on the TWI, see the
EM357 specification.
UART Data Flow
Devices that have a UART interface can connect directly to the pins of the RF module as shown in the figure
below.
System Data Flow Diagram in a UART‐interfaced environment
(Low‐asserted signals distinguished with horizontal line over signal name.)
DIN (data in)
DIN (data in)
DOUT (data out)
DOUT (data out)
Serial Data
Data enters the module UART through the DIN (pin 4) as an asynchronous serial signal. The signal should
idle high when no data is being transmitted.
Each data byte consists of a start bit (low), 8 data bits (least significant bit first) and a stop bit (high). The
following figure illustrates the serial bit pattern of data passing through the module.
UART data packet 0x1F (decimal number ʺ31ʺ) as transmitted through the RF module
Example Data Format is 8‐N‐1 (bits ‐ parity ‐ # of stop bits)
Serial communications depend on the two UARTs (the microcontroller's and the RF module's) to be
configured with compatible settings (baud rate, parity, start bits, stop bits, data bits).
The UART baud rate, parity, and stop bits settings on the XBee module can be configured with the BD, NB,
and SB commands respectively. See the command table in chapter 10 for details.
SPI Communications
The XBee modules support SPI communications in slave mode. Slave mode receives the clock signal and data
from the master and returns data to the master. The SPI port uses the following signals on the XBee:
• 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
© 2010 Digi International, Inc.
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