2. Rf Module Operation; Serial Communications; Uart Data Flow; Serial Buffers - Digi XBee Manual

XBee/XBee‐PRO DigiMesh 2.4 RF Modules
2. RF Module Operation

Serial Communications

The XBee/XBee-PRO 2.4 DigiMesh RF Modules interface to a host device through a logic-level asynchronous serial port. Through
its serial port, the module can communicate with any logic and voltage compatible UART; or through a level translator to any
serial device (For example: Through a Digi proprietary RS-232 or USB interface board).

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
Serial Data
Data enters the module UART through the DIN (pin 3) 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
The module UART performs tasks, such as timing and parity checking, that are needed for data communications.
Serial communications depend on the two UARTs to be configured with compatible settings (baud rate, parity, start
bits, stop bits, data bits).

Serial Buffers

The modules maintain buffers to collect received serial and RF data, which is illustrated in the figure below. The serial
receive buffer collects incoming serial characters and holds them until they can be processed. The serial transmit buffer col-
lects data that is received via the RF link that will be transmitted out the UART.
(Low‐asserted signals distinguished with horizontal line over signal name.)
CMOS Logic (3.0 - 3.6V)
DIN (data in)
CTS
XBee
Microcontroller
DOUT (data out) Module
RTS
Example Data Format is 8‐N‐1 (bits ‐ parity ‐ # of stop bits)
© 2010 Digi International, Inc.
CMOS Logic (3.0 - 3.6V)
DIN (data in)
CTS
XBee
Microcontroller
Module DOUT (data out)
RTS
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