Hardware Design notes
XBee®-PRO 900HP/XSC RF Modules 27
Module operation for the programmable variant
The modules with the programmable option have a secondary processor with 32k of flash and 2k of RAM. This allows module integrators to put custom
code on the XBee module to fit their own unique needs. The DIN, DOUT, RTS, CTS, and RESET lines are intercepted by the secondary processor to allow it
to be in control of the data transmitted and received. All other lines are in parallel and can be controlled by either the internal microcontroller or the
MC9SO8QE micro; see the block diagram in Operation for details. The internal microcontroller by default has control of certain lines. The internal
microcontroller can release these lines by sending the proper command(s) to disable the desired DIO line(s). For more information about commands, see
AT commands.
For the secondary processor to sample with ADCs, the XBee 14 (VREF) must be connected to a reference voltage.
Digi provides a bootloader that can take care of programming the processor over-the-air or through the serial interface. This means that over-the-air
updates can be supported through an XMODEM protocol. The processor can also be programmed and debugged through a one wire interface BKGD (Pin
8).
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