Hardware Description

All unused pins should be left disconnected. All inputs on the radio can be pulled high or low with 40k internal pull-up or
pull-down resistors using the PR and PD software commands. No specific treatment is needed for unused outputs.
For applications that need to ensure the lowest sleep current, unconnected inputs should never be left floating. Use
internal or external pull-up or pull-down resistors, or set the unused I/O lines to outputs.
Other pins may be connected to external circuitry for convenience of operation, including the Associate LED pin (pin 15)
and the Commissioning pin (pin 20). An LED attached to the the associate LED pin will flash differently depending on
the state of the module to the network, and a pushbutton attached to pin 20 can enable various join functions without
having to send serial port commands. Please see the commissioning pushbutton and associate LED section in chapter
7 for more details. The source and sink capabilities are limited to 6mA on all I/O pins.
The VRef pin (pin 14) is only used on the programmable versions of these modules. For compatibility with other XBee
modules, we recommend connecting this pin to a voltage reference if analog sampling is desired. Otherwise, connect to
GND.
XBee-PRO 900HP Board Layout
XBee modules are designed to be self sufficient and have minimal sensitivity to nearby processors, crystals or other
PCB components. As with all PCB designs, Power and Ground traces should be thicker than signal traces and able to
comfortably support the maximum current specifications. No other special PCB design considerations are required for
integrating XBee radios except in the antenna section.
The choice of antenna and antenna location is very important for correct performance. XBees do not require additional
ground planes on the host PCB. In general, antenna elements radiate perpendicular to the direction they point. Thus a
vertical antenna emits across the horizon. Metal objects near the antenna cause reflections and may reduce the ability
for an antenna to radiate efficiently. Metal objects between the transmitter and receiver can also block the radiation path
or reduce the transmission distance, so external antennas should be positioned away from them as much as possible.
Some objects that are often overlooked are metal poles, metal studs or beams in structures, concrete (it is usually
reinforced with metal rods), metal enclosures, vehicles, elevators, ventilation ducts, refrigerators, microwave ovens,
batteries, and tall electrolytic capacitors.
XBee-PRO 900HP Module Operation for 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 Block Diagram for details). The internal microcontroller by default has control of certain lines. These lines
can be released by the internal microcontroller by sending the proper command(s) to disable the desired DIO line(s)
(see XBee Command Reference Tables).
In order for the secondary processor to sample with ADCs, the XBee pin 14 (VREF) must be connected to a reference
voltage.
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).