XBee‐PRO®900HP/XBee‐PRO®XSCRFModules
©2014DigiInternationalInc. 13
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.
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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