XBee®/XBee‐PRO® ZB SMT RF Modules
All unused pins should be left disconnected. All inputs on the radio can be pulled high or low with 30k 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
pad (pad 28) and the Commissioning pad (pad 33). The Associate LED pad will flash differently depending on
the state of the module to the network, and a pushbutton attached to pad 33 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 4mA for pad numbers 3,
4, 5, 10, 12, 14, 15, 16, 17, 25, 26, 28, 29, 30 and 32, and 8mA for pad numbers 7, 8, 24, 31 and 33 on the
module.
The VRef pad (pad 27) 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.
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. A recommended PCB footprint for the
module can be found in Appendix C. 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. With the exception of
the RF Pad variant, 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.
Design Notes for PCB Antenna Modules
PCB Antenna modules should not have any ground planes or metal objects above or below the antenna.
For best results, the module should not be placed in a metal enclosure, which may greatly reduce the
range. The module should be placed at the edge of the PCB on which it is mounted. The ground, power
and signal planes should be vacant immediately below the antenna section. The drawing on the
following page illustrates important recommendations for designing with the PCB Antenna module. It
should be noted that for optimal performance, this module should not be mounted on the RF Pad
footprint described in the next section because the footprint requires a ground plane within the PCB
Antenna keep out area.
© 2010 Digi International, Inc.
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DCD [ DIGITAL CORE DESIGN ]
