Specifications

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the converter appropriately. Th is feature realizes the required auto-ranging capabilities of
this port. If the port is configured in correctly or th e signal is outside the measurable range
of the device for any particular setting, the port also has shunt diodes to r emove the excess
charge before it reaches the MCU.
4.1.5 Digital Measurements
The digital ports were implemented using a CCP and timer pair inside the MCU, and
a line driver. The line driver is the SN74LVC 1T45 [42] used in the SDI-12 module. The
digital ports have hardware support to sample on request, record on event, measure pulse-
widths, and pulse-frequency. This port is protected f rom over-voltage and reverse-voltage.
These ports support one switch-able power port.
4.1.6 Serial Communications
To comply with RS-232C, a MCU UART communication port was selected to drive the
full-duplex asynchronou s message passing. This was coupled with a MAX3221 [43] RS-232
level translator IC. Two excellent features the come with this chip include auto-shutdown
when no line is connected and a valid signal to alert the MCU that a device is connected
to the other end.
4.1.7 ZigBee Radio
The radio module selection process lo oked at ZigBee radios from Cirronet, Maxstream,
Radiotronix, and Aerocomm. These companies are the four leading developers using ZigBee
technology. The different companies vary in their base technology from Freescale’s two
chip ZigBee solution to Ember’s or Chipcon’s SoC ZigBee solution. Every one of these
companies offers distinctly unique solutions that target different end applications. However,
all four modules failed to fully satisfy the needs of this project. The SoC systems are recent
developments in the ZigBee market and boast power ratings of 300µA active, 27mA RX,
and 169mA TX. However, these solutions are not fully released to the public and bugs are
still being worked out of the products. The older and more stable Maxstream XBee-Pro