Technological Arts Inc.

3  HARDWARE DESIGN FEATURES

3.1    3-VOLT OPERATION

One of the nice features of the MCU is that it can operate on 3 to 5 V, while maintaining full bus speed capability.  To support 3 Volt operation, the Docking Module incorporates an adjustable regulator whose output voltage is set by a resistive voltage divider.  The circuit has been designed such that simply inserting a shorting jumper causes the regulator’s output to shift from 5V to 3.3V.  (Note:  If you aren’t using a Docking Module (e.g. you’re just plugging the DIP module into a breadboard), you’ll need to supply your own 3 Volt regulated supply to the Vcc pin to take advantage of 3-Volt mode.)  When operated at 3V, there are a few precautions that should be noted, however:
The logic pins are not 5V-tolerant, so you will need to take the necessary steps to prevent damage to the I/O pins of the MCU.  Also, the maximum VRH voltage is limited to 3.3V, so any external voltage or precision voltage reference you supply for use with the Analog-to-Digital converter subsystem should be scaled accordingly.  If you're using the MAX (40-pin) module, the CAN transceiver will not work--  it is 5-Volt only.  One last point is that some BDM pods (eg. MicroBDM12LX5) will not work with 3V targets, so you should check the specs of the BDM pod you intend to use.  A good choice is USBDMLT, which works with both 3V and 5V targets.

3.2    RESET

Unlike previous HC11 and HC12 designs, the 9S12C MCU has an on-chip low-voltage inhibit (LVI) reset circuit, so it is not necessary to provide such a circuit externally.  A momentary tact switch is provided for manual reset, and the LVI circuit will provide a clean reset signal upon power-up.

3.3    ABOUT THE DOCKING MODULE VOLTAGE REGULATOR

The NanoCore12 Docking Module (also, the School Board) includes an LM1086CT-ADJ voltage regulator.  Housed in a TO-220 package, it is capable of dissipating about 500 mW at room temperature.  Other nice features are:  reasonably low quiescent current, and low dropout voltage--  it will work with an input voltage down to about 5 Volts (or 3 Volts, in 3-Volt Mode), making it quite well-suited to battery operation.  It is also designed to withstand reverse polarity.  One drawback, however, is that it can become unstable and start to oscillate at low temperatures, especially if the input voltage source is connected to J1 via long wires.  If low-temperature operation is anticipated, the on-board 10uF tantalum capacitor can be replaced with a higher value (47uF or 100uF).  To compensate for long lead-in wires, add capacitance of about 100uF at, or close to, the J1 connector.

3.4    ABOUT THE ON-BOARD VOLTAGE REGULATOR

NanoCore12 modules include a tiny LP2981AIM-5 voltage regulator mounted on the underside of the module.  It is a low-dropout 5V regulator capable of supplying the on-board circuitry with the required current, with as much as 50 mA to spare for user applications, if the supplied input voltage to the regulator is limited to 6V.  The higher the input voltage, the less current will be available for the user application before thermal shutdown occurs.

3.5    PLL

While the supplied crystal is only 8MHz, the MCU is capable of running at a much higher speed.  The phase-locked loop feature of the MCU allows you to boost the bus speed by an integer multiple of the crystal frequency, so by enabling the PLL, you can actually run the MCU at 24Mhz.