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68HC812A4 Development Board

By Kevin Ross

This article describes a 68HC12 development board produced by the Seattle Robotics Society. Proceeds from the sale of this board are donated to the SRS to support our website. The board was designed by SRS members Marvin Green, Kevin Ross, and Karl Lunt.

The 68HC812 is a great new chip from Motorola that is destine to replace the 68HC11 for price/performance. A write-up on the 68HC812A4 was done a few months back. Check out An Overview of the 68HC12 for more specfic information about the chip.

What is it?

Many of us are used to seeing Marvin Greens BotBoard controllers based on the 68HC11. The 68HC812A4 board is designed to be a single chip board for the 68HC812A4 Microcontroller from Motorola. Key points of the design are:

The board holds the chip and oscillator circuitry that allows it to run in single chip mode.

All pins of the controller are brought out to a .1" grid, which allows it to be inserted into a standard breadboard.

Its small size allows it to be used in a variety of situations.

The 68HC12 series of controllers have a nice feature which is an onboard debugging system. The Background Debug Mode (BDM) is a special one wire interface on the 68HC12 that communicates with a built-in hardware debugger. Using the BDM interface, you can read/write memory and registers, program EEPROM, and single step instructions. A BDM interface module is required to translate between the one wire interface of the 68HC12 and your host computer. The 68HC812A4 development board supports the standard 6-pin Motorola BDM connector. It also supports a 4 pin connector that serves the same function. (The 6-pin connector has been difficult to obtain).

The board also supports two serial ports by providing connectors S0 and S1. An external RS-232/TTL converter is required. The serial port connectors are compatible with the BotBoard series of Microcontroller boards. See http://www.seattlerobotics.org/encoder/aug97/cable.html for a easy to build converter cable. A connector is provided for power (+5, GND). The board will run in stand alone mode with the standard 68HC812A4 allowing for 4k of code, 1k of RAM, and around 96 pins of I/O.

Board Layout and Parts List

P1 = [gnd, +5, gnd]

S1, S2 = [Rx, Tx, +5, gnd]

BDM4 = [gnd, +5, bdm, reset]

BDM6 = [1 bkgd, 2 gnd, 3 N/C, 4 reset, 5 nc, 6 +5]

C1, C3, C7 = .1uf

C2 = 10uf or higher

C5, C6 = 22pf

C4 = See Text

Resistors are all 1/8 watt

R1, R2, R3, R5, R6, R7, R8, R9= 10k

R4 = 10meg

U1 = MC68HC812A4

U2 = MC34064

RESET = push switch 6mm square (Digikey P8037-ND)

X1 = 16mhz

TP+ and TP- are pads that provide +5 and GND for connecting test equipment. You can solder in a single pin or small wire to connect if you wish.

How to get one (or three!)

The 68HC812A4 development board is being sold through Kevin Ross's homepage. Proceeds after expenses are used to pay for the Encoder online newsletter. Checkout the following website for pricing information.

Pricing and ordering information