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68HC812A4 Development Board

By Kevin Ross

kevinro@nwlink.com

 

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 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