This page describes a possible arrangement of the hardware necessary for the programming of the Motorola MC68HRC908 microcontroller. The design is based on the monitor circuit given in their data sheet MC68HC908JL3/H Rev. 1.0 Section 9.
The circuit requires a simple interface to a microcomputer (via RS232), and the arrangement here requires just a 5 volt supply, (available at the game port). The circuit is arranged to use the low voltage entry to monitor mode and 9,600 baud communication as described in the data sheet.
The chips used in my circuit are not as Motorola's circuit. I used chips that were available to me, ones that I had in my box of bits. The principle of operation is the same. There is an AMD232A. This is used to interface between the TTL level voltage and the RS232 levels. It generates the higher positive voltage as well as the negative from a 5v positive input. To control the data flow I have used a 74LS244. For anyone who knows about these things, it may seem rather extravagant to use an octal device when only 2 of the 8 gates are employed. The fact is that the 244 is the kind of chip I tend to find down the back of the sofa, so is readily available. I am sure there are others where that came from. The only other requirement is a 20 pin socket and a few resistors and capacitors. It was the very simple programming requirements that attracted me to this PIC.
The circuit diagram is available here. Very simple to construct. I have included a PCB layout and there is also a photo of the finished device. The two wires sticking up at the back of the device are my simplified reset switch. You may wish to fit something of a rather more sophisticated nature. Also supplied is a copy of my etch resist diagram for anyone who etches their own boards. It has become a little jaggy in the transition to this location but will I hope, still be useful.
One thing to note about the construction is the choice of the resistor and capacitor for the oscillator. Information in the data sheet implies an 8k resistor with a 10pF capacitor. The variation in the manufacture can result in large variation in oscillator frequency. I found I needed 4k7 with my 10pF. The criterion is to achieve a frequency of 9.8304 MHz +/-10%, on pin 3 of the PIC.
A.J. Donald M.Sc.,D.I.C. --- Dec. 2000 --- E-mail
