Build a Programmer for the 68HC705C8 MicroController
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Introduction  Description  Schematic  Construction  Testing  Software  References Instruction Manual

Sorry folks, but since very few people have expressed any further interest in this product we stopped development on it and no longer offer kits or parts. Please also see the Frequently Asked Questions.


This programmer is used for programming the Motorola 68HC705C8 (or C8A).

The 68HC705C8 has the following attributes;

  • 8K bytes of internal EPROM
  • 304 bytes of internal RAM
  • 24 user-definable input/output ports
  • 7 Input only ports
  • Serial Control Interface (SCI)
  • Serial Peripheral Interface (SPI)
  • 16 bit capture/compare timer system
  • Watch Dog timer

This micro-controller is used in numerous projects available from Midon Design.  Programming is very straight-forward when this programmer is used in conjunction with the easy to use DOS-compatible software also available from Midon Design.  Both One Time Programmable (OTP) and EPROM versions of the 68HC705C8 can be programmed with this circuit.

The PC software used to program the micro-controllers was written and developed by Gil Shultz.


Figure 2 Programmer Schematic Diagram


As you can see from the schematic diagram in Figure 2, this is a very simple circuit built around Motorolas 68HC705C8 8 bit micro-controller. 

The SCI is connected to a MAX233 serial interface driver from Maxim. This five-volt only device converts the TTL level serial communications to standard RS232 levels through the use of built-in voltage converters. No external capacitors or additional power supplies are required to obtain the 12 volts required for standard RS232 levels.

A simple power supply, consisting of U3 and C5, converts 12 volt DC to the 5 volts required for the circuit. A LM78L05 version of voltage regulator was used due to the low power consumption (less than 32mA, worst-case) of the circuit.  The programming voltage, 15.5VDC 0.5V, is connected when requested to connector P2.

Two LED's are available to provide a visual state of programming.



Standard construction techniques may be used for this circuit and a printed circuit board is available if desired. The prototype was point-to-point wired on a Radio Shack prototype PCB (part # 276-168B).

If building the Programmer from the PCB, refer to Figure 3 for component placement. Begin by inserting IC sockets (highly recommended). Follow this with the insertion of all passive components. I like to proceed by inserting all resistors, then all capacitors. Watch the polarity of the electrolytic capacitors. The positive lead of the electrolytic capacitors is the square pad for that component.

There are only 4 types of discrete resistors; 470, 10K, 2.7K and 10M. The color codes for these are shown in Table 1.  Insert the resistors where shown in Figure 3.  The SIP resistor should be mounted as shown, with care to ensure pin 1 is oriented to the square pad on the PCB.

Resistor Color Codes


1st Stripe

2nd Stripe

3rd Stripe

4th Stripe (Tolerance)
















470 Yellow Violet Brown


Table 1. Resistor Color Codes

Programmer Part Layout

Figure 3 Parts Placement

Next, insert voltage regulator U3. At this time, you should also insert connectors P1 and P2. Once that is done, put down the soldering iron and get out a suitable power supply. Connect the power supply to the terminals of P1, being careful to connect the proper polarity to the correct terminal. Power up and then measure voltages between pins 40 and 20 of IC U1. Pin 40 should be +5 volts while pin 20 is ground. If you are wiring this circuit on your own board, then check polarity on the other IC socket as well. Table 2 shows the voltages required on each IC.


Table 2. Power and Ground Table










Turn off the power and get back your soldering iron. To finish up, insert the crystal, the LED's, the switch and the remaining connector. The flat side of the LED's should be lined up with the square pad for it.

You are now ready to prove in the circuit.


Testing the Circuit       

Carefully insert the MAX233 in it's socket, observing polarity while doing so.  Insert a blank 68HC705C8.  Do not apply VPP programming voltage until told to do so.

Install the PROG07 DOS program on your PC.  Connect a serial cable between the PC and the Programmer.  Run the PROG7 program.  Now power up the Programmer.

Use the C,B (Check, Blink) commands on the PROG7 program to verify that the PC is communicating with the Programmer.  The result should be alternating flashing of the PROG and VERF LED's.  A failure mode will be indicated on your PC.

If this does not work, verify 1. you have a good serial connection between the units, 2. that the PROG7 program is communicating on the correct serial port (the default is COM1).  You can also try pushing the RESET button on the Programmer.  If these trouble-shooting steps do not work, verify that the Programmer board has the correct components placed and that there are no solder bridges.

If all is now working, you are ready to start programming your micro-controllers!

The PROG7 commands are shown in Figure 4.

Note that if you run into any problems, the PROG7 software will ask you operate switches S1 and S2.  The S2 switch is the same as the reset switch on the Midon programmer.  The S1 switch does not exist on the Midon programmer.  Turning power on and off on the Midon programmer has the same effect as turning S1 on and off.

       /       \-r<A>m
      \                                                             /-r<O>m
       \         /-r<O>m                            /-<F>lush buffer 
        -<B>uffer                                  /                \-r<A>m 
                 \-r<A>m                   /-<F>ile
                                          /        \             /-r<O>m
                                         /          \-<R>ead File
                       <L>oad buffer from                        \-r<A>m
                                          \        /-r<O>m

                 /        \-r<A>m
<W>rite buffer to 
                  \-<C>hip - Program                      /-<P>rogram Ram
                                                  /       \-<V>erify ram 
                        <P>rogram Chip from buffer
                                                   \        /-<B>lank check
                                                    ---r<O>m---<P>rogram rom
                                                            \-<V>erify rom

<C>heck Programming Module

ESC to exit command!  Your choice ( D L W P C ? Esc S)?__

Figure 4 Available Commands

Parts List and Suggested Sources


Table 4. Included Parts List



Part Number


DigiKey Part Number

JameCo Part #




Capacitor, electrolytic, 10uFd

P6239-ND 94289




Capacitor, 22pFd

 1358PH-ND 15405




Green LED   34761



LED Red LED   94511
1 D3 1N4001 1N4001 Silicon Diode 1N4001GICT-ND 35975




PC Mount terminal strip, 2 pin

277-1027-ND 152346




DB-9 Female PC Mount connector

 A2100-ND 104951




Resistor, 10K, W

 10KQBK-ND 29911
1 R6 2.7K Resistor, 2.7K, W 2.7KQBK-ND 30390



10KSIP 8 pin SIP, 10K, W Q710K-ND 97826




Resistor, 10M, W

 10MQBK-ND 29938
2 R2,3 470 Resistor, 470, W 470QBK-ND 31165




Voltage Regulator

 LM78L05ACZ-ND 51182




RS232 Interface

 MAX233CPP-ND 106163
1 U4 FWB 1A 100V Full wave bridge DF01MIR-ND 103000





CTX068-ND 102621
1 J1 HDR 3 pin header strip ED8250-ND  
1   DIP 40 pin ZIF socket A348-ND 102752
1   DIP 20 pin DIP socket    
1 SW1 SW SPST SPST mom PCB switch   155256

Accessories for this project

1    12VAC (or DC) adapter    Midon Part Number MD3004


PC Programming Software (Prog7) is available from this link.  If you cannot download from the link, please send us an email with your request for the software.


PROG05 User Manual

68HC705C8 Specification Sheet

MAX233 Specification Sheet

LM78L05 Specification Sheet

Specification Sheets are in Adobe Acrobat format. If you wish to download Adobe Acrobat Reader, go here

Frequently Asked Questions

  1. Is a PCB available for this project?
    1. No we discontinued this project in 2002 after lack of demand for kits and no longer have stock of the PCBs.
  2. Can you provide the PCB design files?
    1. No the original PCB was designed using an older, proprietary system and are not available.  In Appendix 1 of the Instruction Manual you will find a graphic depiction of the PCB layout should you choose to design your own.  The image is also available here.
  3. Can this programmer program other processors?
    1. No the hardware and software design is specific to the 68HC705C8 and 68HC705C8A only.
  4. Is the source code available for the programming software?
    1. No the source code is property of the original author (not Midon Design).

Last Updated June 26, 2015

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