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.

Description       

As you can see from the schematic diagram in Figure 2, this is a very simple circuit built around Motorola’s 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.

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.

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.

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<O>m
        -<C>hip
       /       \-r<A>m
      / 
<D>ump 
      \                                                             /-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
                                           \-<C>hip
                                                   \-r<A>m

                          /-r<O>m
                  /-<F>ile 
                 /        \-r<A>m
<W>rite buffer to 
                 \ 
                  \-<C>hip - Program                      /-<P>rogram Ram
                                                   /-r<A>m
                                                  /       \-<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)?__

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

PC Programming Software (Prog7) is available from this link or from our Documents section.