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Build a Programmer for the 68HC705 MicroController

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Inside the 68HC705C8 programmer: chip, schematic and construction

This programmer is a small serial-controlled board for writing code into the Motorola 68HC705C8 and 68HC705C8A microcontrollers. The circuit is almost entirely the target chip itself, a Maxim level converter and a tiny regulator. This page covers the hardware: what the C8 contains, how the schematic works, how the board goes together, and how to prove it before programming a part.

Target68HC705C8 / C8A
Host linkRS232 via MAX233
Supply drawUnder 32 mA
VPP15.5 VDC +/-0.5 V

First published June 4, 2002 · Text rewritten 2026

Isometric illustration of a microcontroller chip with a quartz window under a glowing ultraviolet lamp (AI-generated image)

Why a dedicated programmer

The 68HC705C8 was the workhorse processor behind several Midon Design kits, which is why the company published a programmer for it. The part is a member of Motorola's HC05 family, an 8-bit line that was common in consumer and hobby electronics, and its program memory is EPROM. Writing that memory needs a controlled sequence and a raised programming voltage, so a plain serial cable is not enough.

The design here is deliberately minimal. Instead of building a complex programmer around a separate controller, it lets the 68HC705C8 do most of the work: the chip is placed in its socket, started in its built-in bootstrap mode, and then receives its new program over its own serial port from a PC. The circuit around it only has to supply clean 5 V power, convert the serial levels, switch in the programming voltage and show status on two LEDs.

The same hardware appears in the product line as the PROG05 kit. The kit, its DOS-compatible software, the command set and the programming procedure are covered on the PROG05 software and operation page. Both OTP and windowed EPROM versions of the C8 can be written with this circuit.

The 68HC705C8 at a glance

The C8 and the later C8A share the resources below, which made them a comfortable fit for small control projects that needed a serial link and a fair amount of I/O.

Resource	68HC705C8 / C8A
Program memory	8K bytes of internal EPROM
RAM	304 bytes
General I/O	24 user-definable input/output lines
Input-only lines	7
Serial	SCI (asynchronous) and SPI (synchronous)
Timer	16-bit capture/compare timer system
Supervision	Watchdog timer
Package on this board	40-pin DIP in a ZIF socket

The SCI is the interface that matters for programming: it is the port through which the PC sends the code. The SPI, timer and watchdog are for the finished application and play no part in the programming circuit.

Schematic walk-through

Serial interface

The SCI pins of the 68HC705C8 connect to a MAX233 from Maxim. This 5 V-only driver converts the chip's TTL-level serial signals to standard RS232 levels using built-in charge pumps. Unlike the MAX232, the MAX233 needs no external capacitors and no extra supply to generate the roughly plus and minus 12 V that RS232 expects, so the board stays small. The PC side is a female DB-9 connector.

Power supply

A simple supply built from regulator U3 and capacitor C5 turns 12 V DC into the 5 V the logic needs. An LM78L05, the low-current version of the familiar 7805, was chosen because the whole circuit draws less than 32 mA even in the worst case. A full-wave bridge ahead of the regulator means the input polarity is not critical and a 12 V AC adapter works as well as a DC one.

Programming voltage

EPROM cells need a higher voltage to be written. The programmer expects 15.5 VDC, within plus or minus 0.5 V, on connector P2, applied only when the software asks for it. Keeping VPP on its own connector makes it easy to leave it disconnected during setup and testing.

Clock, reset and status

A 2.0 MHz crystal clocks the processor during programming, and a momentary push-button provides reset. Two LEDs, one green and one red, give a visual indication of the programming state; in the software they are referred to as the PROG and VERF indicators.

Building the board

Standard construction works, on a printed circuit board or on perfboard. The prototype was wired point to point on a Radio Shack prototype board, part number 276-168B, which shows how forgiving the layout is. A PCB was also produced for the kit.

Assembly sequence

Fit IC sockets first. They are strongly recommended, and the 40-pin position should be a zero insertion force (ZIF) socket so chips can be swapped without bending pins.
Insert the passive parts: resistors first, then capacitors. The electrolytic capacitors are polarized, and their positive lead goes to the square pad.
The board uses four resistor values: 470 ohm, 2.7K, 10K and 10M, plus an 8-pin 10K SIP network whose pin 1 must match the square pad.
Add regulator U3 and connectors P1 and P2, then stop soldering and do the power check below.
Finish with the crystal, the two LEDs (flat side to the square pad), the reset switch and the DB-9 connector.

Power check before the chips go in

Connect 12 V to P1, observing polarity, and measure at the empty sockets. On U1, the 68HC705C8, pin 40 must read +5 V and pin 20 is ground. On U2, the MAX233, pin 7 is +5 V and pins 6 and 9 are ground. Anyone wiring their own board should check every socket this way before inserting parts.

Isometric illustration of a large microcontroller chip on perforated prototyping board with a regulator and two LEDs (AI-generated image)

Proving the circuit

Insert the driver. Place the MAX233 in its socket with the correct orientation.
Insert a blank chip. Put a blank 68HC705C8 in the ZIF socket. Do not connect the VPP supply to P2 yet.
Connect the PC. Run a serial cable from the PC to the DB-9, start the programming software, then power up the board.
Run the check. The software's check and blink commands make the PROG and VERF LEDs flash alternately if the PC and the board are talking. A failure is reported on the PC screen.
If it fails. Confirm the serial cable, confirm the software is using the right port (COM1 by default), press reset, then inspect the board for misplaced parts and solder bridges.

Once the LEDs alternate, the hardware is proven and the board is ready for real programming, described on the PROG05 operation page. Programmed C8 parts were used across several kits in the Midon Design product line, and the smaller 68HC705K1 sibling ran the STATX10 X-10 status display.

Questions and answers

Which chips can this programmer write?

Only the Motorola 68HC705C8 and the 68HC705C8A. Both the hardware and the PC software were designed around those two parts, and they cannot program other members of the HC05 family or other microcontrollers. Both the one-time programmable plastic version and the windowed, UV-erasable EPROM version of the C8 are supported.

Why does the circuit use a MAX233 instead of a MAX232?

The MAX233 builds its charge-pump capacitors into the package, so it converts TTL serial levels to RS232 levels from a single 5 V supply with no external capacitors at all. That saves board space and parts on a circuit that is otherwise very small, at the cost of a slightly larger 20-pin chip.

What programming voltage does the 68HC705C8 need?

The programmer expects 15.5 VDC, held within plus or minus 0.5 V, on connector P2. It should be connected only when the software requests it, and it must stay disconnected during the power check and the first communication test. The rest of the circuit runs on 5 V from an LM78L05 regulator.

Can the programmer be built without the printed circuit board?

Yes. The prototype was wired point to point on a Radio Shack 276-168B prototype board. The circuit has only a handful of parts, so perfboard works well, provided each socket is checked for correct +5 V and ground before any chips are inserted and a ZIF socket is used for the processor.

How much power does the programmer draw?

Less than 32 mA in the worst case, which is why a small LM78L05 regulator is sufficient. The board is fed from a 12 V adapter, either AC or DC thanks to the full-wave bridge, and the separate 15.5 VDC programming supply only needs to be present while a chip is actually being written.