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Project Notes

#394 AT89C2051 Programmer

Building a programmer for the Intel MCS-51/8051 compatible AT89C2051, with host software running on MacOSX.



I’ve been meaning to dive a bit deeper into Intel MCS-51/8051 devices and programming ever since I built the LEAP#088 ElectronicClockKit. While it’s long been EOL at Intel, this article By Jon Wilder is a valiant argument for why they are still around.

I have some (still in production) AT89C2051 chips - but before I can do much more, I need a programmer.

Reading the AT89C2051 datasheet, its seems programming is actually a quite straight-forward process - the only complication being the need for a 12V programming voltage in addition to standard logic high of ~5V.

So while I could go out and buy a programmer, the idea of building my own is very attractive for two reasons:

  • it is a bit of a short cut to building familiarity with the chip and datasheet
  • I can make sure it supports my host platform of choice - MacOSX

Inspirations and Prior Art

I found a couple of interesting projects that provided a good starting point. I’d recommend checking these out. In summary:

  1. AT89C2051_programmer
    • by piotrb5e3
    • uses separate 12V programming voltage power supply [negative]
    • open source python host programming software distributed via PyPI (at89overlord) [positive]
    • uses Arduino Uno as a programmer [positive]
  2. An Arduino-based programmer for the AT89C2051 chip
    • by ceptimus
    • uses a charge pump for 12V programming voltage [positive]
    • charge pump control is quite naive (bit-banging) [negative]
    • C# host programming software, open sourced but not in collaborative version control [negative]
    • uses Arduino as a programmer [positive]
    • quite wasteful of GPIO pins, so requires Arduino Mega/ATmega2560 [negative]

Boiling down my wishlist/plan:

  • use an Arduino-controlled charge pump for programming voltage, but control it with PMW similar to LEAP#393 DicksonChargePump
  • assign GPIO pins judiciously so can still use Ardiuino Uno/ATmega328 as the programmer
  • base the Arduino programming sketch on piotrb5e3’s, but modified for charge pump control
  • hopefully maintain compatibility with at89overlord so it can be used as the host programming software, else adapt as required

VPP/RST Power Control

We need to switch between 12V, 5V and 0V on the VPP/RST pin for the range of programming modes. From the datasheet

  • VPP Programming Enable Voltage: 12V ± 0.5V (programming current is only 250µA)
  • for HIGH with 5V supply: actually require 0.7 * VCC i.e. 3.5V minimum
  • for LOW: actually require 0.2 * VCC - 0.1V i.e 0.9V maximum

My goal was to achieve this with as few Arduino pins as possible, and without requiring an external 12V power supply. I can’t find the specs, but I’m pretty sure we need negligible current/power at any of these levels, which simplifies things.

I finally figured out this scheme. It needs 3 Arduino pins:

  • ~16V charge pump, generated with 62kHz PWM from 2 pins (inverted pair of PWM waves)
    • the output is fed to VPP/RST after clamping with a 12V zener diode
  • 0/5V controlled with a digital pin:
    • 5V is connected to VPP/RST via a diode to prevent 12V appearing back on the Arduino pin
    • 0V signal is inverted via a pair of NPN transistors that pull the VPP/RST down hard
    • this avoids ~2V from the unexcited charge pump preventing a true 0V input

And it works;-)

There is one caveat: there’s an “invalid state” that must be avoided lest the magic smoke may be released in a possible high current/burnout. This is avoided in the programming software.

Here’s a breakdown of the control states. In this table:

  • RST_PIN - pin A0 output
  • Charge Pump PWM - pins 3,11 output
RST_PIN Charge Pump PWM Charge Pump Voltage Q1c Q2 VPP/RST
High, 5V On 12V Low Off ~12V
Low, 0V On 12V High On invalid state
High, 5V Off ~2V Low Off ~4.4V
Low, 0V Off ~2V High On ~0V

Possible improvement: I could probably reduce this to requiring 2 pins in total with the addition of an inverter IC to:

  • invert a single PWM signal for charge pump control
  • replace the 2 x NPN inverter for the 0V pull-down

Installing at89overlord

Using a Python3 virtual environment. Requirements are in requirements.txt

python3 -m venv venv
source venv/bin/activate
pip install --upgrade pip
pip install -r requirements.txt

This installs at89overlord 0.4.0 (there was a minor issue with the packaging of 0.3.0 that may cause problems; 0.4.0 works fine though)

Programming a Device

On my system, I have the Arduino plugged in and appearing on /dev/tty.usbmodem14511.

Using the hex file generated with the Blinky code here, programming a chip is as simple as this:

$ at89overlord -p /dev/tty.usbmodem14511 -f ../Blinky/Blinky.hex
# Initializing the programmer...
# Initialized!
# Confirming chip ID...
# Confirmed!
# Erasing flash...
# Done!
# Writing flash...
# Done!
# Verifying...
# Done!

Breadboard Construction




Putting it on a Prototyping Shield

With so many wires on the breadboard, this is not very practical to build every time I need to program an 8051, and it is pretty fragile - just one dodgy wire and programming/verification can easily fail.

It looks like a nice project to build a custom board, with ATmega328 built in … another project for later!

I have a spare prototyping shield for an Uno, so for now I’ve put the circuit on that. Here’s a sketch of the layout I used:


The finished shield:


Now it’s easy to program an AT89C2051..


Credits and References

About LEAP#394 8051
Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.