Elektor_Mag_-_January-February_2021

lektor January & February 2021 41

This was also Dennis Kuschel’s train of thought when he began his
work on building a discrete microcontroller with individual logic
ICs. Perhaps we should call it a ‘macrocontroller’ really. Here we
hand over to Dennis:

“Figure 1 shows my work space at home. I only own a few items
of measurment equipment — an oscilloscope and two digital
multimeters. I don’t need anything else and I’m really not missing
anything either. On my work bench you can see an example of the
MyNOR computer where I have replaced 10 of the ICs with discrete
transistor circuits (the cheerfully-coloured jumper wires).

“For Christmas in 1989 I received my first computer: a Commodore

64. I quickly wanted to do more than simply play games — I
    preferred to write my own programs and learn how such a device
    actually works. Four years later (I was then 17) I built a new computer
    from the salvaged parts of a C64 that I then programmed in assem-
    bler. During my electrotechnology studies in the 90s, the idea of
    building a CPU from simple logic gates and ICs came to me. A few
    years later MyCPU [1] was born. This is a computer built from many
    tens of logic ICs from the 7400-series CMOS logic family that runs
    a version of C64 Basic.

“The MyCPU received a lot of attention at computer festivals and
hobby fairs, but many of those who were interested shied away from

building their own because the cost of the complete MyCPU plus

peripherals quickly approaches €1,000. That is why I decided (after

20 years) to begin a new project: a simple and as-cheap-as-possi-

ble computer that is, of course, entirely built from discrete parts.

It had to meet two conditions. Firstly, it had to do without an ALU

(Arithmetic Logic Unit) for the simple reason that the previously

ubiquitous ALU chip 74LC181 is no longer available and alterna-

Homebrew CPUs

The discrete microcontroller

By Dennis Kuschel (Germany)

Modern microcontrollers

make it possible to cram a

whole lot of functionality into

an extremely small volume.

Things that, in the past,

required a bag full of chips

to accomplish can now be

realised in a single IC without

any great difficulty. Of course,

such a microcontroller still

needs to be programmed and

this is often done with the aid

of high-level programming

languages. But this has the disadvantage that the designer can lose the link to what goes on

at the port or transistor level inside such devices.

homelab

Figure 1: Dennis Kuschel in his home laboratory. The furnishings can be described as almost spartan.

Figure 2: This is the heart of the MyNOR computer: a single, discretely-built

NOR gate.