Retro Computer Build Part 1 – CPU

So as part of the Retro Computer build, lets talk about the core of this computer, which will be a 6303. “What is this?” I hear you say. “I’ve never heard of this before?”  Well maybe you have, but just didn’t realise…So if you were asked had you heard about the 6800 from Motorola, I’m sure that many would say, “Oh, yes, I know it!”. Well back in 1977, Motorola took the 6800 microprocessor and added a few bells and whistles and called it the 680x microcontroller. This was used in thousands of systems back in the day and some variations are still in use now. Then Hitachi came out with their low power version called the HD630x.

Here is a link to wikipedia that gives more information on this chip.

I’m going to use the HD6303RP version of the micro for this computer. It will allow me to run 6800 code, while offering some basic I/O, which will help to get this computer operating.

The system will use about 8K/16K ROM depending on the configuration, and will have about 32K RAM for active memory. We may look at paging out memory at a later date.

To get the micro operational, there are a few things we have to do on the prototype board just to get things moving. It’s not that difficult if we take it one step at a time.

  • Clock
  • Reset Circuit
  • Operating Mode
  • External Interrupts
  • Power

The clock is used to provide timing to the micro – the faster the clock the faster the micro (to some extent). In this computer we are going to use a 4.9152MHz crystal to provide the basis of the clock for the micro. With the crystal, we have to use small bypass capacitors which help to stabilise the crystal while it’s operating.

Reset Circuit

The reset circuit is a very important part of the computer. When power is applied to the micro, it takes a little while for the clock to stabilise and get into a steady state. During this period we want to keep the micro in an inactive state. After a delay we then allow the micro to start working. To do this, we would normally have the reset pin on the micro held low (0, GND) then let it go high. This can be achieved with a simple RC (Resistor – Capacitor circuit).

Operating Mode

This micro has several modes that it can operate in. These normally are used in situations where the software is pre-programmed into the micro and it won’t be accessing any external memory. Other modes allow access to external memory, which is what we want to do for our configuration. To make the micro more flexible for future projects, we can add a small circuit to the system, to allow us to switch the operating mode for when the micro starts. We won’t be covering the different modes here, as we will be using Mode 2/4 for our application. This operating mode section, will allow us to choose, if we want to multiplex the lower address bus and the data bus. This is a good option, as it will free up port 1, which does contains other useful peripherals.

The small circuit below, shows both the reset circuit as well as a possible configuration to allow us to switch the operating mode of the micro.

External Interrupts

External interrupts are input pins, that when active, will cause the micro to stop working on what it is doing and jump to do something different. When it is finished doing this task, it will return back to what it was doing in the first place.

As we don’t want the computer to randomly jump to something else, we have to look after these external interrupts. Normally we just connect them to VCC via a pull up resistor.


This is the simplest of all – just connect a clean 5V DC supply to the relevant pins on the micro. One nice feature of this micro is that it has a sleep mode, where the quiescent current is very low -nothing like today’s low power micro’s.

Schematic & Assembly

Here is the overall schematic for these first few sections of this computer. SW2,3 & 4 are used to set the startup mode for the computer, this mode is only set during the reset cycle. When operating, a quick check that was used to verify that this was operational, was to check the clock signal on pin 40 (E Output).

This image, shows the assembled circuit for this post. Part 1 is now operating and we are ready to move on to the next part.

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