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This time a take a look at how a different project is generating the video RAM timing signals and try to reproduce the concept for myself on a breadboard. I look at the overall idea of how the timing is broken down into 8 stages, making use of logic gates to decide if each signal is on or off at each stage, and then the use of a latch to re-time the signals, elimination most of the propagation ...

In this part I start to look at the concept of sharing video RAM with the CPU. I show a circuit diagram of some registers connected to the video RAM. I explain the concept of how the CPU can write to and read from these registers without being concerned with the video circuitry timing. Finally I show a PCB design that I intend to get manufactured to simplify the wiring during the prototype phas...

This time I finally manage to get some pixel data on the screen and display some characters from a font set. I explain the different parts of the circuit and how it works. We take a look at the characters being displayed horizontally in a row on the screen. Right now each row just gets repeated, so the next step would be to add a vertical character counter. Font taken from: www.jimblimey.com/bl...

This time I connect one of the output ports on the Z80 trainer board to the VGA circuit by adding some d-type latches. Using the program from part 16 with a minor change, I send the 3-bit binary number to the new port. This has the effect of changing the colour of the screen to one of 8 possible values. We have black, blue, red, magenta, green, cyan, yellow and white. I can now press of one the...

Let's do some live coding. What is the worst that could happen? In this video I try coding a short Z80 assembly program to read input from the trainer board's on-board push buttons. Depending upon which button is pressed we should send a number from 0 to 7 to an output port. I have an idea how I might be able to use something like this to control part of the system in the future.

With the full keyboard built, it's time to test it. I start by looking at how the trainer board can be modified to connect to the keyboard. I talk through how to add a 74HC244 buffer IC to connect the keyboard to the data bus. Next I look at the Z80 assembly code, and how it has been modified to read from the keyboard. Finally we get to see the keyboard working on the trainer board. I check eac...

Time to build the full keyboard for the Z80 computer. I have 3D printed a key-switch plate for the full keyboard and the PCBs have arrived so the first thing to do is see if the PCB fits correctly. Next solder the diodes and cable connector. After some fun and games with the switch stabilizers, I finally get to fit all the keycaps. With a ribbon cable to finish things off. But will it work?

Now that I've got the VGA timing circuits working, I want to see if I can control the pixels as the horizontal lines are being generated on the screen. I am using a 74HC165 shift register to load data for 8 pixels, and then shifting the data out on each horizontal clock count. The basic idea does seem to work but there are some issues. I explore some ideas to solve the problems. Simply Put's vi...

I want to start looking at the VGA signals that will be used to produce a graphical output. Being one of the more difficult aspects of the project. Taking ideas from the Ben Eater graphics card, I build up the horizontal timing circuits. I explain an issue I had when trying to use the circuit without the inverters, which resulted in the SR latch getting very hot. So putting the inverters back i...

In this video, I will show you how I complete the keypad by mounting it on a PCB and fitting it inside a case. To interface the keypad with the trainer board, I will be using another 74HC244 buffer chip. Additionally, I will add a 74HC138 to provide input address decoding, allowing us to distinguish between the onboard tactile switches and the external keypad. I will walk you through the hardwa...

Still thinking about input I want to start creating a small prototype for the keyboard. The idea is to go over the design process and see how the parts will fit together. So I have decided to build the number pad section of a keyboard. I start by showing how we can use the KeyBoard Layout Editor website to generate the raw data for the keyboard design. I then show how we can take that data and ...

Taking a look at a slightly more complex input program, I realise that I really need a fast clock option. Ideally, a high-frequency crystal oscillator signal that can be divided down to more manageable speeds. After a suggestion from a viewer, I decided to look at the CD4060 integrated circuit. With this IC I can make the high-speed oscillator circuit and divide it down all with one chip. I go ...

Z80 Computer - Part 8 Testing Input

SN74HC244N Octal Buffer

Z80 Computer - Part 7 Keyboard Layout

Z80 Computer - Part 6 The Stack & Subroutine Calls

Interfacing with an SD Card

Z80 Computer - Part 5 Adding RAM

Z80 Computer - Part 4 I/O Address Decoding

Z80 Computer - Part 3 Output

Z80 Computer - Part 2 Adding ROM

3D Printed Screwdriver Holders

December Maker Mailbag

Z80 Computer - Part 1 The CPU, Clock & Reset

November Maker Mailbag

74HCT04N Crystal Ocsillator Circuit

October Maker Mailbag

AS6C62256 32K SRAM Integrated Circuit

September Maker Mailbag