Z80-computer

The Z80 is an 8-bit CPU designed by Zilog that was first introduced in 1976, and was one of the first microprocessors that allowed the general public, not just large businesses, to own a computer. The aim of this project is to design a Z80 computer with the following features: 256k of read only memory, and 256k of RAM, and output in the form of an LCD screen. I have written some programs for it.

Versions

1.0

• Initial design of computer.
• 32kb of ROM, and an additional 32kb of RAM.
• Input is given by a PS/2 keyboard module plugged into header pins.
• Output is sent to an LCD screen.

1.1

• Simplified clock circuit, now gives a constant pulse.
• LCD_EN is now negated, since the LCD is enabled on a high pulse.
• Switched to dedicated PS/2 connector.
• Removed clock LED.

1.2

• Redid the majority of the schematic. Redesigned the symbols to have a logical as opposed to realistic layout.
• Added ability to switch connections on the PS/2 connector to allow all combinations. Previous version shorted the power.
• Switched to On-Off switch for the reset button.

Components

The Big Three

• Zilog CPU. I am using a modern Z84C00. Its static design means that the clock signal can be as slow or as fast as we want (up to several MHz).
• AT28c256. A 256k EEPROM. Or more specifically, it can contain 32k words, each containing 8 bits. I will be using as the ROM where the instructions for the program is held.
• CY62256NLL-70PXC. A 256k SRAM. Yet again, its really 32k, each containing 8 bits of data.

Power.

The computer can be powered by either using an external power supply, or by plugging in the Arduino.

• LM7805. A 5V linear regulator.

Clock pulse

• NE555 timer. This will be used as the timer.
• 10nf to 100nf capacitor.
• 1M trim pot, to adjust the clock rate.

Output

• A generic LCD display.

Input

• A PS/2 port that a keyboard can be plugged into,
• Arduino Nano. Used to take input from the keyboard, and turn it into a suitable ASCII character.
• 74HC595. A shift register that will be used to send data from the Arduino, to the data bus.

Logic

• 74x04 chip. A NOT gate. This will be used to turn high signals low, and low signals high.
• 74x32 chip. An OR gate. This will be used to output a high signal when either of the inputs are high. Useful for outputting an active low signal, depending on two active low inputs.

Miscellaneous

• A momentary push button. This will be used as our reset button.
• Some 100n capacitors to help clean out the voltage throughout the circuit.
• Some way to write data to an EEPROM. I am using an XGecuPro T48.