Z80uart

Recent Bring-Up Notes

See BASIC_DEBUG_2026-04-11.md for a summary of:

• BASIC workspace/RAM mapping fixes
• toolchain findings (zasm vs z80asm)
• UART and RAM smoke-test ROMs
• current diagnosis of the remaining BASIC startup issue

#My Z80 Computer

Assembler software

There is a Z80 assembler in the Ubuntu universe repos!!!

sudo apt-get install z80asm 

Make sure z80asm is in your executable path. There is also a de-assembler for binaries (z80dasm).

I wrote a simple Python script to automate assembly.

python assemble.py -i test.asm

In order to review binary hex code install bless:

sudo apt-get install bless

Most software and examples are written in TASM, I found a compatible project still maintained called ZASM

http://k1.spdns.de/Develop/Projects/zasm/Distributions/

CPM setup

This page proved very useful

http://cpuville.com/cpm_on_new_computer.html

(Currently validating its usefulness)

------------- Much of the below is now obsolete ------------------

Arduino driven Z80 development

The Arduino is gone now. Development is fun without it. However, the form factor is still there. Arduino could be used for debugging as needed.

The basic idea of this project is to use an Arduino to support the development of a z80 computer system and debug any of its components in the process. The Arduino stands in as ...

a) reset generator b) clock c) ROM for the lowest 64byte 0000h-003Fh

The z80 code is fed from a C array to PORTD, PORTB (with 6pins on an Arduino UNO R3) as address bus, and PORTC simulates the control bus (RD --> PIN4, MRQ --> PIN2, RESET <-- PIN3, CLOCK <-- Pin5). The array index correspondends directly with the z80 system address.

The z80 board is more or less configured as an Arduino shield (even though bigger, the Arduino is plucked in upside down on top of it). In order to run the board without the Arduino, I created a circuit on an Arduino shield PCB with the reset and clock circuit. Thus, it is possible to go back and forth between normal computer operation and running debug code from Arduino.

The Arduino emulation runs somewhat over 600khz after stripping all pinMode routines out of the Arduino code. (I wonder whether the if ... else ... control structure could be replaced with bitwise logic as well).

After Arduino reset, the Aruino code will perform a z80 reset as well (which needs at least 4 clock cycles).

~~The clock speed can be modified by nserting Arduino delays in the code which is very useful for debugging. Serial.println proved also very useful even though it might conflict with z80 data bus on PORT D. For this reason it needs to be placed strategically (e.g. at the high phase of the clock cycle).

A new idea is to use the Arduino as a simple IO device at the Z80 bus. At a 4Mhz clock it would require to use the WAIT signal.

Prerequisites

EEPROM

I got a very cheap, very universal EEPROM programmer called MiniPro TL866xx. It works perfectly with following project: Install the project if you want to use this project to directly write XPROMS with the code below.

https://github.com/vdudouyt/

Tested with [XICOR 28C64] (https://www.jameco.com/Jameco/Products/ProdDS/74827.pdf). The python code below assumes this 8k by 8byte device. Change code accordingly for other devices or more flexible use.

Example use:

minipro -p X28C64 -w d88_rom.bin

RAM

http://ee-classes.usc.edu/ee459/library/datasheets/628128.pdf

Current configuration

The circuit follows very closely http://www.z80.info/gfx/z80test.gif.

Differences:

• I am using the more original ZILOG z80 PIO.

Addresses:

Memory:

0000h - 003Fh Arduino ROM emulation
or
0000h - 7FFFh Space for ROM
8000h - FFFFh (static) RAM

I/O:

00h - 03h PIO 00h PORT A - ctrl
01h PORT A - data
02h PORT B - ctrl
03h PORT B - data
80h - 87h UART - TL16C550A, documentation: http://www.pci-card.com/TL16C550.pdf

Using the 74HCT138 for address decoding we can address 8 devices (2 of the lines are still unused). The wiring is a little bit unusual here: A - A7, B - A6, C - A5

Example code in this project

piobasics.asm

Just turns bit 0 at port B on if addresses are configured as above. (any speed, originally 1 to 10hz :) )

pioextended.asm

Blinks bit 0 at port B if addresses are configured as above (optimized for, i.e. visible at 600kHz clock). Also requires RAM at ffffh for sub routine call.

d88.asm

A somewhat unsuccessful attempt to drive a I2C 8x8 LED display from Adafruit. The program is able to output a "picture". However, the module used requires continous I2C clock which is hard to provide without timer interrupt. Might pick this project up once CTC interupts are available.

serial.asm

Basic implementation of a serial communixcation. Minimal working (output) setup using 9600-8-N-1 configuration:

To set the baud rate, we need to access the divisor registers, which is done by setting the most significant bit of the line control register to 1.

• set register 83 to 80 (1000 0000)

• now set dividers:

  • set register 80h to 0ch (12 for 9600)

  • set register 81h to 00h

• now configure line control register

• set register 83h to 03h

• now transmit data by setting value of register 80h

• check register 85h bit 4 will tell whether transmission register is clear

bootloader.asm

• load file from serial port

• assembled code needs to start on 0800h

• file end is determined by time out

• jump to start program

keyboard.asm

• simple serial reflector of keyboard activity

• load with bootloader

Documentation Links

[Zilog User Manual] (http://www.z80.info/zip/z80cpu_um.pdf)

CPM User Manual

###Explore

http://www.ecstaticlyrics.com/electronics/SPI/fast_z80_interface.html

CPM 2.2 installation: This looks pretty reasonable. I like the idea to use an actual IDE hard disk and do not care about its compatibility outside the system.

[http://www.glynstore.com/content/docs/ftdi/DS_VDIP2.pdf]: Explore this option to get USB sticks and FAT32 going relatively easily and somewhat lame.