Questions tagged [z80]

The Zilog Z80 is an 8-bit microprocessor designed by Zilog. It powers the popular ZX Spectrum computer.

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Why do these resistors decouple the two databuses?

The ZX Spectrum is a computer with a Z80 accessing ROM and two separate areas of RAM, one of which is also accessed by a ULA which generates video. If the Z80 wants to access the video RAM, it may be ...
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Is the RFSH signal really needed in a Z80 CPU?

The Z80 CPU has a RFSH (active low) signal used to signal when it's refresh time for dynamic RAM chips. The Z80 datasheet states that RFSH alone shouldn't be used to generate a RAS only bus cycle in a ...
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2 answers
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Will diagonal PCB traces affect a sensitive CPU circuit?

I am trying to design a Z80 Computer Boards from scratch, just relying on the Z80 datasheet and some assembly language tutorials (I have no experience with the Z80 and never used them before but I am ...
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1 answer
78 views

Interrupt deduplication with discrete components

I am playing with interrupt mode 2 on a z80. Once pulled low it will disable interrupts and make a subroutine call to 0x0033. Before returning from the interrupt handler, interrupts should be ...
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ZiLOG Z80 Stack Pointer Location in Memory?

I am trying to design a board for a Z80 based computer, and am having trouble designing the memory map. I have found that the Program Counter defaults to address ...
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1 vote
2 answers
228 views

How to reset flip flop in ATF16V8?

I'm designing a Z80 microcomputer with dynamic memory (DRAM), and I want to control the memory with 3 chips; an ATF16V8 Generic-Array Logic, and 2 74LS157 quad 2:1 multiplexers. The logic diagram of ...
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5 answers
2k views

Crystal oscillator using 7404 is unstable

I am currently building a small Z80 microprocessor system and require some kind of clock generation. It is not my first design: Earlier I used an Arduino pin that generated a slow clock or a 555 timer....
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Can the MINI Wemos D1 ESP32 handle 5v on its GPIO pins?

I need to connect multiple data and address pins of a Z80 processor, which runs like the rest of my circuit on 5V, to the gpio pins of a mini wemos D1 Esp32. But I am not sure if the wemos can handle ...
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2 answers
277 views

Z80B very hot with slow clock?

I haven't done any hardware work in a lot of years, but I've run up against an oddity. I'm doing some retro-stuff with a Z80 from scratch and I'm finding that the chip is heating up pretty fast; so ...
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2 votes
1 answer
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Z80B starts and stops?

I've decided to take some time to refresh my electronics knowledge after 30 years out of college. I've run into a small problem that I can't quite figure out and need a pointer. TLDR: The Z80 ...
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2 answers
202 views

What are some good replacement ICs for DIP UARTs? [closed]

Recently, I have been developing an SBC based on the Zilog Z80 microprocessor. Since my first design, a serial connection has been essential for the board to communicate with a PC or terminal. For ...
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15 votes
3 answers
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Why am I Seeing A Weird "Notch" on the Data Line For Some Logical 1s?

I'm attempting to build a Z80 homebrew computer for some retrocomputing fun and to teach myself the basis of electronic design. For proof-of-concept, I've already assembled a basic system on ...
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How to wire up multiple Z80 PIO chips?

From my last question, I now understand how to use one Z80 PIO chip. I'm having trouble understanding how I would wire multiple PIOs up, though. The more I think about it, the less sense it makes. I ...
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2 answers
101 views

Better way to selectively enable memory chips

I'm designing a computer using a Z80 CPU. I have one EEPROM chip and two RAM chips, so far. I have to do things like this: To ensure that this specific RAM chip is only enabled when the 13th and 15th ...
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2 votes
1 answer
2k views

What exactly does the Z80 PIO do?

I've been reading the datasheet for the Z80 PIO chip a lot in the last few weeks, and I still don't fully understand what it's supposed to do. Take this diagram, from the Z80 CPU datasheet: This is ...
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11 votes
4 answers
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Why is the RESET pin set up like this in this Z80 schematic?

I've found the following schematic: Which after a lot of datasheet reading I mostly understand. The main thing I don't understand, however, is what's going on with the RESET pin. First of all, I ...
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1 vote
2 answers
392 views

Z80 RD and WR to RD/WR?

This may sound like a very stupid question. However, I am new to the Z80 stuff. I am planning on how to connect the Z80 control signals to a SRAM. But the Z80 has seperate RD and WR, while my SRAM has ...
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2 answers
613 views

Does pin A15 on a Z80 tell if the CPU is addressing ROM or RAM?

I am designing a simple, hobbyist single board computer similar to an Arduino using a Z80 CPU. The trouble I am running into is how the CPU addresses memory. I know that the Z80 uses pins A0-A15 to ...
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Crystal oscillator circuit not reaching full voltage

I want to generate a clock for a z80 CPU using a crystal oscillator but somewhere between 8 and 13.5mhz the voltage drops and with 20mhz it's about 1 volt peak to peak. I have tried numerous circuits ...
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8 votes
8 answers
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Measure square waves without oscilloscope?

I have a Z80 which is misbehaving and want to check the signals. However I do not own an oscilloscope, so I'm looking for another way of checking the square waves on the pins. I basically just need ...
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Z80 computer design peer-review [closed]

this is a re-post as I was unable to get any help on other forums. I have designed and built a simple Z80 computer system with 32K RAM, Z80B at 4mhz, 32K ROM, and a UART. The only problem with it is ...
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-2 votes
2 answers
459 views

Bidirectional TTL to CMOS

Is there a way to have bidirectional ttl logic levels to cmos logic levels conversion in a unique IC. Because right know im trying to create a z80 computer but using a avr as a peripheral as serial ...
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1 vote
3 answers
1k views

Z80 I/O explanation?

I have been following the two Great Z80 Project guides and the only thing I don't get is the I/O. He uses a 6522 I/O V.I.A. and I don't quite understand the idea of I/O ports. So my question is, how ...
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-1 votes
2 answers
1k views

How to check that Z80 is dead [closed]

I've bought three Z80 processors (SGS-Z8400B1-Z80CPU datasheet) In order to check them I use a setup like the one in the following picure (ref). The only difference is that I use a manual clock and ...
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2 votes
2 answers
313 views

Prevent bit-bang data-in line from back powering device?

I'm building a Z80 machine from components, to talk initially to a PC via bit banged I/O through an FTDI USB cable, and I've little idea how best to avoid back powering the Z80 board from a 5V data in-...
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Z80 - adding numbers in U2 - settings of register flags

We have Z80. This processor do following operation in U2: 200 - (-56). What will be values of following register flags: S - the ...
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19 votes
3 answers
4k views

SRAM isn't blank on powerup, is this normal?

I had finally finished my Z80 memory board, but I was disappointed to see that it wasn't working properly (simple NOP test with the address lines connected to some LEDs) as the counter quickly ...
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3 votes
2 answers
399 views

Logic family for 3.3 V microcontroller interface

I need to generate a bunch of chip selects for a SPI bus that I want to use on a project. I will use a 36 position (similar to PCIExpress = cheap) card edge connector and etc. The bus is simple : <...
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2 votes
1 answer
267 views

Will any rom file for a Z80 computer work with any hardware?

A few months ago I made up some PCBs for a Z80 computer using designs from allthingsmicro.com Unfortunately their website has been down and there seems to be no way of contacting them. I have had ...
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2 answers
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How can z80 uses a 4-bit ALU and return results in a single clock cycle?

According to Ken Sheriff's blog post Z80 ALU is 4-bits wide. If we take a look at page 8 and 9 of Z80 User Guide we will find the following information: Clock states T3 and T4 of a fetch cycle are ...
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1 answer
316 views

8bit: Gate array or dedicated chip?

I'm designing a Zilog Z80A-based computer for fun and profit, limiting myself with what components were available around year 1984+/-3, and trying to get the lowest possible cost that could be ...
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Z80 Binary Front Panel Design

Hello fellow engineers, I need some design design help with a Z80 based computer I am building. It will feature a binary Front Panel. The following functions may be performed via the front ...
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2 answers
658 views

Memory design with 4 256K X 1 Bit IC's to a Z80

I have a couple of memory IC's lying around (16 of them) which I would like to use in my homebrew Z80 project. These IC's were produced by Samsung in 1988, the KM41256A 256K x 1 bit. I realise I can't ...
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1 answer
2k views

Can a Z80 be powered at 3.3v and still run reliably at 3-4MHz?

I have various Z80 CPU's in my parts bin. Some are from Zilog and some are from other companies (IIRC). Some of them are hard to see the part numbers so I can't tell who made them but I'm sure they ...
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3 votes
3 answers
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How was the Zero Flag implemented on Z80 ALU?

Z80 was a popular 8-bit processor with a 4-bit ALU. Implementing a zero flag for a register should be straight forward, it would be a logical NOR of all the bits ...
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1 vote
1 answer
837 views

Z80 control pins not reading a defined logic level

I'm having trouble getting my simple Z80 based project to work. I asked for help here last week, and received an excellent response on troubleshooting the Z80; here is the relevant part: There ...
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3 votes
1 answer
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Z80 Simple Computer Troubleshooting: IORQ pin not going low

I'm following the instructions at http://www.reocities.com/SiliconValley/2072/z80proj1.htm to build a "proof of concept" Z80-based computer. I have wired everything on my breadboard, and programmed ...
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Z80 Instructions with accumulator as both operands?

Currently I am reading up on some documentation on the Zilog Z80 instruction set. There are a few instructions that has one operand that involves the Accumulator, which is denoted as an 'A' in the ...
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4 votes
2 answers
4k views

What components are needed for a 20MHz square wave clock?

I would like to create a 20Mhz clock that generates a square wave for the home-brew Z80 computer I' making. I already have several 20MHz crystal oscillators but am at a loss when it comes to circuit ...
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3 votes
1 answer
633 views

How do I know if I fried my Z80?

I'm working on a free-running circuit like the one here: http://www.z80.info/z80test0.htm The only differences are that I'm using a 4049 clock circuit with a 1 uf cap, and I've added an extra LED at ...
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11 votes
4 answers
8k views

Confused about IN/OUT architecture of z80 chip

I am reading the manual, and it's a bit complex. I can't understand the machine code encoding for the life of me, but that's not my main concern ... my main concern is the address/data buses. I do ...
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6 votes
4 answers
2k views

Can I connect TTL gates directly to the address bus of a Z80 CPU?

I need to know when the 14 MSB bits from the address bus of a z80 CPU are high. To do so my first idea is to use 4 74LS08 ICs (TTL AND gates) reading those lines but I don't know the maximum amount of ...
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3 votes
4 answers
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Video Controller Design

I'm designing my own computer based on the Zilog Z80. It has 256KiB of static RAM with paging as well as a few megabytes of flash memory. Everything is going along pretty well until * BAM * I hit a ...
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24 votes
8 answers
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How do I build a computer with a Z80 microprocessor?

I've got the chip, so how would I add: a clock, RAM, hard drive (maybe EEPROM?), a screen (LCD graphical screen?), input method (keyboard, mouse) ?
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