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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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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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55 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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110 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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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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122 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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166 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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1answer
118 views

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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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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258 views

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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2answers
198 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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3answers
398 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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1answer
248 views

How to check that Z80 is dead

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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2answers
188 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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1answer
126 views

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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3answers
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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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2answers
230 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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1answer
205 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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1answer
692 views

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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1answer
246 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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2answers
696 views

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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2answers
481 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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847 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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3answers
2k views

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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1answer
513 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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1answer
1k views

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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1answer
359 views

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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2answers
3k 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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1answer
442 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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4answers
5k 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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4answers
1k 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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4answers
819 views

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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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) ?