Articles tagged “Z80 computer”
Adding 11KB of RAM to a CP/M 3 system with a single NAND gate chip
It's been quite a while since I posted about my Z80 Computer project. This is a home-made Z80 computer I built back in 2010 that features a 10MHz Z80 CPU with 64KB RAM that runs CP/M 3. It can drive an internal LCD, TV or VGA monitor at 320x240 (monochrome only) and unfortunately is a project I was never too happy with due to several compromises I had to make in its design – though at the time I was happy enough I got it to work at all! The video output was limited by both my choice to use an internal graphical LCD and the limitations of the dsPIC33F I chose to use to drive it and the software was all a bit half-baked.
A parallel port and a demonstration of the Z80 computer
The last piece of hardware to add to the computer was a parallel port. These have eight data lines and nine assorted control and status lines. My last two 8-bit I/O expanders provide sixteen of these seventeen lines, and the final one was provided by the DS1307 real-time clock chip which happily has a spare pin on it that can be used as an output. This parallel port can be used to print from the computer. Some software has printing capabilities built in (such as the text editor VEDIT Plus), but by pressing Ctrl+P in CP/M any text sent to the display will be simultaneously sent to the printer.
A clock and a serial port for the Z80 computer
At the end of the previous entry I mentioned that I was going to start developing my own programs for the Z80 computer. The first is a graphical clock, taking advantage of my implementation of the BBC Micro's VDU commands and the ability to use those commands to draw graphics onto the screen as well as text:
I have uploaded the code and binary to my site for anyone who is interested, though it will only work on a machine running CP/M 3 and that is equipped with a display that implements a handful of BBC Micro VDU commands.
A useful Z80 computer in a project box
Work continues on the Z80 computer. The two final modifications to the box itself are the holes for the status LEDs and the power switch. The green LED indicates power and the orange one disk activity. Unfortunately, the project box is fairly scratched on the outside (one scratch on the front is my own fault, but the sides and back were fairly scuffed and scratched when I bought it). If anyone has any tips for polishing scratches out of ABS I'd be glad to hear them; the usual household polishing abrasives (such as toothpaste) remove most of the light scuffs and result in a lovely mirror finish, but don't do anything to the deeper scratches.
Mounting circuit boards and rear panel connectors
One of the fun things about working with electronics is that you can end up with a physical product at the end of your hard work. To this end I have started moving my Z80 computer from its current breadboard to a more permanent enclosure. Large project boxes can be quite expensive (around £40, it seems), but the one I picked out was a slightly more reasonable £7. It's not the prettiest enclosure I've seen but it should be large enough to house the computer and provide space on the lid for the LCD and on the rear surface for a collection of connectors (as you'd expect to find on the rear of any computer).
Integrating the dsPIC33 VDC with the Z80 computer
The ultimate goal for the video display controller module I have been working on is to drive the display in my Z80 computer project. As I have now got a pretty good set of features I thought it would be a good idea to join the two projects together. The big board in the lower middle of the above photograph is the main body of the computer, including the Z80, its RAM, the ATmega644P that is used to handle I/O, an SD card for storage and a DS1307 real-time clock. The small board in the bottom left of the photo is the power supply (supplying both 5V and 3.3V) and clock generator (providing a 20MHz and 10MHz clock).
Comments dsPIC33 VDC dsPIC33FJ128GP802 Electronics PIC Z80 Z80 computer
Booting CP/M 3 from an SD card
Up to this point I have been running CP/M 2.2 on the Z80 computer. CP/M 3 adds a number of useful features, including the following:
Support for more than 64KB RAM via banked memory. Standardised access to real-time clock for file date and time stamping.Improved text entry on the command-line when using the memory-banked version, such as the ability to move the cursor when editing and recall the previously entered line.Support for disks with physical sectors larger than the default record size of 128 bytes.
Comments ATmega644P AVR CP/M DS1307 Electronics SD card Z80 Z80 computer
Keyboard input and RAM disks make CP/M more useful
The hardware for the computer has changed in (mostly) subtle ways since the last post, with the exception of a PS/2 socket for connection to a keyboard. PS/2 keyboards (which use the same protocol as the older AT keyboard) communicate with the host by clocking data in either direction (keyboard to host or host to keyboard) over two wires, appropriately named "clock" and "data". An AVR pin change interrupt is used to detect a change in state of the clock line and either input or output a bit on the data line depending on the current direction of data transmission.
Comments ATmega644P AVR CP/M Electronics Keyboard Z80 Z80 computer
Combining a Z80 and an ATmega644P to boot CP/M
I've been working on a new Z80 computer over the last few days. I would say that I had been working on the existing Z80 computer were it not for the fact that this a completely new design. The previous computer had two 32KB RAM chips to provide a total of 64KB RAM. To run a user program you need to get it into RAM somehow, so I also included a 128KB ROM chip which occupied the lower 16KB of the Z80's address space to provide the fixed operating system that could be used to load programs. By adding memory banking hardware I could select one of eight 16KB pages of ROM.
Thinking about CP/M
It's been some time since I worked on my Z80 computer project, but the recent electronics projects I've completed have got me thinking about it again. Click to watch the video on YouTube I did record a video to demonstrate the basic parts of the computer and some of its flaws a few months ago, which can be seen above. However, I'm now thinking of a more radical redesign than fixing the I/O board's shortcomings. One of the reasons for my lack of motivation is that even if I did get something working I wouldn't have much software to run on it; it would be a lot of work to write software that only ran on that one particular machine.
Decoding SIRCS commands with a PIC16F84
Some time ago I was working on a simple Z80-based computer. It has a PS/2 keyboard and mouse port for user input, and these are implemented using a large number of discrete parts - transistor drivers with all manner of supporting latches and buffers. The AT protocol (which the PS/2 keyboard and mouse inherit) is entirely implemented in software by the Z80. On the one hand this design has a certain purity, but it ties the CPU up every time data is to be transferred. The keyboard sends data when it feels like it, so if you wished to perform some function based on a key press event you'd need to poll the port periodically, assuming that if communications time out there's no key waiting.
Z80 computer - Lines, cubes and inverted text
I've made a few additions to the operating system for the computer. The Console module, which handles text input and output, now supports "coloured" text - that is you can set the text foreground and background colours to either black or white. This functionality is exposed via the BBC BASIC COLOUR statement. If you pass a value between 0 and 127 this sets the foreground colour (0..63 is white, 64..127 is black) and if you pass a value between 128 and 255 this sets the background colour (128..191 is white, 192..255 is black).
Fixed and scaled CHIP-8/SCHIP interpreter
The CHIP-8/SCHIP interpreter now seems happy enough to run games, though the lack of settings to control how fast or slow they run makes things rather interesting. First of all, I've hacked together a painfully simple read-only file system. Each file is prefixed with a 13-byte header; 8 bytes for the filename (padded with spaces), 3 bytes for the extension (padded with spaces) and two bytes for the file size. The above file listing can be generated by typing *. at the BASIC prompt. I've written a new sprite drawing routine that scales sprites up to double size when in CHIP-8 mode; this allows CHIP-8 games to fill the entire screen.
64KB RAM and a CHIP-8/SCHIP interpreter
The only major hardware modification since last time is the addition of another 32KB SRAM. Click to toggle labels This appears as two 16KB pages in the $4000..$7FFF slot. Currently only the first page is used for OS variables and scratch space, freeing up the upper 32KB entirely for BBC BASIC's use. One other minor hardware addition is support for a dual-coloured LED on the control port. This LED will be used to signify file access - reads by a green LED and writes by a red LED. As such I haven't implemented a proper file system, but typing SAVE "FILE" or LOAD "FILE" at the prompt will transfer data between the Z80 RAM and a 24LC256 32KB EEPROM.
Times, backlights and off-page calls
Dates, times and backlights
I'm using a DS1307 real-time clock to provide the computer with real-time date and time functions. It's a great little chip - all it needs is power, two lines for I2C communications, a 32768Hz crystal between two pins and a back-up battery to keep it ticking when main power is removed and it's happy. That accounts for seven pins; the last remaining pin can be used as a one-bit output (you can set it to a high or low state in software) or it can be configured to output a square wave at 1Hz, ~4kHz, ~8kHz or ~32kHz.
Bank-Switching Memory and I2C
Cheers for the comments. As EasilyConfused pointed out, I have done calculator programming in the past, which makes this much easier - learning Z80 assembly to program a calculator influenced the choice of CPU in this computer, and porting BBC BASIC to the calculator showed that with a minimal amount of code to sit between it and the hardware you'd have a decent operating system with very little work. And if a Terminator-related name is good enough for the UK military, it should be good enough for this project...
Running BBC BASIC on a home-built computer
This computer needs a name - I'd welcome any suggestions! I have built a circuit on another piece of stripboard that will handle memory, clock signal generation and the Z80 itself. A few posts ago I was wondering about how I'd partition memory. To date I've been using a very simple circuit where the lower 32KB of addressable memory is mapped to ROM and the upper 32KB is mapped to RAM. As my ROM chip is 128KB and I have two 32KB RAM chips, this seems a bit wasteful. The memory layout I'm now using is quite simple: the upper 32KB is still mapped to RAM.
2MHz should be enough for anyone
LCD Timing
Last time I discussed the hardware I mentioned I had LCD timing issues. I have finally resolved them, but this has been the most time consuming part of the project so far. The first thing to sort out was the LCD's E pin. Once you have set up the LCD's input pins to a state where they're ready to read or write data, you need to drive this pin high. I had had some success by holding it high permanently and relying on the Z80 to set all the other to the right state at roughly the same moment, but this was inaccurate and resulted in occasional display glitching.
Comments Electronics Emerson LCD PG12864LRS Z80 Z80 computer
Z80 computer with a primitive I/O board
A computer needs some way of interacting with the outside world via input and output devices. It's about time, then, that the Z80 computer project acquires a section dedicated to I/O. The Z80 differentiates between memory and I/O devices, though both share the data bus and the address bus. You can control I/O devices using the in (input) and out (output) instructions. When you input or output you must specify a device address and a value or target register. For example,
in a,($20) ; Read a value from device $20 and store it in A.
Z80 Light-flasher
Now armed with a flash programmer, I thought it about time to try and build a Z80-based system. Click for video (829KB WMV) Not much to look at, and it doesn't do much either. The large IC in the bottom-left, prominently marked Z, is the Z80 itself. To its left is a 555, generating a ~220Hz clock signal (yes, Hz, not MHz or even kHz). Above the Z80 is another large chip - this is the 128KB flash ROM. The eight parallel wires between them are the address bus - only A0 to A7 are connected. This only lets the Z80 address 256 bytes, but that should be enough for testing.
Emulators and neatened wiring
I've decided to switch to a regular 10MHz Z80 rather than a Z180, given the difficulty of using an SDIP 64. I now have a DIP 40 Z80 ready for use, but as I don't have the programmer for the Flash chip (which will hold the OS) there's not much I can do with it physically. I have therefore cobbled together a basic emulator to help develop some of the software beforehand. To cut hardware costs I'm going to try and handle input in software. One bit of hardware I'm planning on having is an eight-bit open collector I/O port.
Comments Electronics Emulation PICAXE PICAXE-28X1 Z80 Z80 computer
Experimenting with a 32KB RAM
The next component I thought I'd experiment with is the RAM. The project is an analogue recorder - a circuit that samples an analogue input periodically and saves it in RAM, and can be configured to play the recorded signal back afterwards. Yes, it says plating. A single RAM chip offers 32K with an eight-bit word size. This requires fifteen lines to address it, A0..A14. The PICAXE-28X1 that is to control the circuit does not have enough output pins to be able drive this address bus and a data bus (to transfer values to and from RAM) and a still have enough pins left over to control the various components.
Back to Hardware
I enjoy dabbling with low-level programming, but have never actually built a computer to run these programs. I think it's time to correct that, and as the BBC BASIC project has required me to develop an almost complete Z80 OS (the only thing that's left for the TI-OS to do is manage files) I thought a Z80 computer would be a good start. The planned specs are (as a starting point):
10 MHz Z80180 CPU;64KB RAM (2 32K×8 SRAM chips);128KB Flash ROM;Graphical LCD;Simple joypad input;Keyboard input (AT using either software AT routines or dedicated microcontroller).
Comments Electronics LCD PG12864LRS PICAXE PICAXE-28X1 Z80 computer