A useful Z80 computer in a project box

Saturday, 14th August 2010

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.

Status LEDs 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. I'll probably invest in the finest grade wet-and-dry sandpaper I can find and have a go with that followed with a Brasso polish, and if that doesn't help (or makes it worse) just sand the whole thing down and paint it.

Pin header connector

The circuit board inside the case needs to be attached to the case-mounted components somehow. In simpler projects I've resorted to soldering these connectors directly to the board, but this can make maintenance a problem (to remove the circuit board one would have to cut and resolder the wires). For this project I've left pin header strips on the board. The external connectors have leads soldered to them terminated with pin headers cut to size using some wire cutters and a rotary tool to polish them off; these headers are pictured above.

Circuit board mounted inside the case

The main circuit board can then be easily installed or removed from the case as required. The small circuit board for the video display controller is connected to the main circuit board in the same way.

Z80 and SRAM pin numbers marked

A Z80 computer can't live up to its name without some sort of a Z80 inside it, so I thought that that was the most obvious part to add next. Computers also generally need access to memory so I decided to add the 128KB SRAM chip at the same time. The Z80 communicates with the memory over an eight-bit data bus, a sixteen-bit address bus (to indicate which address in memory it is reading from or writing to) and a number of control lines (to indicate whether the current operation is a memory read or a memory write, for example). This provides a fairly tedious amount of soldering work; each pin on the memory needs to be connected to the corresponding pin on the Z80. To aid in the construction I stuck masking tape to the bottom of the perfboard around the outline of where the two chips would go and wrote the pin numbers onto the tape, shown in the photograph above.

Z80 and SRAM address, data and control buses

I put the two chips close together so I could put all of the bus wires on the inside of the IC holders rather than going around the outside. This saves a bit of space and avoids having to route the wires around the chip holders which gets a little untidy. The above photograph shows all of the wires in place before the chip holders were soldered in. Adding those in should be a quick and easy job, at least...

SRAM IC socket soldered in the wrong way around

Well, you'd have thought so, but somehow I managed to solder in the 32-pin SRAM socket the wrong way around. Each socket has a notch to help you align the chip using its corresponding notch. As you can see in the above photo the notch points right when it should point left like all of the other sockets. It wouldn't affect the operation of the circuit (as long as the SRAM chip was inserted with the notch to the left) but it looks untidy and I may as well do the job properly.

SRAM IC socket soldered in the correct way around

On the positive side I suppose I got to practice my desoldering skills.

Z80 and AVR data bus connections

The computer design uses an AVR microcontroller to manage the I/O devices (such as the keyboard, video display controller and SD card) and to load the OS into the Z80's memory on reset. To achieve this the Z80 and the AVR need to be connected together. The above photograph shows some new wires between the AVR (bottom left) and Z80 (bottom middle) to connect the Z80's data bus to the AVR's PORTA and a number of other wires to connect the Z80's control lines to several other I/O pins on the AVR. A number of pull-up resistors have been added to control lines on the Z80 so that when nothing is driving the control bus they rise high (the de-asserted state). If left disconnected ("floating") the other components connected to the control bus may think these lines had gone low (asserted) and treat that as a read or write operation, corrupting data.

I/O expanders Soldering detail of the I/O expanders

The AVR also needs to be connected to the Z80's address bus. This would take another sixteen pins if driven directly by the AVR; sixteen pins that aren't available to me! I am therefore using two MCP23S08 eight-bit I/O expanders, pictured above, to drive the address bus from the AVR. These are controlled over the SPI bus, which only takes up three pins on the AVR (these pins are shared with other SPI peripherals, such as the SD card) plus a single chip select pin that is unique to the I/O expanders. Four pins is better than sixteen, at any rate.

All ICs to date installed Computer in its project box

I keep mentioning chips even though the sockets are quite clearly empty in the above photographs. As I was approaching a useful computer circuit at this point I plugged all of the chips into their sockets to test the connections. As there was no SD card, real-time clock or keyboard I had to modify the boot loader on the AVR quite considerably; I started with a test program that wrote random data to blocks of memory then read them back to verify that they had written correctly. Once I had verified that the AVR was able to access memory correctly I reprogrammed it to copy a small Z80 program to memory and then let the Z80 take over. This Z80 program repeatedly output the string 'Z80' to the console output port. With everything plugged in I switched on the computer and saw the screen fill with Z80Z80Z80… so I was pretty certain that I'd wired everything up correctly!

DS1307 and battery clip

At this point I could start reintroducing the various peripherals to the computer. A DS1307 is used as a real-time clock. This clock needs to keep running when the computer is switched off, so I've added a 3V battery connector to the computer to keep it ticking.

SD card slot

As the computer uses a 512MB SD card for storage, I have added a pin socket strip to the board to plug in the SD card slot I scavenged from a card reader. The card is connected to the SPI bus along with the I/O expanders used to drive the Z80 address bus. SD cards run at 3.3V rather than the 5V that nearly everything else on the board uses so I've used a series of voltage dividers to drop the voltage on each input pin from 5V to around 3V (the resistor values I have don't allow me to get to 3.3V; 3V is the closest I can manage without going over 3.3V). The video display controller board also runs on 3.3V so I do at least have a suitable voltage supply for the card!

Keyboard connector

The final part of the computer that was on the breadboard prototype but not yet in the final build was the keyboard connector. This is simply a four pin header on the board that is connected to the PS/2 port screwed to the case. However, when I tried to use the computer, the keyboard didn't appear to work. Pressing Num Lock, Caps Lock or Scroll Lock would toggle the associated LED and hitting Ctrl+Alt+Del would reboot the computer but no other key worked. This implied that the AVR was handling the keyboard correctly but the Z80 wasn't receiving any notification of key presses. A bit of digging identified the problem; I'd forgotten to connect the Z80's interrupt pin to the AVR! When a key is pressed the AVR triggers an interrupt to let the Z80 know that a key is available. By soldering a wire between the two chips it started working as intended.

Z80 computer in its enclosure

The computer is now up to the same standard as it was when assembled on the breadboard, but is much more practical to work on. I hope to add a serial and parallel port to the computer soon, and would like to mount an LCD into the lid of the project box, but for the time being I am happy that I have managed to get this far.

Z80 computer running VEDIT

One of the advantages of running CP/M on the computer rather than my own operating system is the availability of existing software. The above photograph shows the computer running VEDIT, which is an excellent visual text editor.

VEDIT for CP/M

Zork for CP/M

With the hardware in a decent configuration I can start writing my own software. I think the first CP/M program I'll write is a graphical analogue clock, as this is the sort of program that can be left running for long periods as a way to check the stability of the computer.

Mounting circuit boards and rear panel connectors

Monday, 9th August 2010

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.

Project box outside Project box inside

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).

Perfboard shown inside the computer.

The first challenge was how I intended to mount the circuit board within the box. The perfboard I will use for the main computer circuit doesn't fit the marked mounting posts on the bottom of the project box; it's too narrow and too deep. What the photo doesn't show very well is that the perfboard is not able to lie flat in the box due to the curve at the rear of the box. To raise the board above the bottom of the box I decided to use four PCB spacers, which required two new holes to be drilled into the perfboard away from its corners.

Two new holes for PCB spacers Underside of the perfboard showing PCB spacers

I decided that the video display controller, which resides on its own board, should be mounted on the main circuit board using PCB spacers too.

Holes drilled to support the VDC VDC mounted on the main circuit board

This required four more holes to be drilled into the main circuit board. I tried to align the small video display board so that its 16-way pin socket for connection to the LCD was as close to the horizontal centre as possible.

Holes drilled to support the main circuit board Using the main circuit board to find the position of all of the screw holes

The base of the project box needed to have four holes drilled into it to support the main circuit board. Once the two nearest the front edge had been drilled, I screwed the circuit board to the back of the project box to mark the position for the other two holes to ensure that they lined up exactly with the holes drilled in the circuit board.

Both circuit boards mounted inside the box

Screws come through the bottom of the project box to hold the main circuit board in place. Some sticky foam feet are provided with the project box which will raise it off the surface it is resting on to prevent these four screws from leaving scratches! Due to the curve at the back of the box the circuit board is only a few millimetres above its surface, which is why I reversed the screws holding the video display board to leave the long threaded ends pointing upwards.

Power supply Power supply soldering detail

As working on the enclosure is a fairly noisy activity I switched my attention to the electronics for a brief spell. The first part of the circuit I assembled was the power supply; this just uses a pair of voltage regulators to provide 5V and 3.3V from an external power supply (I use a cheap wall wart affair rated at 7.5V DC).

Oscillator

I decided that the next part to tackle would be the oscillator. This uses a 20MHz crystal and a 74LS04 according to the design on z80.info to generate a 20MHz clock signal which will be further divided by two to produce a 10MHz clock signal for the Z80. I had some real problems with this design; it would run at 20MHz until I attached a load to it, at which point it would generate a fairly random-looking signal or stop oscillating entirely. I experimented with a few different capacitors and found that if I remove the 120pF capacitor and replace it with a 33pF capacitor on the other end of the crystal it works reliably. I'm not entirely sure why this is, but it's the design I've been using for a while with the computer on a breadboard so I'm happy to keep it this way for the time being.

ATmega644P

I added a D flip-flop to divide the 20MHz clock to 10MHz and then added the ATmega644P microcontroller to the board. This has a jumper next to its clock input allowing for the selection of either 20MHz or 10MHz operation; a pin header to the left of this jumper allows for it to be programmed in-circuit.

VDC reinstalled in the case

With those new parts in place I reinstated the video display board to check that everything still fit. My main concern now was how far the connectors screwed into the rear of the case would intrude and whether there'd be any problems with them getting in the way of the circuit boards.

Rear panel marked for mounting connectors

I sketched a design of how I saw the connectors would fit on the back of the case and then copied the layout to some masking tape stuck to the case. The computer naturally needs a power supply and keyboard input, and the video display board accounts for the VGA connector and an RCA connector for composite video (which I neglected to mark). I also hope to include a serial port and a parallel port in the final design (though neither are currently supported by the software) so left space for those two connectors.

Hole drilled for the keyboard connector Keyboard connector mounted in the case

The 6-way mini-DIN connector for the keyboard is the deepest one to contend with so I decided to start with it. I cut the hole in the case by drilling a small hole in the plastic which I then enlarged with a burr tool to the correct shape and size.

Keyboard connector screwed in

Fortunately it looks like there's plenty of room in the case for connectors!

Connectors for the serial port, composite video output and DC input Inside view of the case with some more connectors installed

The next few connectors confirm this. I really do not enjoy cutting the holes for D-sub connectors (such as the one for the serial port); they don't have much of a metal lip to hide a botched hole, so I have to cut very slowly and very carefully, taking a very long time to slowly enlarge each hole until the connector fits. I'm therefore not really sure why I decided to have three D-sub connectors in this computer design; maybe I'm just a glutton for punishment.

Completed rear panel Rear panel as seen from the inside of the case

Finally, the rear of the case is completed. I will leave the masking tape on there as scratch protection until I have finished the front of the case (this will be significantly simpler — just a power switch, power LED and disk activity LED). Once that is done I can resume working on the electronics!

Integrating the dsPIC33 VDC with the Z80 computer

Saturday, 31st July 2010

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.

Z80 computer with dsPIC33 VDC

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).

At the top of the photo is the video display controller, connected to a 320×240 graphical LCD. A pin header is used to connect this VDC board to the rest of the computer. Three pins are required for power; 0V, 3.3V (dsPIC33 and output buffer) and 5V (LCD). The VDC is connected to the computer's ATmega644P I/O controller using the two-wire I2C bus (the same bus that is used to access the DS1307 clock). Rather than run a series of graphical demos, the VDC now waits for commands to be written to the I2C slave address 0xEE which it acts on to control what is shown on the screen. I'm aiming for these commands to work in the roughly same way as they did on the BBC Micro VDU, which should make porting the enhanced TI-83+ version of BBC BASIC to this computer a bit easier. The BBC Micro's VDU could be accessed by calling OSWRCH (assuming it was being used as the current output stream), which typically has an address of &FFEE — hence my choice of 0xEE as the I2C slave address!

Detail of the LCD connected to the VDC

A handful of these VDU commands have been implemented, which is sufficient to run simple CP/M software. The generic CP/M version of BBC BASIC does not, naturally, support any hardware-specific features and as such lacks advanced text or drawing support (one can send commands directly to the output stream with the VDU statement but this isn't very user-friendly). I will need to work on this now that the hardware is coming together! The current VDC code can be downloaded here if you are interested in the changes that have been made.

The above photo shows the newly constructed VDC hardware. All of my previous projects have been assembled on stripboard; as the projects have become more complex or simply smaller I've found stripboard to be increasingly awkward to work with. ICs can only really be orientated in one direction, and to reduce the size of circuits I've had to start cutting the tracks between holes (rather than the usual method which is to drill out an entire hole). The supplier I normally acquire parts from, Bitsbox, recently added three different sizes of perfboard to their catalogue so I thought I'd give it a go. I've found it much more pleasant to work with than stripboard, though not as easy to correct if you make a mistake and need to desolder a connection. You can certainly perform some interesting space-saving tricks on the underside of the board!

The underside of the VDC showing the soldering technique

The Kynar insulation on the wire I switched to using also has the advantage of not melting when heated with a soldering iron, as I've had problems in previous projects where tightly-spaced wires will end up getting shorted together as the insulation between them melts.

I have mentioned that one pin header is used to connect the VDC to the computer. There are three others on the board; the two-pin one is for the composite video output, the six-pin one is for connection to a PICkit to reprogram the dsPIC and the four-pin one for the VGA output.

Detail of VGA output from the VDC

Now that I have moved the VDC onto a permanent circuit board I feel that I can start moving the rest of the computer in the same direction. The software is far from complete and the hardware is pretty rudimentary but it does basically work and having a more robust system to work on should make life a bit easier.

VGA output for the dsPIC33 VDC

Sunday, 25th July 2010

I have spent quite a while working on different projects that generate PAL video signals in software. This may seem a bit odd if you consider the fact that I don't own a TV, so tend to rely on a video capture card or VGA box to see the output of these projects on a computer monitor — something I do have a fair number of.

This reliance on another piece of technology between my project and the display device is not something I'm too keen on, so have spent some time adding native 640×480 60Hz VGA output to my dsPIC33 video display controller.

VGA monitor showing the output of the dsPIC33 VDC

Another advantage of using a VGA monitor directly is that individual pixels are shown very crisply, unlike my video capture card or VGA box which tend to blur the image horizontally. This is shown in the zoomed in part of the above photo.

Generating a video signal for a VGA monitor is easier than generating a composite video signal for a PAL TV, as there are distinct pins for the image data, horizontal sync and vertical sync. One problem I did have, however, is with the length of the vertical sync pulse. I started with a very brief pulse (the same duration as the horizontal sync pulse) which worked fine with my old analogue CRT monitors but didn't work at all with my modern LCD monitor. The documentation I was using for timing information indicated that there were "two scanlines" for vertical sync so I extended the pulse to last for those two frames, which worked on the LCD but didn't on the CRTs. My final compromise has been to assert the vertical sync pin for the duration of a single scanline, which seems to work on all of my monitors.

dsPIC33 VDC on a breadboard

When connected to a TV two microcontroller pins are used to drive a single load (composite input). When connected to a VGA monitor, however, a single microcontroller pin is used to drive three loads (red, green and blue inputs). I thought it prudent to check the datasheet for the dsPIC before connecting this increased load to the output pin where I was surprised to discover that the maximum source or sink current for each output pin is a measly 4mA — not even enough to drive an LED! I have added a buffer to each video output pin to protect the dsPIC — any buffer capable of sourcing up to 30mA or so should be sufficient (I'm using a 74F125, which can be seen in the bottom right of the above photo). I had previously been occasionally using the video output pins as inputs to check if there is a load on the output or not (such a load would indicate whether a TV or VGA monitor is plugged in or not) but I can no longer do this with the external buffer IC so have had to revise the circuit somewhat. Updated source code featuring the new VGA output code and an accompanying schematic are available for those who are interested!

Text and filled shapes for the dsPIC33 VDC

Thursday, 22nd July 2010

The dsPIC33 video display controller project I am working on needs to support several common text output and drawing operations offered by existing BBC BASIC implementations. The previous demo included basic point, line and circle outlining functions, but I also need to output text and outline (or fill) rectangles, circles, ellipses and triangles. On top of that the drawing operations need to support multiple colours and plotting modes. Owing to processing power and memory limitations the output is black and white only but different "shades" can be implemented with dither patterns. The plotting modes allow you to perform logical operations between what you are drawing and what's currently on the buffer — for example, you could fill a circle that is logically ORed with the existing background or draw a line that inverts every pixel along its length rather than applying the new colour.

dsPIC33 VDC text output demo
Filled rectangles and text output produce the above image.

Finding suitable algorithms for some of these routines has been a little tricky at times. Due to the way that filled shapes can be set to invert (rather than overwrite) what's on the background there has to be zero overdraw and the outline of filled triangles should exactly match the outline of a triangle drawn by plotting a line between its three vertices; this makes combining triangles to form more complex shapes possible, as you can guarantee that the overlap between the two shared vertices of a pair of triangles covers the same pixels as a line drawn between those two vertices.

dsPIC33 VDC spinning cube demo
Filled triangles produce a solid cube.

I ended up writing a program in C# that would plot a random triangle using the triangle filler I was attempting to write and then compare its outline to that of a triangle drawn by plotting lines between the three vertices. The final code is chock full of special cases and workarounds but has been tested against hundreds of thousands of random triangles and seems to be working!

Due to a shortage of memory there is only a single frame buffer, which (naturally) means there is no double-buffering and hence smooth animation becomes a little tricky. When connected to a TV one can take advantage of the vertical blanking period to update the buffer (this is a period below and above the active display where you only need to feed sync signals, not image data, to the TV) and still get decent effects as long as you don't try to do too much. The LCD has no such vertical blanking period and so some of the demos look rather flickery.

I have captured a video of the output of the circuit when running the demo which can be seen above. The horizontal grey lines are a limitation of my video capture card; these lines appear correctly as alternating black and white pixels on a real TV set! You can download the code for this demo from my site along with a PDF of the schematic. As this is a work in progress I'm sure there are plenty of bugs left to squash but I think it's getting there, slowly but surely!

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