Multicomper

This is a hobby project and comes with no official support. The PCB schematic and board design files are offered 'as is' with no claims made about their fitness for any specific purpose.

A motherboard for the EP2C5T144C8N Cyclone II dev board and 'Multicomp'-based FPGA computers with Z80, 6502 and 6809 soft cores.

The Multicomper motherboard is a simple platform for creating a UK101 functional or 'Multicomp' computer based on the design work of Grant Searle:

http://searle.wales/uk101/uk101.html

http://searle.wales/Multicomp/index.html

Full schematics and PCB files here: https://easyeda.com/linker3000/multicomp-cyclone-ii

The main additional part needed is a low-cost Cyclone II development board from your favourite auction site or other supplier - search for EP2C5T144C8N mini board, but do read the notes below BEFORE ordering.

The Multicomper has the following features:

A prototyping area.
All spare FPGA pins are broken out to headers so you can use them for your own purposes.
A ‘key’ (reset) button in parallel with the one on the FPGA board.
An optional regulated 5V power circuit in case you need it.
A micro-SD card slot – See notes below.
Two serial port headers (NB: 3.3V logic like all the I/O).
VGA and PS/2 keyboard ports.
Composite video header.
A second SPI port with its own CS header pin (link it to the FPGA pin of your choice).

Notes:

FPGA development board – IMPORTANT

Stand by with the soldering iron! You should purchase a version of the board that does NOT have the header pins already in place because they need to be fitted on the underside as shown below:

On the dev board, FPGA pins 26 and 81 are directly connected to 1.2V by 0 ohm resistors (which you can remove if you want to use these pins for your own purposes). The default device setup in the Quartus software links all unused pins to GND - which means 26 and 81 can be shorted to ground; this makes the FPGA and the 1.2V regulator get very hot! Once you have loaded a project for the board, go to Assignments...Device...Device and Pin Options...Unused Pins and set this to "As input tri-stated with weak pull-up".

Also make sure that the internal pullup resistor is enabled on the reset ('key') tact switch pin: In the Quartus software, with a design project open, go to Assignments...Pin Planner, right click in any field and select Customize Columns. Add Weak Pull-Up Resistor to the columns to show and click OK. Next make sure that the Weak Pull-Up Resistor is enabled for n_reset, which is pin 144.

You will need a programming tool for the development board. Many suppliers will also sell you a cheap 'USB Blaster' compatible device and cable for this purpose. There's plenty of advice 'out there' for using a USB Blaster; the main thing to remember is that you can program the board in JTAG mode where the programming is lost when the board is powered down (ideal for testing), or AS mode where the programming persists until changed or erased. The programming mode determines which of the dev board connectors you use with the USB Blaster - check the board connector markings for which is which.

Quartus development environment

To view/modify the code and to program the dev board, you'll need to install the Quartus package from Intel; it's a big download, but at least it's free! The last version to support the Cyclone II FPGA fitted to this dev board is Altera Quartus II 13 sp1 - go seek out the Web Edition which, at the time of writing is here: http://fpgasoftware.intel.com/13.0sp1/?edition=web

5V power circuit

The optional 5V regulated power circuit is there in case you need it for the prototyping area(*) If you want to fit this circuit you need to solder a 2-pin male header to the FPGA dev board – the board has holes for this (see the pic above - look to the left of the DC power connector), but the header is not usually supplied as standard so you'll have to provide your own.

The 100uF 10V smoothing capacitor C8 should be bent over and fitted flush with the PCB otherwise it will hit the bottom of the dev board above it.

The Cyclone II is a 3.3V device and the I/O is NOT 5V tolerant so you will need to ‘level shift’ or clamp the voltages if you are hooking up to stuff such as LSTTL/74HCT logic.

The Multicomper’s 5V regulator is fed from the 2-pin header mentioned above. The main supply to the dev board's DC socket needs to be increased to at least 7V if a standard (non low drop-out) ‘7805’ is used on the Multicomper. During testing, the DC input voltage was raised to 12V to see if the 3.3V regulator complained - it was OK on two boards, but your mileage may vary according to the quality of the part fitted.

(*)You could use a 3.3V regulator for the prototype area if you wish - or just jumper in-out where the voltage regulator should be fitted and then the voltage at the '5V' header will be whatever's being fed to the FPGA board.

Micro SD card

Grant’s VHDL code does not support SDHC micro-SD cards so if you want to use one for your Multicomper build (i.e.: You want to run more than the basic UK101 configuration), replace the sd_controller.vhd file in the Multicomp project's sub-folder Components/SDCARD with the one from the following project: https://www.retrobrewcomputers.org/doku.php?id=builderpages:rhkoolstar:mc-2g-1024. To do this, download and unzip the ZIP file on the above page then locate and copy the file mentioned.

The easiest way to solder the micro SD card socket to the board is to use a fine soldering iron tip - I use an 0.5mm chisel bit (an Antex 55 bit on a trusty XS25 iron). Position the socket so that the connections and shield ground tabs are correctly aligned with the board pads, then use the soldering iron to tack down one of the tabs at the edge of the board while holding the socket in place. Check the alignment of everything again, melting the solder & repositioning the socket as necessary. Once everything lines up, solder the electrical connections and then the remaining shield tabs. Once done, check the connections for solder bridges, loose solder splashes and dry joints.

Keyboard

As per the original Multicomp designs by Grant, the keyboard is powered from the dev board's 3.3V supply rather than the normal PC standard of 5V. This works fine with many (but not all!) keyboards, including USB ones with a PS/2 adapter.

If you need to modify the Multicomper to run the keyboard from the optional 5V supply rail, you will need to clamp the clk and data lines to 3.3V with a pair of reverse-biased zener diodes because the circuitry inside a standard keyboard has pullups to the power rail - which you're now running at 5V and, remember, the FPGA is not 5V tolerant.