2 layers290.3 × 119.1 mm
Keyboard
crankery/crank68
One day I thought to myself I want to build a computer with the simplest 8 bit CPU I can think of". That CPU is the Motorola M6800 and this is the computer.
162 partsMIT
2 layers100 × 100 mm
Math
crankery/crank68One day I thought to myself I want to build a computer with the simplest 8 bit CPU I can think of". That CPU is the Motorola M6800 and this is the computer.
- Size
- 100 × 100 mm
- Copper layers
- 2
- Thickness
- 1.6 mm
- Parts
- 26
- Nets
- 93
- Pads
- 485
- Tracks, vias
- 0, 0
- Pours
- 1
- Routed
- Not yet
- KiCad
- KiCad 9.0 (file 20241229)
- Created
- 14 Mar 2026
- Updated
- 3 Nov 2026
Overview
Math is an open-source printed circuit board by crankery, from the crankery/crank68 repository on GitHub, published under the MIT licence. It is a 2-layer board of 100 × 100 mm with 26 parts on 93 nets.
It is built around the 74LS00SC, 74LS139PC and 74LS85DC. Its parts include 5 capacitors, 4 ICs, 1 connector, 1 switch and 1 crystal.
Main components
3
74LS00SC
-0℃~+70℃ 15ns@5V,50pF 2 2.7V 2V 4 4.75V~5.25V 400uA 500mV 74LS 800mV 8mA NAND Gate - Logic Gates ROHSTexas InstrumentsLogic Gates
74LS139PC
PDIP-1622ns 74LS 8mA Decoder Single Supply PDIP-16 Signal Switches, Multiplexers, DecodersonsemiSignal Switches, Multiplexers, Decoders
74LS85DC
CDIP-16-55℃~+125℃ 10mA 24ns Four channels TTL CDIP-16 Comparators ROHSTexas InstrumentsComparators
README
README.mdC68
One day I thought to myself I want to build a computer with the simplest 8 bit CPU I can think of". That CPU is the Motorola M6800 and this is the computer.
It's going to be pretty useless but it's fun to play around with all of this.
Hardware
So, this isn't intended to be historically accurate or anything. It's a 1970s CPU but I'll use modern components where it makes things simpler. So, for whatever reason I thought it'd be nice to use things like the MC6850, MC6821 and other components that would have been available when the M6800 was being used in the industry. But really, I'm just trying to avoid using microcontrollers that would render the actual CPU hopelessly redundant.
One place I used a microcontroller is on the terminal board. It's just being used to produce DVI video and render text. Oh, it also does the keyboard matrix scan. It has its one job and that's to replace a glass terminal. So, since it's busy doing dumb terminal stuff it doesn't really matter that it's anacronistic... right?
The computer is split up across a bunch of 100x100mm boards. This is an attempt to reduce the cost of production as that's the maxiumum size the Chinese fabs will produce at bargain pricing. The one exception (thus far) is the actual keyboard; its dimensions are dictated by the keyboard layout.
I'm currently using some SPLDs as there were analogs of these back in the day. I am using them for decode logic for the most part but I'm trying to replace them with simple 74 series logic slowly.
Boards
As previously mentioned, the computer is built upon a set of cards. These cards are generally single function so there's a lot of them.
Backplane
The backplane is a simple board with a bunch of slots on it. Each board will plug into this board. The boards are setup to be chained together. The slots are 13.5mm apart giving about 12mm of component height for each board. If a board grows taller than this or has multiple layers, it will block the next slot. This is tall enough to allow for most common components though.
The left side of each board is allowed to host connectors. This allows for connecting I/O devices to individual boards such as parallel ports, serial ports, video output or sound output.
Status: C68BP.01 sent to production
Processor
This board hosts the Motorola M6800 CPU and the MC6875 clock generator along with an oscillator running at 4MHz. This will result in a 1MHz system clock.
Status: C68CPU.01 sent to production
Memory
Lower 48KB of RAM (in FRAM), 8KB ROM (EEPROM)
Status: C68MEM.01 sent to production
Banked Memory
1 MB of batery backed SRAM presented in two four KB windows in the main address space.
Status: Finalized design
Math
AMD AM9511 FPU
Status: design in progress
Sound
This is a sound board. I'm still trying to decide which chip(s) to use. The YM2149 OPL2, Philips SAA1099, GI AY-3-8910 SGM
Status: spitballing
IO
2xACIA MC6850, 2xPIA MC6821
Status: design completed
Time
A DS12885 RTC and a MC6850 PTC.
Status: design completed
Front Panel
blinkenlights and switches and buttons.
Status: in progress
Terminal
Using a Raspberry PI Pico with PicoDVI to generate the terminal's video and to perform keyboard scans. Uses an on-board MC6850 for communication to the host system.
Status: sent to production
Software
The software for this project includes some old stuff and some new stuff. The old stuff is being "translated" into a VASM 6800 standard syntax while the new stuff is being written that way.
Emulator
emu is a very basic M6800 emulator and is being used to verify software development. It isn't complete yet and will halt when it encounters an unimplemented instruction.
It is designed to match the actual computer's IO and memory access and will do the minimum to make things appear as they will in reality. So, the ACIA status and control registers set the flags appropriately while the actual rate of transfer isn't worried about.
New Stuff
C68 BIOS
Old Stuff
MIKBUG2
Micro Basic
Tools
Everything is being done using open source tools. This includes, but is not limited to:
- kicad
- vasm
- vlink
- galasm
- minipro
- GCC
Parts
26| Qty | Value | Footprint | References |
|---|---|---|---|
| 5 | 100nF | 0-Footprints:C_Disc_D5.0mm_W2.5mm_P5.00mm | C1, C2, C3, C4, C5 |
| 3 | Grid | 0-Footprints:Grid | QC1, QC2, QC3 |
| 1 | 10K | 0-Footprints:R_Array_SIP5 | RN1 |
| 1 | 2.000MHz | 0-Footprints:Oscillator_DIP-14 | Y1 |
| 1 | 74LS00 | 0-Footprints:DIP-14_W7.62mm | U3 |
| 1 | 74LS139 | 0-Footprints:DIP-16_W7.62mm | U4 |
| 1 | 74LS85 | 0-Footprints:DIP-16_W7.62mm | U2 |
| 1 | AM9511A | 0-Footprints:DIP-24_W15.24mm | U1 |
| 1 | Buss | 0-Footprints:Card | J1 |
| 1 | Logo | 0-Footprints:Crank | G1 |
| 1 | Logo | 0-Footprints:crankery_small | G2 |
| 1 | Slot | 0-Footprints:SW_DIP_SPSTx04_Slide_9.78x12.34mm_W7.62mm_P2.54mm | SW1 |
| 1 | ~{CS} AM9511 | 0-Footprints:TestPoint_THTPad_D1.5mm_Drill0.7mm | TP6 |
| 1 | ~{EACK} | 0-Footprints:TestPoint_THTPad_D1.5mm_Drill0.7mm | TP1 |
| 1 | ~{EACK} | 0-Footprints:SolderJumper-2_P1.3mm_Bridged_RoundedPad1.0x1.5mm | JP1 |
| 1 | ~{END} | 0-Footprints:TestPoint_THTPad_D1.5mm_Drill0.7mm | TP3 |
| 1 | ~{PAUSE} | 0-Footprints:TestPoint_THTPad_D1.5mm_Drill0.7mm | TP4 |
| 1 | ~{SVACK} | 0-Footprints:TestPoint_THTPad_D1.5mm_Drill0.7mm | TP2 |
| 1 | ~{SVACK} | 0-Footprints:SolderJumper-2_P1.3mm_Bridged_RoundedPad1.0x1.5mm | JP2 |
| 1 | ~{SVREQ} | 0-Footprints:TestPoint_THTPad_D1.5mm_Drill0.7mm | TP5 |
Questions
How big is the Math PCB?
It measures 100 × 100 mm and has 2 copper layers, 1.6 mm thick.
What parts does the Math use?
26 parts, built around the 74LS00SC, 74LS139PC and 74LS85DC. The full list of its parts is on this page, with links to each part's symbol and footprint where PCBKit's library has them.
Where can I download the Math KiCad files?
From the crankery/crank68 repository on GitHub, at the commit this page was made from: the board is Math/Math.kicad_pcb. PCBKit opens the board in the browser, with nothing to install.
Can I use the Math design in my own project?
Its repository publishes it under the MIT licence: you may reuse it on that licence's terms (keep the author's notice, and for a copyleft licence share your changes alike).
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crankery/crank68
One day I thought to myself I want to build a computer with the simplest 8 bit CPU I can think of". That CPU is the Motorola M6800 and this is the computer.
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RTC
crankery/crank68
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Time
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