8 Bit Breadboard CPU

A home-brew 8 bit Microprocessor built on a breadboard
64K Address Space, IRQ, and DMA
16 Bit Stack Pointer, 4 8 bit Registers or 2 16 bit

Description

This will be an 8 bit CPU built to interface with a RAM, ROM, and I/O. It will be built out of standard TTL logic on solderless breadboards.

CPU Includes (current design):
- 64K address space (16 address lines).
-Two 16 bit Registers that can be used as 4 8 bit registers.
- 16 bit program counter.
- 16 bit stack pointer.
- 8 bit status register, high nibble used as instruction step counter (carry, zero, equal and negative)
- 8 bit Instruction register (256 possible opcodes)
- IRQ input
- DMA input
- Reset input
- Tri-state address and data line buffers
- AB, XY, PC, SP, and instruction step counter built out of up/down counters for quick incr / decr.

Details

I was doing research for a new hobby project. I wanted to build a 68000 system on a bread board with I/O and possible video. As I dug deeper I started shifting towards maybe starting with an 8 bit CPU (6809) and then do the 68000 after. I'm pretty rusty at hardware, haven't done anything serious since the early 90's.

While doing this research I discovered a brilliant You-tuber named Ben Eater https://www.youtube.com/channel/UCS0N5baNlQWJCUrhCEo8WlA . He is an excellent presenter and educator. I watched his series on building an 8 bit bread board computer. He goes through step by step building a functioning 8 bit computer out of TTL logic. I'm sure many of you might be familiar with his work.

I've never really had a good understanding of the internals of a CPU. I knew that it involved state machines and registers but didn't really grasp how it functioned. In his series he broke down the computer into easy to understand blocks. After watching his series I was inspired to try and build something of my own.

Instead of building a simple computer, I want to build a self contained CPU that could be connected to a complete system (RAM, ROM, and I/O). I will use RAM and some I/O to build it but design it so it can be separated. I want to have some more advanced features on my CPU while trying to avoid feature creep. I do want to actually finish.

I've ordered a bunch of Breadboards TTL chips, resistors, LED's, DIP Switches, EEPROM's, RAM chips, hookup wire (8 colours), capacitors, 555 timers, toggle switches, and some other bits. I found a good deal on the long (63 row) breadboards on amazon. 3 boards for $12CDN with free shipping. I just received 9 of them today and they are superb quality.

I can't wait to get the rest of the supplies so I can start tinkering.

I've drawn up a block diagram for the basic design.

https://docs.google.com/drawings/d/1BW84zwO5effvYcdKjxzyki9cuvuHsM98z6nAamw4iTM/edit?usp=sharing

I've also started an instruction list which also has a page that lists the full names for all the control lines shown in the block diagram. Like I said I'm pretty rusty and not up to speed on normal naming conventions.

https://docs.google.com/spreadsheets/d/1kbxYGDeIOszgvjPYRdes-bELOBlojT_tjxe_RD9MVUQ/edit?usp=sharing

I've started an upverter project where I will be posting all the schematics.

Idea's and advice are most welcome.

Files

8bit Breadboard CPU.circ

Someone asked for the .circ file. I'm not sure if it's fully working I haven't looked at it in a long time.

circ - 248.52 kB - 03/02/2021 at 23:03

Download

Project Logs

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No more loopy wires
2-Zons • 02/17/2019 at 20:47 • 4 comments

Finished routing all the control signals to a single board. Need to build a few more ribbon cables for the address bus, data bus connections and LED's. Looks much neater now. Modules completed thus far (from left to right in picture): RAM / ROM module, ALU, XY register, stack pointer, and AB register.
Added RAM and ROM module
2-Zons • 02/10/2019 at 04:24 • 2 comments

I'm back from my little vacation in Vegas and have been busy building. I bought a sheet of 1/4" acrylic to mount all the boards together. I was tired of trying to clip them together with wires or headers. It's working nicely. I wanted to start running all my control lines into their positions so I can get rid of all those big loopy wires. I couldn't do that until I had a place for them all to go (the empty board at the bottom). I have room for another column of breadboards on the left side of the data bus.
I finished the RAM / ROM module and tested it. I used a ZIF socket for the 32K ROM chip to make reprogramming quick. I have more of them for the microcode ROM's. The address bus is removed in the picture as well as the data bus connections to the RAM ROM module. The RAM / ROM module is technically "external" to the CPU and my DI control line would be the R/W output. The DI control line would control the tri-state buffers between the internal and external data bus.
Connectors and Glue (the sticky kind)
2-Zons • 01/25/2019 at 23:03 • 1 comment

I've been working on cleaning things up and building connectors to make things easier. I'm using headers and ribbon cable to replace a lot of the bundles of lose wires. I've also improved my power situation by using a 850W ATX power supply. 850 watts is total overkill but it's what I had lying around. I also finished building the stack pointer (it's on the top right) I've also built some bridges to hold the breadboards together and connect them using headers and hot glue. I've been using a lot of hot glue, truly magical stuff. I'm going on a trip for a few days so I will be taking a break from construction.
Combining Modules
2-Zons • 01/20/2019 at 03:51 • 3 comments

I've been working on combining the modules I have so far. It's been I little trickier than I'd hoped but It's coming along. I built a temporary data and address bus driver with LED readout. In my initial testing I just had the dip switches connected to the data inputs of the modules. This wouldn't let me test the modules output to the bus. So by separating the dip switches with their own bus driver I can now test what the modules put out on the data bus. I did the same with the address bus. I'w worried that all these dip switches are going to draw too much current though. I think if I make a front panel for the system I will use toggle switches that simply switch between VCC and GND.
On another note. Upverter sent me an email. They chose my project for a case study. They want to help me get it manufactured (PCB and components).
Status Register
2-Zons • 01/16/2019 at 22:48 • 0 comments

Finished this last night. This was more difficult than I expected. It's only a 4 bit register but it took quite a bit of translation from logisim to reality, trying to keep the chip count low and minimize unnecessary gate delays. My satus register (SR) has 4 flags: Equal EQ, negative NE, zero ZE, and carry CA. SR is set by the ALU during any ALU operation. It can also be set by loading a value from the data bus. The CA flag will affect subtraction or addition in the ALU. All 4 are used by the instruction decoder for conditional jumps. The SR can also be sent out to the data bus for saving it on the stack or for logical functions in the ALU. I was able to get by using only two 2 way bus driver IC's. One for both in from, and out to the data bus, and the other for buffering the flag outputs of the ALU. This is what made it tricky to make sure that the internal bus in the SR would never have more than one source based on the control signals: FL-O, Flags out to data bus; FL-I, flags in from data bus; and FL-O, store flags from ALU (an operation is taking place).
In the block diagram and in logisim SR is an 8 bit register, the first 4 bits being the instruction step counter. Since each 4 bits needs it's own IC (a counter and a D-flip flop), and they work completely independently of each other I will move the step counter to the same location as the instruction decoder. Keep the wire runs shorter. The step counter is never loaded from or sent to the data bus.
I've got enough modules completed to start fitting them together. Which is what I'm working on now
AB Register
2-Zons • 01/15/2019 at 21:14 • 1 comment

Finished the AB register. It's the same as the XY register minus 16 bit in and out to address bus. I'm getting much better at the wiring. For previous modules I would wire up just enough that I could test it, and then add a bit at a time testing as I went. This time I wired up the entire thing before testing, and only had one mistake when I fired it up (two bits crossed over on A). It was a quick to spot and quick to fix. My productivity should go up now. I've also received all of my orders so I have everything I need to complete the entire system. I dug up my label maker to tag the control lines.
I'm getting enough modules complete to start wiring them all together. I can better judge now how many boards it will take for the whole system. I wish I could buy the power rails separately. I will be needing quite a few for data and address bus.
ALU
2-Zons • 01/14/2019 at 02:36 • 1 comment

ALU is mostly complete and working. Using 2 x LS181's. Top right LED's show contents of input register tied to input B on 181's. Databus (Dipswitches) tied to input A. Blue LED is carry not. Bottom Right LED's show contents of output register. Showing A XOR B. All loopy wires are temporary for testing, as well as the 5 dipswitches on the board, they will be control signals when hooked up to the bus. Only need to implement zero, negative, and equal flags. I hope I can use the A=B output on the 181's and'ed but when I tested it in logisim it didn't work properly. I haven't tested on this setup yet.
Completed and Rebuilt XY Register
2-Zons • 01/12/2019 at 04:32 • 0 comments

I started over and completely rebuilt the XY register. I was having trouble with the bus drivers. I switched to 74HC245's from 74ACT241N's. The 241's were actually dual 4 bit buffers each with their own enable line (one active hi and one active lo). The enable lines couldn't be tied together and each 4 bits was output on opposite sides of the chip which made wiring really messy. I also got new wire delivered which is the same guage (22AWG) but has a thinner insulator and is much nicer to work with. I've tested out all the functions and they all work: X inc and dec; Y inc and dec; XY inc and dec; load XY from address bus; load X or Y from data bus; push X or Y to databus; push XY to address bus; and reset. Only thing left is some control signal logic which I am missing some OR gates to complete. I neglected to order them earlier. I think I'll build the ALU next.
XY Register
2-Zons • 01/08/2019 at 04:37 • 4 comments

Working on the XY register. This is the register with the most control signals and features. Separate hi, lo byte, and word increment and decrement. Hi or lo byte in from or out to databus. Word in from or out to Address bus (Address bus is used for 16 bit register to register transfers). The address bus side of in and out is finished and tested. Need to wire data bus in and out and lo byte to hi byte up down logic. I've already used over 40' of wire just for this one register. I'm going to need a lot more. The yellow loops are control signals tied hi or lo for testing. Dip switches are acting as the address bus.
Construction has started
2-Zons • 01/06/2019 at 00:18 • 3 comments

Finally received the delivery I was waiting for to start building something. This is the clock circuit. It's pretty much an exact copy of Ben Eater's 8 bit computer clock. Starting from the left: A 555 circuit to generate and adjustable square wave clock that can run as slow as 0.5 Hz. Second, another 555 circuit to debounce a pulse that will be fed into the output logic for single stepping. Third, a final 555 circuit to debounce the enable circuit that feeds into the output logic to select from the single step or clock. Red button is single step pulse, beside the toggle for the select. Next are two quad nand 7437 for the output logic. Second chip is not used yet. Will be used to generate inverse clock, and halt for DMA.
No need to comment on the crappy soldering job on my power input.

View all 24 project logs

Discussions

Andre Baptista wrote 03/17/2019 at 15:27

Great job! It has many similarlies with my breadboard computer.

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Mario wrote 01/11/2019 at 02:27

Nice and educative work! By adding TTL Analog to Digital Conversion (ADC) IC at the input and implementing DSP algorithms using e.g. fixed-point arithmetic (execute calculations with real numbers using integer arithmetic circuits) it might be possible to implement even DSP processors in TTL logic.

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Peabody1929 wrote 12/11/2018 at 17:34

Do you have a goal for the instruction cycle time? This could be the clock rate times the number of clocks it takes to execute an instruction.

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2-Zons wrote 12/11/2018 at 22:36

I have a 16 step instruction counter, so the max instruction will take is 16 cycles. I have a control line to reset it so I will be able to program each instruction to take the minimum amount of cycles. I haven't started the microcode yet but from what I'm thinking most 8 bit instructions will take 3 cycles.

As for the frequency of the clock: I will build a 555 circuit with a pot for initial construction. Nothing too fast, probably max about 50Khz. My plan is that once I've got the design to a certain point I will write a program to test every instruction and then see how fast i can get it to run without errors. I've got 20MHz crystals, if it handles that I would get faster ones. I would be happy If I could get it running at 10Mhz, but I'm totally new at this. We will see.

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Steve Toner wrote 12/08/2018 at 01:00

OK, just glancing at the register schematic, I see a couple of issues:

1. I wouldn't drive an LED directly from the output of an LS chip. There should be a current-limiting resistor in there. Don't depend on the output resistance of the chip itself. Because a TTL chip can sink more current than it can source, you typically pull the cathode of the LED down rather than trying to pull the anode up. CMOS is different, and can typically source and sink (approximately) equal amounts of current.

2. You can't reliably drive an HC chip from an LS chip, especially if there's an LED pulling down the output level of the LS chip. HC series expect a higher input voltage to indicate a logic '1'

3. Pullup resistors: typically a current-limiting resistor is used when pulling a pin to logic level '1' rather than connecting it directly to Vcc. There are people who will tell you it's OK to connect a CMOS input directly to Vcc, buut it doesn't hurt to include it. And if someone somewhere down the line might substitute a TTL chip for the CMOS part, then it is most certainly a good idea to include it.

4. U76A: Don't leave the unused inputs floating. Ground 'em.

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2-Zons wrote 12/08/2018 at 01:09

Thank you for the info. These are the chips I have. What would you recommend to interface the HC with LS? I can go through my inventory and see what other HC I have, but I think I got the HC because I couldn't source the LS. I will have go through my parts list again.

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Steve Toner wrote 12/08/2018 at 01:55

You would typically use a pullup resistor on the LS output/HC input. However, with the LED there you're still going to have problems. Say it's a red LED that has a forward voltage of 2V. The way the circuit is drawn (with the input of the HC chip connected directly to the anode of the LED), the HC chip is going to see 2V at its input even with a pullup resistor in there. And it needs something like 3.5V. If you were to drive an HC chip directly from an LS chip so that the HC is the only load (i.e., no LED), you'd probably be OK because the HC input draws minimal current so won't pull the nominal 5V output level down...

HCT parts are TTL compatible. But the LEDs may still cause problems if they are not buffered.

Additional reading:

http://www.ti.com/lit/an/scla011/scla011.pdf

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roelh wrote 12/08/2018 at 22:15

I agree with Steve. LS parts can not deliver much current in high state. You could however use low-current LEDs that light up below 1 mA, the series resistor can then be something like 2K2.

For the part that receives this logic signal, you could use a HCT instead of HC part. A HCT will see a logic high when the voltage is above approx. 2.4 volt.

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Yann Guidon / YGDES wrote 12/07/2018 at 01:26

For PC and SP, you might like https://hackaday.io/project/8121-discrete-yasep/log/27843-the-registers
This system uses cascaded 74'193 up/down counters for several registers, which would help keep your system simple.

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Yann Guidon / YGDES wrote 12/07/2018 at 01:31

My bad ! https://cdn.hackaday.io/images/2606371544094274163.png shows '193 already :-D

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[deleted]

[this comment has been deleted]

2-Zons wrote 12/06/2018 at 23:08

Not sure. I didn't mention you, should I have?

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Yann Guidon / YGDES wrote 12/07/2018 at 01:21

the site's server has some hicups...

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roelh wrote 12/06/2018 at 12:40

Hi Martin,

nice to see another homebuilt cpu here !

I do have a few questions and some advice....

- I do not see logic instructions (AND, OR, XOR) in your ALU. Will you add them ?

- Do you have ideas about the instruction set that you will implement ?

- I suppose you will have a microcoded control. You have a lot of control signals, 48 total in your list. I suppose the microcode will be smaller than that, and the microcode will go to a few decoders to produce the control signals ?
My experience is, that a datapath with registers and ALU is in most cases not so hard to design. The complexity comes when you want to control the whole stuff.

- Your design for the 16-bit register seems much too complex for what it does, having 11 IC's, while it could be done with only two 74HC574 (with some loss of functionality). Incrementing would be useful for PC, and inc/dec for SP. Allowing only 16 bit inc/dec (so dropping 8 bit inc/dec) for these will already simplify your register. For SP, you could consider the simplification to have the upper byte fixed, as in the 6502.

- For registers other than PC or SP, you can consider doing inc/dec with the ALU, in order to simplify these registers.

- It seems that your internal databus is 8 bit wide and the ALU 16 bit wide. I would suggest to make them the same size, 8 bit would be enough for a first design.

- I would suggest that you first build a software model of your processor (or a Logisim version), and make a few programs for it. That will show if things are missing, and will show if there are unused features.

For inspiration you could also look at the gigatron (https://hackaday.io/project/20781-gigatron-ttl-microcomputer) or my design for an ALU (https://hackaday.io/project/160506-4-bit-ttl-alu) and for the simplest TTL CPU ever designed (https://hackaday.io/project/161251-1-square-inch-ttl-cpu).

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Dave's Dev Lab wrote 12/06/2018 at 18:35

@roelh - oh oh oh, my feelings are sooo hurt! you didn't include my stuff! hehe

https://hackaday.io/project/158510-ddl4-cpu

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roelh wrote 12/06/2018 at 19:36

Sorry Dave !

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2-Zons wrote 12/06/2018 at 18:48

@roelh Thanks for the feedback.

1 - I wasn't planning on having anything but add / subtract in the ALU. I haven't really wrapped my head around an ALU with more functions. I know how to implement the add / sub with zero, carry, and negative status. I looked at your ALU design, and it seems quite simple for all those functions. If I can wrap my head around how it works I might try and implement it, that might mean more control lines however, but they would be exclusionary so it might be OK (see 3).

2 - If you look at the other page of that google docs spread sheet you will see a list of instructions that I have come up with, based on the control lines I have, that I can think of. If I missed any, I would like to know.

3 - As for the microcode. I plan on using EEPROM's to decode. You are correct with the 48 lines. I am simplifying the logic by considering exclusionary groups (see spreadsheet). E.g. all the data out controls. Only one of those control lines can be used at a time. There are 12 of them. So I can use 4 bits of combinational logic output that will drive a 4 bit mux to get up to 16 exclusionary control lines. There are only 4 address out controls so I will use a 2 bit mux for those. Total reduction of 10 bits (2 less ROM chips). If you can think of other obvious ways I could reduce the control logic that would be awesome.

4 - I don't mind the complexity of the registers. The SP, and PC will be a lot simpler because they won't have any logic for isolating the inc/dec of the individual bytes. They will also be only inc and dec respectively. having incrementing registers will make the code much more efficient IMHO. Having it in the register will allow it to be done in one cycle and added to load / store instructions with no extra cycles. Having a register auto increment /decrement every time it is used as a pointer without extra cycles is a huge for software efficiency IMHO. As for an 8 bit SP: I've always thought that a 256 byte stack is too limiting for software. Probably the only problem I have with the 6502, I know I'm not the only one. Also with my register / instruction design B and Y are selectable page pointers, setting the page with A or X. I guess I'm a software guy at heart, and I don't mind making the hardware more complex if it makes for simpler more efficient code.

5 -Inc / dec in the ALU.. see 3 and 4.

6. Having an 8 bit ALU would be less chips, slightly more complex IMHO. It might even be the same amount of chips. I might design both and see.

7. Although I do like the idea of running a sim of my design, personally I think it will be easier to test on the actual circuitry. I will build each block (AB, ALU,XY, etc) on it's own bread board, starting with a clock circuit that can be paused and single step. Then a register, Then the ALU etc. Using LED's I can even test parts of each block. I might be wrong but to me this feels easier.

8. I was aware of the gigatron (one of the inspirations for this project). Thanks for the links to your ALU design. I was writing this response point by point and then saw your link. Had to change my response to your ALU question. I was originally saying if I could see a design for an ALU with more functions. That square inch CPU is insane. I think it's a good example of, if you simplify something too much it makes it can make it more complex in some ways.

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roelh wrote 12/06/2018 at 19:34

Thanks for your explanations. I overlooked the instruction tab in your spreadsheet. It does however not show addressing modes for the instructions. For instance, will ADD add from a memory address, from an immediate, or from another register ?

I do not know a single CPU (except the square inch cpu) that has no logic operations. At least a NAND or NOR should be present I think, since other logic connectives can be derived from that.

Having inc/dec capability will indeed win you a few cycles, but that will be of limited use when the other parts of the cpu are not optimized for speed.

Maybe my own focus is too much toward lowest component count solutions....

The square inch cpu is indeed not very fast or practical, but I think not much has to be added to make it useful.

And before I get a complaint from @Yann Guidon / YGDES that he is not mentioned, you might find his Processor Design Principles a good read: https://hackaday.io/project/46000-pdp-processor-design-principles

There are even famous processor designs named after this: PDP-8, PDP-11 !

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Yann Guidon / YGDES wrote 12/06/2018 at 19:46

@roelh me ? complain ? come on, never ever !!! :-P

I'm looking with interest (like all the CPU projects here) and wait for the perfect timing to invite our new friend to #Hackaday TTLers

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8 Bit Breadboard CPU

Description

Details

Files

8bit Breadboard CPU.circ

Project Logs

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No more loopy wires

Added RAM and ROM module

Connectors and Glue (the sticky kind)

Combining Modules

Status Register

AB Register

ALU

Completed and Rebuilt XY Register

XY Register

Construction has started

Discussions

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