Sprout24 CPU

Sprout24 is a "know-your-roots" computer processor — designed and built from the ground up as a teaching tool to help introduce the basics of computer science and demystify how computers work.

early code execution test

We've heard that computers work entirely in ones and zeroes... but what does that even mean? How is that really enough to let modern computers, phones, and devices do all the amazing things that they can?

Let's take a look together at how that all works, using the design of the Sprout24 along the way to illustrate how simple pieces working together like an orchestra can make beautiful things possible.

At this point, the CPU is largely completed and running programs, but I'm doing extensive testing and still working out bugs. The tentative plan right now is to debut the project publicly with a presentation at Penguicon in April 2024. I'm looking forward to sharing more with you!

~ Autumn

January 28, 2024 — Turns out the value of what the carry flag should be during a subtraction is not well-defined:

"While the carry flag is well-defined for addition, there are two ways in common use to use the carry flag for subtraction operations: ... subtract with borrow ... and subtract with carry".

I'm seeing occasional mismatches of the carry flag between my CPU and emulator, only happening on some subtractions. I suspect I may have implemented it one way in the ALU hardware and the other way in the emulator without realizing it. If that turns out to be the case, then either way is valid, so I can just update the emulator to match the physical ALU and document the behavior in the ISA docs.

That leaves me with three open issues and no other known problems:

  1. Conditional jumps — needs an updated Control Unit board to select correct jump type
  2. Register timing — needs an updated Register File controller to latch on falling edge of clock
  3. Carry flag — may just need to update docs and emulator

Getting close!

February 29, 2024 — The new Control Unit and Register Files are ordered. Plus, since we really don't need a constant r1=1 register anymore, I redesigned the Register01 board into just a Register0 board and ordered that too. That'll free up one more general-purpose register for use. With that change, and swapping the opcodes for JMP and JX to make the Control Unit logic cleaner, I had to update the emulator and assembler, and reassemble all of the existing test programs.
January 27, 2024 — I found the reason for the conditional jumps acting all weird. The "jump taken" signal was looking at instruction bits 18–16 instead of 19–17 to determine what type of conditional jump it is. That's effectively just a transcription error when transcribing the design from emulator code to schematic. I'll need to design an updated control unit board, but I know exactly what to fix now.
January 26, 2024 — I think I've solved the register timing issue. The clock pulse idea didn't work reliably, but waiting to select the new writeback register until the second half of the clock cycle does the trick. With that out of the way, it's behaving more predictably, making it easier to isolate the remaining problems. Now I'm seeing two separate issues: first is a carry flag sometimes being set when it's not supposed to, and second appears to be an issue with the "jump taken" signal not always getting set correctly for some types of conditional jumps. I will definitely need a revision B for the RegisterFile board, and quite likely one for the Control Unit as well.
January 10, 2024 — Tracking down some issues with ALU flag values, and it looks like it may be related to the workaround that delays the clock signal going to the registers. I think the right way to solve that register clock timing issue is to instead convert the clock signal the registers see into just a quick pulse at the clock rising edge so that the write signal doesn't slew over to the next instruction's output register. Waiting on a parts order to be able to better test that theory.
January 5, 2024 — I had some fun with my first workbench livestream tonight! I did some live troubleshooting and tracked down a bad ShiftBusReader board. Removing it from the chain allowed all the rest of them to work as expected, and results against a couple of test programs are matching up 100%. Next step is to test more complicated programs and see if conditional jumps are all working correctly.
December 12, 2023 — Oops, never mind. I just forgot to reconnect a bodge wire, so the VCPU pin on the expansion slots was dead, making the shift registers on the test interface only halfheartedly drive the memdata bus with whatever voltage leaked through them from the signal pins. Plugging the bodge wire back in immediately solved it. Phew.
December 10, 2023 — Installed the new pinadapters and backplane connector, but something along the way broke either the memdata bus or the test interface board. I think I'm going to have to tear everything apart to troubleshoot, which at this point looks like it will have to be after the holidays. Frustrating indeed after being so close.
November 27, 2023 — Ordered a revision B for the Southeast board that adds the missing IRIN pulldown and breaks out more internals to headers for automated monitoring. Created and ordered Protocard for making experimental expansion cards, Pinadapter to adapt sketchy blade pins to square pins, and Backplane Connector to replace ribbon cables between CPU and backplane.
November 21, 2023 — Testing is progressing much faster with the help of the new test interface. I've identified and worked around two hardware bugs this week — one was a clock skew issue that was allowing bad data to seep into general registers, and the other was from the IR input bus missing its pulldowns so the IR sometimes clocked garbage data during a memory-access stall cycle.
November 14, 2023 — Revised Northwest board is testing good. The backplane and test interface are working. Now to keep moving forward with extensive testing of the CPU as a whole. There is so much complexity to test, but the new test interface board will make it a lot easier than having to wire every bit of the test connections by hand.
November 7, 2023 — Backplane, debugger, and revised Northwest board have arrived.
October 30, 2023 — Placed order for the backplane, debugger, and revised Northwest board.
October 23, 2023 — Trying to debug everything is becoming more and more of a ratsnest of wires. I would eventually need to design a backplane anyway, so I just finished a basic 8-port backplane and am starting on a debugger expansion board that will interface between the backplane and a Raspberry Pi to drive the testing. The plan is to order the updated Northwest board, the backplane, and the debugger board in the same order to save on shipping.
October 13, 2023 — We had a lot of family adventures and outdoor fun over the summer. Now that it's cold outside again, I'm back to working on this project in the evenings. Tonight I created revision B of the Northwest board that fixes the lookahead carry for the PC and SP incrementors. I also updated this board from KiCAD 5 to KiCAD 7; it took a surprising amount of work to adjust for the changes that came from that.
April 6, 2023 — Hello! This project has suddenly received a lot of attention that I wasn't expecting... so welcome, new friends! This has been just a quick introduction page until now, but now that I have a fully-assembled CPU, I'm going to start sharing some more technical details. I hope you'll continue to check in as I keep moving forward with it.

Some quick highlights:
24-bit CPU
Load/store architecture
RISC-style instruction set
53 instructions and interrupt support
Lightly pipelined
Working assembler and emulator
25 PCBs and 2,634 components
1,054 ICs, many of which are single logic gates
Approx. 9,700 total logic gates or 29,300 transistors
730 LEDs
April 5, 2023 — She's now officially been powered on and executed her first code. I'm so excited for this!

I did find a design oops on the carry-lookahead adder that increments the program counter, so it goes haywire after counting to 256, but I think I will be able to bodge that to be able to finish testing.
April 4, 2023 — All boards are tested as well as they can be individually. Tonight I assembled all the boards together, and she's beautiful. I'll be powering the whole thing up and starting to test it in the coming days.
April 1, 2023 — Assembled ALU and Register 01 boards are complete and match emulator in testing. Register File and Control Unit are complete and pass spot-check testing.
March 30, 2023 — The order is in, and all 25 boards that make up the CPU are ready to test and assemble!
March 8, 2023 — Placed order for all remaining CPU boards, including the Southeast and East Bridge boards. Now we wait.
March 3, 2023 — My bank flagged the PCB order as suspicious, so I had to call them to confirm it was legit. In the meantime I noticed an omission in the Register File board, so I put the order on hold and made a revision B for the Register File and West Bridge boards. At this point, I'm close enough on the layout of the Southeast board that I'm going to wait to try the order again until I can include the Southeast and East Bridge boards too.
February 28, 2023 — The Register File, Register 01, Northwest, and West Bridge board designs are now finished, and I just placed an order for those plus the Control Unit and ALU Core boards. There will be a lot of things to test once this order shows up.
February 1, 2023 — The Control Unit and ALU Core boards are ready to order, though I'm going to wait until I have a couple others ready to save on shipping costs.

Next to design are the "Northwest" board that includes components related to the program counter and stack pointer, and the "Southeast" board that relates to the instruction and status registers.

And I now have a tentative physical layout for how the individual boards will fit together into a complete CPU. Two additional helper boards with no components will sit below the rest of the boards for physical support and for routing busses between the boards above.
December 22, 2022 — The CPU's Control Unit is working and tested in the emulator, and now it's time to turn it into hardware. More details and a demo in this video.
November 22, 2021 — Ordered ALU Adder, ALU Logic, ALU Shifter, ShiftBusRead, ShiftBusWrite
October 2, 2021 — Ordered Register rev.B
March 1, 2020 — Ordered Register
February 5, 2020 — First KiCAD PCB sketch — for register board
September 27, 2019 — More design goals and ideas; sketches of board layouts
September 25, 2019 — Start of project notebook: description, what, why, first sketches of opcodes and isa, instruction encoding, register tower.
register complete
register01 need to design revision B
registerfile revision C ready to order
alu-core complete
alu-adder complete
alu-logic complete
alu-shifter complete
controlunit revision B ready to order
northwest complete
southeast complete
westbridge complete
eastbridge complete
extra-cpu boards
backplane complete, with bodge
backplane connector complete
ram parts ordered for prototype memory board
rom will initially emulate rom with a pico on a protoboard
isabridge tbd
shiftbus-read complete
shiftbus-write complete
test interface complete
protocard complete
pinadapter complete
instruction set complete
emulator working
assembler working
automated testing of completed alu