PicoRAM 2090

A Raspberry Pi Pico (RP2040)-based 2114 SRAM Emulator & Multi-Expansion for the Microtronic Computer System

* RetroChallenge 2023/10 *

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A Raspberry Pi Pico (RP2040)-based 2114 SRAM Emulator, SD Card Interface, and Multi-Expansion for the Busch 2090 Microtronic Computer System from 1981.

Features include:

- SD card interface
- Comfortable UI
- 16 user memory banks
- Mnemonics display
- Hardware extensions: speech synthesizer, Real Time Clock (DS3231 RTC), sound, ASCII text and graphics OLED display
- Extended op-codes for driving these extensions

PicoRAM 2090

A Raspberry Pi Pico (RP2040)-based 2114 SRAM Emulator and Multi-Expansion for the Busch 2090 Microtronic Computer System.

Final Demo Video on YouTube:

I have written about the Microtronic here before, and done a few Microtronic-related projects:

The final PicoRAM 2090 in the Microtronic: 

The firmware is attached to the project, as are the Gerber files. Please also have a look at the Github repo.

Early Prototype Pictures: Schematics

Latest News / Updates 

It's done! PicoRAM 2090 is finished. The GitHub repo has been updated and now contains an extensive README, the Gerbers, and the firmware. I'll upload the source code a little bit later. But for now, Makers / builders have everything they need to replicate it!

Here's the final demo video on YouTube:

Log Entries for RetroChallenge 2023/10 Participation

It's done - the RetroChallenge 2023/10 October month is coming to an end! This is my final video for the challenge which wraps it up - a full demo of most of the implemented features (with the exception of the RTC - see Log Entry 4 for that instead). I implemented everything I promised here:

All RetroChallenge 2023/10 Log Entries

Some more details in the final RetroChallenge log entry:

DS3231 Real Time Clock:

Text, Speech, & Graphics Demo:

Extended Op-Codes, Bank Switching:

SD Card Memory Dumps:

Overall Idea and Setup:

By now, the SD card interface, banked memory, graphics, text, speech, and sound extensions are working! Here is a little demo:


This project started out as an attempt to emulate the 2114 SRAM found in the Busch 2090 Microtronic Computer System - a fully compatible "drop in" replacement using the Raperry Pi Pico. A Pico-based SRAM emulator facilitates SD card-based storage of Microtronic memory dumps / its program memory, and hence of programs. Saving to and loading back of programs to SD card is much faster and more convenient than the Microtronic cassette interface (also, the files can be edited and created on the PC / Mac).

Later, this project became more ambitious - utilizing the first core of the Pico for SRAM emulation, the second core is now being used to implement a co-processor / IO extension for the Microtronic. By now it features a banked memory expansion (16 memory banks!), OLED text and graphics output, speech, and sound output. See the latest video above for a short demo.

Original Background & Context / Historical Account of the Project

This is the Microtronic Computer System from 1981, a 4bit educational computer - on the left you see the cassette interface. It also has an Arduino-based speech synthesizer connected to its output ports (an older project of mine, ~ 2016): 


The 2114 was a popular and widely used 1024x4 Bit SRAM chip. It was used in many early arcade games from the late 1970s and early 1980s. Pinout:

2114 Pinout

The Microtronic is a relatively slow educational 4bit microcomputer, so timing is not critical. A standard Pico is more than fast enough for the Microtronic. Note that the Microtronic doesn't...

Read more »


PicoRAM 2090 Firmware for the Pico (RP2040)

uf2 - 227.00 kB - 11/25/2023 at 22:46


PCB Gerbers

x-zip-compressed - 440.61 kB - 11/25/2023 at 22:45


  • Firmware Source Committed

    Michael Wessel12/12/2023 at 02:47 0 comments 
    I am omitting the TTS code for now though (i.e., the SPI Interface to the Epson S1V30120).

  • PicoRAM 2090 got a link on the official Busch homepage!

    Michael Wessel12/05/2023 at 02:43 0 comments
  • Final Demo Video:

    Michael Wessel11/26/2023 at 18:30 0 comments

    It's the end of the project as we know it - thanks for hanging on, folks!

  • GitHub updated! ‚Äč

    Michael Wessel11/25/2023 at 17:44 0 comments

    The GitHub repo and page have been updated. The repo also includes the Gerbers and firmware by now; firmware sources will come a bit later as I am still refactoring things a bit:


  • Final PicoRAM 2090 PCB

    Michael Wessel11/24/2023 at 05:15 0 comments

    The third revision of the PCB is the final one for now - I have fixed all shortcomings and am satisfied with it for now. Now, I'll make one more final demo video, and then upload of the sources and Gerbers to GitHub.

  • First PCB Assembled

    Michael Wessel11/19/2023 at 16:50 0 comments

    The good news is - it works!

    The bad news is: I'll need one more round of PCB revisions; problems are: 

    • the power switch is blocking the Pico's USB port - I hence had to put in a temporary micro slide switch which is very flimsy and a bit of a hack.
    • I also learned that the double throw 6pin DPDT power blue & white switches come in 7x7 and 8x8 mm; the 7x7 that I currently have don't fit the KiCAD footprints without pin bending. So I ordered the 8x8 ones.
    • the silkscreen text for buttons and button legend is too small. 
    • the SRAM 2114 data lines are in reverse bit order - I had to change the firmware for now by including a reverse_bits on the data lines, but have fixed this already in the PCB. 
    • the voltage divider for the Microtronic's DISP line is 1k-1k instead of 1k-2k... I had to cut a trace and use one of the spare 2k resistors in one of the isolated resistor network DIPs with a bodge wire. 
    • I made a stupid mistake when I crimped the 2114 connector cable - it took me 2 hours before I finally identified the badly crimped cable shorting every adjacent pair of wires, questioning my design, as the main culprit why it wouldn't work at all at first. I had to de-solder the crimp connector from the board and messed up a trace, requiring one more bodge wire.

    Well, the 2nd revision of the PCB is already in production, and this should be the final one! 

    Here are a few pics of the mess ups:

  • PCB Designed and in Production!

    Michael Wessel11/12/2023 at 16:19 0 comments

    Waiting for the PCB to arrive... 

  • RetroChallenge 2023/10 Log Entry #6 - Final Wrap-Up Demo Video

    Michael Wessel10/29/2023 at 07:48 0 comments

    I achieved everything I wanted for this year's RetroChallenge 2023/10 - here is a final wrap up demo video:

    So the last week I mostly spent time writing the demo programs you see in the video. Some of these graphics demos (involving trig) were created with the help of a bunch of little Lisp functions (see below) - the Microtronic cannot do trig functions (sin, cos) reasonably fast, so these were pre-computed with Lisp and materialized in the code. The Lisp-program generated Microtronic instructions were just put in a .MIC file and then put on the PicoRAM's SD card. In the end, it turned out to be a good design decision to utilize an SD card interface for memory dumps / .MIC file storage, as opposed to, say, an EEPROM - given that I can easily create and exchange .MIC programs / content with and from the PC using the SD card.

    It might be a good idea to actually add extended op-codes just for trig, i.e., if I wanted to add simple Logo-turtle like graphics capabilities as op-codes, I will need fast trig from register memory as well. For now, I've run out of available vacuous op-codes, but it is of course possible to implement different sets of vacuous op-codes. There could be one set which includes op-codes for sound and speech, like the current set, and another set that utilizes the same vacuous op-codes for trig functions instead. I could use one of the PicoRAM push buttons to cycle through different op-code sets, just like I am doing now to swap memory banks.

    Interesting side note: running some of these graphics demos, I noticed that the 2nd core, which is running the op-code extensions, display, user interface, SD card interface, speech, sound, .etc., wasn't fast enough during certain display line drawing operations to keep up with the Microtronic. As the PicoRAM has no way of halting the Microtronic (it only snoops the RAM bus!), if the extended op-codes such as drawing a line to the display take too long, the 2nd core might miss the next extended op-code if it's still busy executing the current operation (e.g., drawing the line). Hence resulting in (graphics) glitches.

    Solution: I overclocked the Pico to 250 Mhz - all glitches are gone! This problem never occurred for the SRAM emulation running on the 1st core, only the 2nd core was problematic as it was implementing the extended op-codes, driving the display, etc. In general, the Pico is a great platform for this kind of work - fast and powerful. I don't think this would have been possible with the old AVR MCUs - the 2nd core is a blessing. And overclocking and using the 2nd core is insanely easy, even from C! It's definitely a platform / MCU I am going to use for future projects. Despite the slight annoying required level shifters, it's close to perfect.

    Speaking of the op-codes - writing all these demo programs I tweaked the firmware and its op-codes here and there, just to make the programming more convenient. You don't really know how well a programming language or a set of op-codes works until you eat your own dog food! For example, I removed the original

       50D SOUND PLAY FREQ <X><Y> 

    op-codes because I found it more useful to have an op-code that enables auto-echo to the speech synthesizer of text. With speech echo enabled, I can use the


    op-code to drive the text display as well as the speech synthesizer.

    The final (hah - or let's say latest, current) set of op-codes is hence the following: 

       504 DISP REFRESH
       505 DISP CLEAR LINE  
       506 DISP SHOW CHAR 
       508 DISP SET CURSOR  
       509 DISP PLOT   
       50A DISP LINE    
       50B DISP LINE FROM   
       50C DISP LINE TO     
       50F SPEAK BYTE 

    And here is the Lisp program that I used to generate...

    Read more »

  • RetroChallenge 2023/10 Log Entry #5 - DS3231 for Microtronic

    Michael Wessel10/22/2023 at 17:12 0 comments

    In my original RetroChallenge 2023/10 participation announcement, I promised a battery backed-up Real Time Clock (RTC) for the Microtronic. Well, here it is! I have hooked up the DS3231 RTC.

     I did not use any of the DS3231 Pico SDK C libraries found online literally, but I had used this module before in some of my previous projects (e.g., and hence didn't have a lot of issues getting it to work. Most helpful I found and I took bits and pieces of it, but heavily refactored - thanks, good code, Bryan Nielsen (! Nice and simple - I really dislike when people create a huge CPP framework for something that is not rocket science.

    So how does that work? See, the Microtronic already has an internal real time clock in its firmware, and there is even an op-code F06 ("load time") that can be used to load the current time to register memory (registers for A-B for hours, C-D for minutes, and E-F for seconds). Two built-in ROM programs (PGM 3 and PGM 4) also allow you to set and see the current time (without using the F06 op-code).

    However, the Microtronic RTC is not battery backed-up, and hence if you want to use the real time in your programs, you will always have to set the clock manually when you turn it on (using the ROM program PGM 3). This of course means limited utility.

    How can the DS3231 RTC help here? Well, using similar ideas as explained in my previous log entry, instead of adding extra side-effects / semantics to vacuous op-codes, I can also intercept a non-vacuous op-code such as F06 ("load time"), and do anything I want by switching first into a dedicated memory bank to not interfere with the current program, then execute this program with the Microtronic, and finally return to the original program. When F06 is intercepted, the Pico first retrieves the current time from the DS3231, and then materializes a set of MOVI nx = Move Immediate Value n to Register x (op-code 1nx) into the current memory bank that will, when executed by the Microtronic, write the current DS3231 time into the A to F registers, overwriting the values that F06 had already left there. After returning from the banked-in sub-program, the registers A-F contain the correct time.

    Here is a demo video:

    Again, the underlying mechanism that loads the DS3231 time is entirely transparent to the calling program - from a user program's perspective, there is no difference in accessing the DS3231 vs. accessing the internal Microtronic RTC - the same single op-code is used, nothing else is required. This is the program from the demo: 

    00 F6A # display "time" registers A..F on the LED display 
    01 F06 # load time from RTC 
    02 FF0 # wait for keyboard input 
    03 F08 # clear registers
    04 C00 # goto 00 

  • RetroChallenge 2023/10 Log Entry #4 - DEMO

    Michael Wessel10/21/2023 at 14:06 0 comments

    Here is a first "all-in-one" demo - SD card interface, graphics, text, speech!

    Here is the program:

    00 F08 # clear registers 
    01 F20 # display registers 0 and 1 on the LED display 
    02 50A # enable draw line (x1, y1) - (x2, y2) mode; display (0,0) - (x7F,x1F) 
    03 000 # send x1 low nibble: 0 
    04 3F0 # send x1 high nibble: value in register 0 
    05 000 # send y1 low nibble: 0 
    06 000 # send y1 high nibble: 0 
    07 000 # send x2 low nibble: 0
    08 3F1 # send x2 high nibble: value in register 1 
    09 0FF # send y2 low nibble: F  
    0A 011 # send y2 high nibble: 1 -> x1F (32 4*8 = 32) 
    0B 520 # add to to register 0 
    0C 980 # reg 0 = 8? 
    0D E0F # yes, then goto x0F 
    0E C02 # no, goto x02 
    0F 100 # reg 0 = 8: clear register 0 (to 0) 
    10 521 # add 2 to register 1 
    11 981 # reg 1 = 8? 
    12 E14 # yes, then goto x14
    13 C02 # no, goto x02 
    14 50E # register 2 = 8, end of nested reg 0, 1 loop. enable speak echo mode.  
    15 506 # enable print ASCII char to display with speech echo 
    16 0DD # send ASCII codes for MICROTRONIC (x4D, x49, x43, ...); M 
    17 044 
    18 099 # I 
    19 044
    1A 033 # C 
    1B 044
    1C 022 # R 
    1D 055
    1E 0FF # O 
    1F 044
    20 044 # T
    21 055
    22 022 # R 
    23 055
    24 0FF # O 
    25 044
    26 0EE # N
    27 044
    28 099 # I
    29 044
    2A 033 # C 
    2B 044
    2C 0AA # LF (0x0A) -> speech synth will speak MICROTRONIC 
    2D 000
    2E F00 # end of program, halt 

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