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

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PicoRAM 2090

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

Final YouTube Demo Video

Full Feature Demonstration (Breadboard Prototype)

PicoRAM 2

PicoRAM 1

Latest News

August 20th 2024

New assembly & demo video by "Mein Elektronik Hobby" (in German)!

Mein Elektronik Hobby

August 10th 2024

PicoRAM 2090 won the "Grand Prize" in the RetroChallenge 2023/10! Thanks much, guys - it's the project that I am most proud of, and I feel very honored that it got recognized with the renown and reputable "RetroChallenge Grand Prize Winner" label. After all, the RetroChallenge gave us fantastic retro computer projects such as the RC2014.

RetroChallenge 2023/10 - 1 RetroChallenge 2023/10 - 2

Older News

January 28th 2024

Firmware update - version 1.1. Still need to update the source code folder.

Version 1.1 brings an improved .MIC file reader. It is now possible to specify the address, use lower case hex characters, and the reader accepts common typos (e.g., o -> 0). Have a look at this example file that demonstrates the extended .MIC format. The file format is particularly useful if you are developing Microtronic programs on the PC.

There is also a new interesting demo programs: a recursive implementation of the infamous text-book example, towers of Hanoi. This works for up to 4 disks. I emulated the stack using the 2nd register bank and shift operations, and also implemented a return stack, using integer labels and conditional branching. This is a fully general pattern for implementing recursion on the Microtronic. Obviously, the "emulated" stack is limited, but still useful and surprising, as towers of Hanoi demonstrates.

December 4th 2023

Mr. Jörg Vallen of Busch was so kind to include links to my PicoRAM 2090 as well as the Busch 2090 Arduino Emulator repositories on the Busch Microtronic main page.

History of the Project

PicoRAM 2090 started out as a simple project to emulate the Microtronic 2114 SRAM in early September 2023, and evolved into a powerful and versatile multi-expansion for the Microtronic. It reached its current state end of November 2023 (Thanksgiving weekend).

As a contribution to the RetroChallenge 2023/10 I developed the firmware to maturity, still using the breadboard prototype; see my Hackaday IO page and YouTube videos.

Final Breadboard

The project got covered by a number of sites:

About

PicoRAM 2090 is the ultimate expansion for the Microtronic.

It offers:

  • SD card interface: loading and saving of programs (full SRAM memory dumps) and easy file exchange with the PC (FAT32 filesystem).

  • Comfortable UI: 5 buttons and OLED display.

  • 16 user memory banks: the currently active memory bank can selected manually via the UI or by program; each bank hosts a full Microtronic RAM.

  • Mnemonics display: PicoRAM can show the current Microtronic instruction, address, and even mnemonics on its OLED display. Various display modes are offered - the mnemonics display greatly facilitates programming, debugging, and learning of the Microtronic machine language.

  • Hardware extensions: speech synthesis (DECtalk-based), battery backed-up Real Time Clock (DS3231 RTC), monophonic sound, ASCII text and even graphics output on the OLED display. Extended "vacuous" op-codes (see below) are used to access (drive) these extensions.

  • Full integration: for example, the Microtronic's GET TIME op-code (F06) is intercepted so that the actual time from the RTC is loaded instead of the Microtronic's (volatile, not battery backed-up) clock.

  • Easy build & installation: requires only simple modifications to the Microtronic PCB, and PicoRAM uses pre-assembled off-the-shelf modules and through-hole components only.

Demo Videos

Final YouTube Demo Video

Full Feature Demonstration (Breadboard Prototype)

Theory of Operation

2114 SRAM Emulation

PicoRAM 2090 plugs into the 2114 SRAM socket of the Microtronic. The 2114 has a capacity of 1024 4bit words, i.e., it has a 10bit address bus and a 4bit data bus. The tristate (HighZ) capability of the 2114 is not utilized by the the Microtronic, so CS is not connected.

SRAM 2114 Pinout

Interestingly, the "CPU" of the Microtronic, the mask-programmed TMS1600 Microcontroller, does not cater for external RAM or ROM, so the 2114 is connected via GPIO to the TMS1600. This can be seen clearly in the Microtronic schematics:

Microtronic Schematics

Naturally, the WE (Write Enable) line is utilized to distinguish read from write accesses to the 2114.

Microtronic's RAM is organized as 256 12bit words. Thus, three 2114 memory locations are required to store one Microtronic 12bit instruction word. Interestingly, this also leaves 256 memory locations of the 2114 unused. To the best of my knowledge, these SRAM locations are just void.

As can be seen in the schematics, the "address" bus to the 2114 is utilizing general-purpose TMS1600 output ports - 6 bits from the R port, and 4 bits from the O port. These are also shared with the 6-digit 7-segment LED display and keyboard! Since CS is not utilized in this design, it is thus challenging to distinguish Microtronic's SRAM accesses from keyboard scanning and LED display driving activities on these ports (see below).

For the purpose of serving the SRAM, the Pico just runs a tight loop and presents the content of its "C memory array" on the 4 data lines as reactively as possible.

Luckily, for the mere purpose of SRAM emulation, it is not necessary to distinguish the "real SRAM" accesses from the ones resulting from driving the display and scanning the keyboard - the 2114 is actually presenting data for these addresses as well, but the Microtronic firmware just ignores them (it obviously knows whether it addressed the SRAM, the display, or the keyboard).

This situation isn't different with the Pico RAM emulating the 2114. It, too, reacts to all presented addresses and presents data for each of them on the data port; some of them are just meaningless and are ignored by the Microtronic OS. Ideally, the CS signal would have been used to unambiguously identify the "real" SRAM accesses from the involuntary ones, but as explained this is not possible with the Microtronic design.

As for write requests, the situation is much simpler - we have a clear signal in form of the WE signal going low. This tells us when to update the PicoRAM's C array holding the memory contents. PicoRAM simply stores the current 4bit value on the data lines (L1, L2, L4, L8) into its memory C array when WE goes low.

Banked Memory

Given that the Microtronic memory is just a big C array, it is straight-forward to support banked memory (given the abundance of SRAM on the PicO!), simply by adding one more index / dimension to this array: the bank number. Switching the currently active bank does not require any copying, but merely changing the value of the "active bank" index variable:

  val = ram[cur_bank][adr];
  gpio_put_masked(data_mask, ~ (val << DATA_GPIO_START));

PicoRAM offers 16 user RAM banks that can be selected via the UI (OK button), or programmatically (extended op-code 70x). Moreover, a few temporary banks are used for implementing and managing the extended op-codes (see below).

The Challenge of Identifying the Current Instruction

Identifying the 12bit instruction words that is currently addressed (executed, displayed, ...) by the Microtronic is not straight-forward due to the missing 2114 CS signal.

Even though the Pico sees all activity on the 10bit "address bus" and 4bit "data bus" (bus in double quotes here because these "buses" are really just TMS1600 GPIO lines, as explained) it is not straight-forward to distinguish true SRAM accesses for fetching the current 12bit instruction word from "involuntarily" accesses that happen as a side effect whilst driving the LED display or scanning the keyboard.

However, by monitoring the last four addresses on the address bus, a necessary condition for identifying the current Microtronic 12bit instruction word was found by analyzing the address bus:

  if (adr & (1 << 8)) {
    if (adr3 & (1 << 9)) {
      if ( (adr & adr3 ) == adr4) {                    

where adr is the current address on the address bus, and adr4 to adr are the last four addresses that have been seen.

If the Pico detects such a sequence of addresses, then the Microtronic has fetched a 12bit instruction starting at Microtronic memory address adr4 & 0xFF. Note that three 4bit 2114 SRAM words make up one 12bit Microtronic op-code / instruction word. Moreover, these 4bit words are not at consecutive memory locations in the 2114. Instead, set bits 8 and 9 are used to group the three 4bit words into one 12bit word.

Unfortunately, this mechanism does not work for Microtronic word at Microtronic address 00. Moreover, there are still false positives that are caused by display multiplexing! In principle, these false positives are indistinguishable from real SRAM accesses from PicoRAM's external perspective - only the Microtronic firmware knows whether it is addressing the SRAM or the keyboard or display.

To eliminate these false positives, and also turn this necessary condition into a sufficient condition, it has to be strengthened by adding one more input signal from the TMS1600 - the R12 line. As can be seen in the schematics, the TMS1600 GPIO port R12 is used to drive the individual digits of the display. We can hence eliminate all addresses for which R12 is active, provided that the display is on, i.e., at least one of the six 7segment digits is shown on the LED display.

PicoRAM hence requires the R12 (DISP) signal to robustly identify the current instruction that is executed (or on the LED display). Without the extra DISP wire for robust operation it will still perfectly function as SRAM emulator and SD card storage device, but extended op-codes and hardware extensions (sound, speech, text and graphics display, RTC) will not function properly, and neither will the op-code OLED display. Hence, I strongly recommend to add the extra DISP wire to the Microtronic PCB; it's a simple mod (see below).

It should be clear by now that extended op-codes will not work reliably for programs that turn off the LED display using the FO2 (DISP OUT) op-code. Moreover, there can be no extended op-code at address 00. These are not severe restrictions, but have to be followed for programs that wish to take advantage of the hardware extensions and extended op-codes.

Extended Op-Codes

Vacuous, extended op-codes are used to access the hardware extensions. A vacuous op-code is a Microtronic op-code that does something, but basically boils down to a convoluted no-op. These op-codes are being executed by the Microtronic and leave the register contents unchanged; hence, no real Microtronic program is using them (moreover, in case a NOP is required, the F01 (NOP) op-code can be used instead). PicoRAM monitors the current instruction, detects these vacuous op-codes, and puts them to work by implementing extra side effects for them, i.e., for driving the PicoRAM hardware extensions.

A simple example is the op-code 502 = ADDI 0 to register 2, which means "add zero to register 2". This is semantically a vacuous, meaningless operation - a convoluted no-op. No existing Microtronic program uses it. It is hence available for use in PicoRAM! PicoRAM detects this op-code and implements an extra side-effect semantics for it: clear the OLED display.

Unlike 502, many extended op-codes require operands / arguments. For example, the PicoRAM's extended op-code 50D (Play Note) initiates a sound output command. This command requires arguments - the octave and note number to be played. The Pico then awaits additional vacuous op-codes that specify these arguments, and when all required arguments have been supplied, the extended op-code is executed by PicoRAM (e.g., the specified musical note is played on the speaker).

To specify these operands / arguments literally in the machine code ("immediate" style), the vacuous op-codes 0xx, mnemonic MOV x->x, are used: "copy content of register x (0 to F) onto itself". The number of operand nibbles x required varies for the different extended op-codes (usually, from 0 to 8, see the Table below). For example, the op-code sequence 50D 011 022 plays note 2 from octave 1; 2 nibbles are supplied.

Specifying arguments directly (literally, immediately) in-code via 0xx op-codes is fast, but lacks flexibility - these arguments can not be computed at runtime! Since the Microtronic is a Harvard architecture, it is not possible to modify the program memory by program. Instead, registers have to be used as program-writeable memory.

In order to feed the contents of a register as an operand / argument to an extended op-code, e.g., say to play note number x in register 0, we need a special trick. The problem here is that PicoRAM does not have access to the register memory, as Microtronic registers are stored directly on the TMS1600 chip, not on the 2114! So how can PicoRAM get to know the current content of a register? Answer: By temporarily "banking-in" a register interrogation program. This interrogation program shows certain address access patterns characteristic for a register x having a value of y. The Microtronic executes this interrogation program and the Pico observes the addresses that are accessed by the Microtronic. Certain observed target addresses are then indicative of the value y in register x.

Here is an example. Suppose we want to supply the (maybe computed) note number to the 50D (Play Note) extended op-code from register 0. Instead of using 0xx to supply the nibble immediately in the code, we are now using the vacuous op-code 3Fx = ANDI F x ("do a logical AND of the immediate value F with the content of register x") to mean supply the content of register x as an argument to the currently active extended instruction.

When the Pico detects 3Fx, it now immediately switches to a temporary memory bank containing the interrogation program, i.e., materializes this program for the Microtronic, and then presents a jump to address 00 (C00 = GOTO 00) to the Microtronic as the next instruction. The Microtronic has now left the user program and starts executing the interrogation program from the temporary memory bank, starting at address 00. The interrogation program performs a binary search to determine the current register value. The Microtronic cannot simply write the current value of the register into program memory for the Pico to see, due to its Harvard architecture. It can, however, do a number of compares and conditional branches to determine the register value via binary search! The Pico observes the addresses that are reached during the execution of the banked-in interrogation program, and watches for characteristic, unique target addresses that are indicative of the register value y. There is one target address for each possible value y. Once the target address has been detected, and Pico knows the value of the interrogated register, and it supplies it as next argument for the currently active extended op-code waiting for it. When all arguments have been supplied, PicoRAM executes the extended op-code. Now that the register value has been inferred and supplied to the extended op-code, PicoRAM instructs the Microtronic to resume execution of the original program simply by restoring the original memory bank, and by materializing a GOTO to the address after the 3Fx. Microtronic executes the jump, and continues the execution of the original program as if nothing had happened.

From the user program's point of view, this register interrogation process happened transparently. However, it is quite slow - determining the current value of a register takes almost one quarter of a second or so, as a few dozen operations have to be executed from the interrogation program (the Microtronic is a very slow machine indeed; the TMS1600 is clocked at 500 kHz, and the Microtronic OS firmware is a complex program that uses a lot of TMS1600 cycles for itself, so not much processing power is left for executing Microtronic machine language - after all, this is a complex virtual machine and "interpreted" instruction set).

Note that immediate / code-supplied and register-supplied arguments can be mixed. For example, here is a program that uses register 0 to supply the note number to be played, but specifies the octave number as a constant in the code:

00 F10 # display register 0 on display 
01 50D # sound output op-code
02 011 # supply value 1 (= octave 1) immediately 
03 3F0 # use content of register 0 for note number 
04 510 # increment register 0 
05 C01 # jump to address 01 (50D, ...) 

Dual Core Operations

PicoRAM utilizes both cores of the RP2040 (Raspberry Pi Pico).

The first core of the Pico is implementing the SRAM emulation, including bank-switching, identifying the current address and instruction, etc.

The second core is driving the OLED display, implements the UI, extended op-codes, and access to the hardware extensions.

The Pico is overclocked to 250 Mhz - this is still well within range and not problematic at all (some folks have successfully overclocked the Pico to frequencies as high as 1 GHz!).

List of Extended Op-Codes

This is the current list of extended op-codes; note that future firmware versions might contain additional sets (and different sets might be selectable from the UI).

In the following, <CHAR>, <NOTE> and <OCTAVE> represent single bytes, in little endian order (a sequence of two nibbles: <LOW>, <HIGH>). Moreover, all graphics coordinates X,Y,X1,X2,Y1,Y2 are bytes and require 2 nibbles each. In contrast, TX, TY are text display cursor location (text screen column / row coordinates), and only require a single nibble (<LOW>), or two nibbles (<LOW>, <LOW'>) each:


Op-Code # Operand / Argument Nibbles Explanation
0xx 0 Enter Literal Data Nibble x
3Fx 0 Enter Data Nibble from Register x
500 0 Hexadecimal Data Entry Mode
501 0 Decimal Data Entry Mode
502 0 Clear OLED Display
503 0 Auto or manual OLED display updates
504 0 Refresh OLED Display
505 1 Display Clear Line <LOW>
506 2 Display Show ASCII Character <CHAR>
507 1 Display Set Cursor at Line <TY>
508 2 Display Set Cursor at Pos <TX><TY>
509 4 Display Plot <LOW><HIGH> (=X) <LOW'><HIGH'> (=Y)
50A 8 Display Line <X1><Y1><X2><Y2>
50B 4 Display Line From <X><Y>
50C 4 Display Line To <X><Y>
50D 2 Play Note <OCTAVE><NOTE> (Sound Model Only)
50E 0 Enable TTS Display Echo
50F 1 Send <CHAR> to TTS (Speech Mode Only)
70x 1 Select Memory Bank x

Please have a look at the provided example programs.

Here is an example program demonstrating graphics, text, and speech output - ensure that TTS is enabled:

Micronet

F08
F20
50A
000
3F0
088
000
000
3F1
0FF
011
520
980
E0F
C02
100
521
981
E14
C02
50E
506
0DD
044
099
044
033
044
022
055
0FF
044
044
055
022
055
0FF
044
0EE
044
099
044
033
044
0AA
000
F00

Operating Instructions

The following instructions should explain how to operate PicoRAM from a user perspective.

Power Supply

Power to the PicoRAM is supplied either directly from the Microtronic (2090 VCC), or from an optional external and stabilized standard 5V power supply with positive center polarity (EXT VCC). The LEDs 2090 VCC and EXT VCC indicate which power sources are available.

Power Options

Note that the EXT VCC LED will not come on if you have the wrong polarity! In this case, do not power on the PicoRAM! External VCC is not fed into the Microtronic - only GND is shared. Use the SEL VCC switch to determine the power source.

The Microtronic PSU is strong enough to drive PicoRAM, so an external additional PSU is usually not required. But you may choose to use one to be on the safe side anyway. Before powering on the Microtronic, make sure PicoRAM 2090 is running! Turn on PicoRAM by pushing the POWER button. The PWRLED on the MikroE speech daughter board should come on immediately, as well as the OLED display.

PicoRAM 3

Audio

Use a standard mini stereo jack connector cable to connect the MikroE TextToSpeech click board output to the LINE IN mini stereo jack. Determine the VOLUME with the potentiometer.

PicoRAM is equipped with a PAM8403 class D audio amplifier powering a little (4 or 8 Ohm) loudspeaker of your choice.

Speaker

PicoRAM offers either TextToSpeech (TTS), using the MikroE click board for DECtalk, or sound (generated by the Pico itself). Unfortunately, only one at a time can be used due to a shortage of GPIO pins on the Pico. Hence, use the switch labeled TTS or SOUND to select either. A push to the RESET button is required for the new audio mode to become effective.

The current audio mode is also shown on the OLED display (TTS-, TTSE or SND indicators in the first display line).

Reset Button

The RESET button resets the Pico and hence all emulated memory banks. Memory banks can be saved to SD card if required.

SD Card

Use a FAT32 formatted micro SD card. Note that ERROR 2 will be displayed if PicoRAM boots without SD card, the SD card is write protected, not properly formatted, or faulty in any way.

OLED Status Display & Display Modes

The PicoRAM OLED display looks as follows:

Display Explanation

The first line shows

  • the number of the memory bank the PicoRAM is currently serving: #0.

  • the current Microtronic address: 00.

  • the current Microtronic 12bit instruction word / op-code: 000.

  • the current audio mode: SND, TTS-, or TTSE. SND means sound output, else TTS is active. See the TTS or SOUND switch. TTSE means "TTS Echo", i.e., everything printed to the OLED display is automatically sent to the TTS as well, and uttered when an end-of-line character (CR or LF) is sent. TTS Echo is enabled with the op-code 50E.

  • whether extended op-codes are enabled (*) or disabled (-).

The second line shows the current bank, address and instruction using Microtronic mnemonics.

The third line is shown when extended op-codes are enabled, and mostly useful for single stepping and debugging of a program. It shows the status and kind of the current extended op-code (i.e., it's internal op-code type and arguments):

Display Ex-Op Codes Explanation

The fourth line is used for file operations, displaying the current time of the Real Time Clock, etc.

It is important that the OLED display is switched off when a program with extended op-codes is running - not only might the Microtronic program want to utilize the display for text or graphics output, but also from a performance point of view, as the 2nd core will have less cycles available to implement the extended op-codes when it also has to update the OLED display.

Using the CANCEL button, the modes of the display are:

  • Display off: this should be the default when the Microtronic is running a program with extended op-codes, as updating the display requires cycles from the 2nd core which might then glitch on extended op-codes.

  • Op-code display: first line only.

  • Op-code & mnemonics display: first and second line. The third line showing the extended op-codes status is only displayed when extended op-codes are enabled. This should be the default for programm development, single stepping, and debugging of Microtronic programs.

User Interface Buttons

UI

The button legend on the PCB silkscreen lists the button labels (first column), as well as their primary and secondary functions (second and third column).

The primary function is selected with a short press to the button, and the secondary function with a longer press (i.e., hold the button down for about half a second before releasing it).

The button labels are indicative of their functions during file creation and file selection operations:

  • In file load mode (UP button), the UP and DOWN buttons are used to select a file from the list of files on the SD card. OK is used to confirm loading of the current file, whereas CANCEL is used to abort the load process.

  • In file save mode (DOWN button), the UP and DOWN buttons are used to determine the next ASCII character of the filename under constructions. A short press of NEXT/PREV advances to the next character, and a longer press jumps back to the previous character. OK and CANCEL have their normal meaning. The buttons have analog functions for setting the RTC (see secondary function of CANCEL).

In a nutshell, the primary functions of the buttons are (if executed from the main loop / context of PicoRAM):

  • UP: Load a memory dump from SD card, using the file selector.

  • DOWN: Save a memory dump to SD card, using the file name creator.

  • NEXT/PREV: Increment the current active memory bank number by 1; 16 user banks are available. These banks are pre-loaded with some programs, see below for the list. The currently active memory bank can simply be erased using Microtronic's standard clear memory procedure: HALT-PGM-5 (or HALT-PGM-6 for NOP op-codes). Note that this only affects the currently active bank! All banks are reset to default contents upon reset or power cycle of the Pico.

  • OK: En-/disable extended op-codes. When extended op-codes are enabled, PicoRAM acts as a co-processor and the already discussed vacuous op-codes are utilized to drive the hardware extension, i.e., for sound, speech, text, graphics output, and programmatic bank switching. A * in the (first line of the) status display indicates enabled extended op-code, and a - means they are disabled. When extended op-codes are enabled, and a Microtronic program is running, the OLED display should always be turned off, as explained above.

  • CANCEL: toggle OLED display mode (off, op-code display, op-code & mnemonics display).

The secondary functions of the buttons are (if execute from the main loop / context of PicoRAM):

  • UP/DOWN: Test the audio output - either sound or speech (depending on the current audio mode, see TTS or SOUND switch).

  • DOWN: List all the .MIC files on SD card.

  • NEXT/PREV: In TTS mode, speak the current time of the RTC.

  • OK: Show the current time of the RTC on the OLED display.

  • CANCEL: Set the current time of the RTC using the OLED display and buttons.

Pre-Loaded Memory Banks & Programs

The default / power-on memory bank programs are as follows. A * indicates that op-code extensions must be enabled for this program to work properly. All programs are started normally via HALT-NEXT-0-0-RUN:


Bank # Extended Op-Codes? Description
0 * Demonstrates F06 (GET TIME) op-code via RTC
1 17+4 Blackjack
2 Nim Game
3 Three Digit Counter
4 Electronic Die (Random Generator from 1 to 6)
5 Three Digit Counter
6 Scrolling LED Light ("Lauflicht")
7 Digital Input Port Test (DIN Op-Code)
8 Lunar Lander Game
9 Prime Numbers
A Tic Tac Toe
B Car Racing
C Blockade
D * Regload Test Program
E Empty
F Empty

These programs may change without notice in future version of the firmware.

Using PicoRAM with the Microtronic

The standard operation-sequence for loading a bank / program from SD card should look as follows:

  1. Load a memory dump from SD card using the UP button; select a .MIC file using UP, DOWN, and OK (or CANCEL to quit).

  2. Set the Microtronic to address 00: HALT-NEXT-0-0.

  3. Disable the OLED display using the CANCEL button.

  4. Ensure that extended op-codes are enabled: hit the OK button until OP-EXT ON (*) is shown.

  5. Now start the Microtronic program: HALT-NEXT-0-0-RUN.

  6. Important note: when the Microtronic program has finished, DISABLE extended op-codes, else you might encounter weird behavior in the Microtronic monitor.

The last step is important because some of the extended op-codes require temporarily banked-in auxiliary program fragments. If the program is interrupted whilst running one of the banked-in "helper" programs, then the currently active memory bank is no longer the user bank. The Microtronic will appear to have a lost its program! But don't panic - if you don't find your program in memory, simply disable extended op-codes (hit OK until OP-EXT OFF (-) is shown), set the Microtronic monitor to a well-defined address (i.e., HALT-NEXT-0-0), and make sure the right user memory bank gets re-selected (toggle through the memory banks with the NEXT/PREV button). The will restore the original user program.

Note that your program will also appear to "disappear" if you hit the NEXT/PREV button accidentally - you have accidentally changed the active memory bank. Simply re-select the original memory bank with the NEXT/PREV button.

Example Programs Demonstrating Extended Op-Codes

Example programs demonstrating the hardware extensions via extended op-codes are here.

They are also demonstrated in the YouTube Breadboard Prototype Demo.

Schematics, Gerbers, and Firmware

PicpRAM PCB PicoRAM Schematics

Here you will find the firmware, the PDF schematics, and the Gerbers.

Firmware sources will be provided soon. Note that the sources are not required to flash the Pico; the firmware file suffices.

Assembly Notes

The Microtronic's 2114 SRAM socket must be replaced with a standard (ideally, machined) DIP socket so that the ribbon cable can be plugged in:

SRAM Socket

DIP Socket

Moreover, the R12 (DISP) line requires a pin-connector on the Microtronic PCB - there is a via on the Microtronic PCB which can be exploited:

R12 Via

The modified Microtronic PCB should then look as follows:

PicoRAM 5

PicoRAM is easy to assemble: all components are through-hole, and readily available and pre-assembled off-the-shelf modules are used. No SMD soldering skills are required.

I recommend using a machined DIP socket for the ribbon wire cable connector (these crimp connectors are hard to come by, btw):

PicoRAM 4 PicoRAM 5 PicoRAM 6

Bill of Material


Reference Description
C1 100 uF Polarized
C2 100 nF
R1, R7, R8 2k Ohm
R2 330 Ohm
R3 620 Ohm
R4 1k Ohm
R5 3.3k Ohm
R6 120 Ohm
RN1, RN2 20k Ohm Isolated Resistor Network (DIP-16 W=7.62 mm)
RN5 1k Ohm Isolated Resistor Network (DIP-16 W=7.62 mm)
RN6 2k Ohm Isolated Resistor Network (DIP-16 W=7.62 mm)
RV1 101 (100 Ohm) Potentiomer / Trimmer
SW1, SW2, SW6 DPDT 8.5 mm Switch (GRAY, NOT BLUE!!)
D1, D2 3.0 mm LEDs
J5 Stereo Socket
LS1 Loudspeaker
J1, J2 PAM8403 Class D audio amplifier
Brd1 SSD1306 128x64 OLED Display
U1 RTC DS3231
U2 Raspberry Pi Pico
U3 MIKROE-2253 TextToSpeech Click!
U4 AdaFruit MICROSD Module
U5 Machined Socket (DIP-18, W=7.62mm)

For SW, SW2, SW6, note that the common pin of these switches needs to be in the middle! The blue and gray switches from Amazon have a different pinout - get the gray ones, the blue ones don't work for my Gerbers / schematics! Use the 8.5 x 8.5 mm version of these switches - the 7 x 7 mm version doesn't fit the footprint, and the 8 x 8 mm "blue" version has an incompatible pin out.

Moreover, I am using Mounting Feet and crimpable 18-pin DIP rectangular cable assembly connectors (hard to find!). But ordinary DuPont cables will also do.

Acknowledgements

  • Harry Fairhead for his execellent book

  • Hans Hübner (aka Pengo) for motivating me to abandon the BluePill, ATmegas and Arduinos, and for helping to get started and troubleshooting!

  • The authors of the libraries that I am using:

Dedication

Dedicated to my late father, Rudolf Wessel (26.12.1929 - 15.10.2023).

R.I.P, Papa - I will always fondly remember the days following Christmas 1983 (for which you and Mom got me the Microtronic) when we entered the dauntingly big "Lunar Lander" program together. In loving memory!