HIPI Board — HP-IL Pico Interface

HIPI is a small touch-screen box on a Raspberry Pi Pico 2 (RP2350) that sits on the HP-IL loop of an HP-41, HP-71 or HP-75 and acts as a whole set of classic HP peripherals at once:

Cassettes are LIF image files on a micro-SD card. Besides the device views (display, plotter, tape drive, printer) there is an HP-IL analyzer, a loop map, an HP-IL signal view, screen savers, screen dumps, and a mirror of the screen to a PC for recording videos.

The project started as a C++17 port of J. Chilla's MicroPython project JCILD (HP 82163 video display and HP 82161 drive emulator), including its RA8875 display driver (HIPI's first, 5" panel; after 1.3.0 HIPI has only the 7" LT7683 panel).

Display SPI wiring (same as the original share.py):

The build makes build/hipi_7_pico.uf2. The details, including the PicoLED library, are in GUIDE.md § 1–3.

1. Install the Raspberry Pi Pico extension.
2. pico-sdk 2.2.0: if the extension hasn't installed it, clone it and point PICO_SDK_PATH at it:

   git clone -b master https://github.com/raspberrypi/pico-sdk.git ~/pico-sdk
   cd ~/pico-sdk && git submodule update --init
   export PICO_SDK_PATH=$HOME/pico-sdk        # or $HOME/.pico-sdk/sdk/2.2.0

3. Import the project — not File → Open Folder, but Ctrl+Shift+P → Raspberry Pi Pico: Import Project, pick the folder with CMakeLists.txt, and board pico2 (or pico2_w).
4. Build with the Build button in the status bar or Ctrl+Shift+B. The first build takes a minute or two.

export PICO_SDK_PATH=$HOME/.pico-sdk/sdk/2.2.0   # or ~/pico-sdk
cmake -G Ninja -B build -S .
cmake --build build

Hold BOOTSEL while plugging in (or resetting) the Pico 2 and drop the .uf2 onto the drive that appears — or use picotool:

picotool load -f build/hipi_7_pico.uf2

Later updates can be started from the menu: More → System → Bootsel mode.

Everything the SD card needs is in HIPI-SD-card.zip in the project folder (and with every release):

1. Format a micro-SD card as FAT32 (exFAT works too).
2. Unzip HIPI-SD-card.zip onto it, keeping the folders.
3. Put your own cassettes (.dat or .lif files) in lif/.

The card then holds:

README.txt      what's what
resources/      Tape view pictures: hp82161a.bmp, tape-in.bmp, open.bmp, leds.bmp, reels.bmp
                Printer view pictures: hp82162a.bmp, prtparts.bmp
lif/            cassettes (*.dat or *.lif, LIF) — one example to start with
CONFIG.TXT      settings — created by HIPI on first start
screenshots/    screen dumps — created by the first Screendump
text/           the display's text (display_<n>.txt) — created by the first "Save as text"
logs/           analyzer logs — created by the first "Log to file"

The zip is made by sdcard/make_sd_card.py and remade by the build whenever one of its files changes. The script can also copy straight onto a mounted card: sdcard/make_sd_card.py --to /media/$USER/HIPI. To change what's on the card, edit sdcard/ (README, example cassettes) or the pictures in resources/.

The button strip and the logo are built into the firmware, so HIPI starts and its menus work without an SD card.

The User Manual has the full story; these are the essentials.

Buttons. Touch the right edge of the screen to slide in the button strip (it hides again after 5 s):

Touch. Top-left corner: info box. Bottom-left corner: the devices on the loop and their addresses. Swipe left/right: next/previous device view.

Menus. OK opens the menu of the view on the screen (e.g. Clear plotter in the Plotter view). Its last row, More, leads to HIPI's own menu:

More >
├── Go to view >   Display · Plotter · Tape · Analyzer · Loop map · HP-IL signals
├── Settings >     Screen · Screen saver · Devices · PC link
└── System >       Connect to PC · Loopback test · Hardware test · Demo · Character table ·
                   About · Bootsel mode

While a menu is open HIPI keeps working: nothing the HP-41 sends is lost, and the view underneath is up to date when the menu closes.

Settings are saved at once to CONFIG.TXT on the SD card as key=value lines; unknown keys are ignored, so old files keep working. All keys are listed in the User Manual § 12.

Characters. The display font (hp82163_font.hpp) is an 8×16 font with the HP-41 characters plus Å Ä Ö å ä ö. txtWrite() decodes UTF-8, so a string like "Fänge" in the source code shows correctly.

CMakeLists.txt   build: the firmware (hipi_7_pico)
include/, src/   the firmware (below)
lib/             no-OS-FatFS (SD card), vendored, unmodified
PicoLED/         WS2812 library for TFPIXEL, vendored
resources/       artwork: Tape and Printer view pictures (SD card), buttons and logo (built in)
sdcard/          the SD card: README.txt, example cassette in lif/, make_sd_card.py
HIPI-SD-card.zip everything for a new SD card (made by the build)
scripts/         build/flash helpers, bmp_to_rgb565.py (turns BMPs into C++)
tools/           PC programs: hipiview (display mirror), ilctrl (HP-IL controller), pixeltest,
                 printertest; images/ (how the Printer view pictures are made)
documents/       USER_MANUAL.md, GUIDE.md, images
pcb/             hardware: README, BOM, schematics, PCB

The firmware, by area (.h/.hpp in include/, .cpp in src/):

• Global state from share.py became constructor arguments.
• DisplayTransport is a virtual SPI/GPIO interface, so the platform is easy to swap (PicoSpiTransport, LinuxSpiDevTransport).
• delayMs() replaces time.sleep(); _write_reg/_write_reg16 became the public writeReg/writeReg16.
• The display runs in 16-bit RGB565 instead of 8-bit RGB332.
• New: the touch UI, persistent settings, the extra HP-IL devices and views, the LT7683 (7") driver, and the Swedish characters.

The HP-41 sends compact command strings to TFLEDS, for example "1B0:100:900" (LED 1 blinks forever, 100 ms on, 900 ms off). A command is <leds><command>[<parameters>]; several commands are separated by spaces.

LEDs: 1–5 select single LEDs and can be combined (135 = LEDs 1, 3 and 5); 0 selects all.

01 LBL "TEST"
02 3
03 SELECT
04 "1B0:100:900"
05 OUTA

More examples:

"0C"                  all LEDs off
"12O 345C"            LEDs 1 and 2 on, LEDs 3, 4 and 5 off
"0S50 0F+2000"        all LEDs: brightness 50 %, then fade on over 2 s
"1B3:100:200 2F+500"  LED 1 blinks 3 times; LED 2 fades on over 0.5 s

• Put a space between commands when a number is followed by the next LED digit: "1B5 23C", not "1B523C" (that is LED 1 blinking 523 times).
• \n, ; or flush() also end the last command.
• S and the F commands need PWM (CLED_NO_PWM not defined, see leds.h); without it, brightness does nothing and fades switch at once.

• MicroPython share.py + RA8875.py: J. Chilla, March 2026
• HP 82163A video interface for the HP-41
• HP-IL interface specification: https://literature.hpcalc.org/community/hp82166-is-en.pdf
• Control the world with HP-IL: https://literature.hpcalc.org/community/control-with-hp-il.pdf
• RA8875 (CircuitPython driver): https://github.com/adafruit/Adafruit_CircuitPython_RA8875
• 7" panel (LT7683): https://www.buydisplay.com/spi-7-inch-tft-lcd-dislay-module-1024x600-ra8876-optl-touch-screen-panel
• pico-sdk: https://github.com/raspberrypi/pico-sdk
• Pico VS Code extension: https://github.com/raspberrypi/pico-vscode