HP RedEye IR Receiver (RP2040 → UART)

This repository contains an implementation of the HP RedEye infrared protocol (receiver) using the RP2040 microcontroller.
The firmware receives IR data from an HP calculator, decodes the RedEye protocol, and forwards the decoded data to UART0.

The project was developed and tested with an HP 28C calculator, but it may work with other HP devices that use the RedEye IR protocol.

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Overview

The system works as follows:

1. An IR signal is transmitted by the calculator (e.g. HP 28C).
2. A TSMP58000 IR receiver module captures the modulated IR signal.
3. The RP2040 decodes the HP RedEye protocol in firmware.
4. The decoded bytes are transmitted via UART0.

This effectively converts HP RedEye IR communication into a standard UART data stream, making it easy to interface with:

• PCs (via USB-UART bridge)
• Logic analyzers
• Embedded systems
• Data logging setups

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Hardware

Microcontroller

• RP2040 (e.g. Raspberry Pi Pico or compatible board)

IR Receiver

• TSMP58000 (33 kHz IR Photo detector and preamplifier in one package)

Connections (Typical)

Signal	Pin	Notes
TSMP58000 OUT	GPIO 2 (DATA_PIN)	IR signal input
TSMP58000 VCC	3.3V
TSMP58000 GND	GND
UART0 TX	GPIO 0	115200 baud, connect to RX of target device
GND	GND	Common ground with target device
GPIO 3 (HALF_BIT_RECEIVED_PIN)	optional	Debug probe — toggles on each half-bit, useful with a logic analyzer

Pin assignments can be changed by editing the #define constants at the top of src/main.cpp.

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Protocol

This project implements the HP RedEye IR protocol, which was used in several classic HP calculators for:

• Printer communication
• Data transfer
• Peripheral connectivity

The firmware performs:

• Pulse timing capture
• Frame synchronization
• Bit decoding
• Byte reconstruction
• UART transmission

The UART output contains decoded RedEye payload bytes in real time.

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Examples

Print LCD pixel-by-pixel (byte 27 triggers drawing mode):

Print RPN stack and a single numeric value:

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Building & Flashing

Requires PlatformIO (e.g. via the VS Code extension or CLI).

# Build and upload
pio run --target upload

# Open serial monitor (115200 baud)
pio device monitor

The USB serial port (Serial) outputs debug info. Serial1 (UART0, GPIO 0) outputs the decoded bytes.

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Project Status

This is a hobbyist project created for experimentation and personal use.

It was developed to interface with an HP 28C calculator but may require modification for other HP devices or timing variations.

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Disclaimer

This project is provided as-is, without any warranty.

• It is an amateur implementation of the HP RedEye protocol.
• Timing accuracy may depend on clock configuration and firmware settings.
• There is no guarantee of compatibility with all RedEye devices.
• You are responsible for verifying electrical compatibility before connecting hardware.
• Improper wiring may damage your RP2040 board or IR module.

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Motivation

Many classic HP calculators use the RedEye IR protocol, but modern computers do not natively support it.
This project bridges that gap by converting legacy IR communication into a simple UART stream that can be easily captured and analyzed. I also implemented graphics mode, so you could "print" the whole LCD and transfer using UART.

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Contributions, testing reports, and protocol improvements are welcome.