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Inputronic Keyboard - How it works

The Inputronic Keyboard is built around the TCA8418 chip by Texas Instruments. When using our board, you are essentially communicating with the onboard TCA8418 directly via I²C communication.

TCA8418 chip on board
TCA8418 chip on the board

Datasheet​

For an in-depth look at technical specifications, refer to the official TCA8418 Datasheet

TCA8418 Datasheet

Detailed technical documentation for the TCA8418 keypad scanner

How the scanner works​

The TCA8418 is a keypad scanning controller that continuously monitors a matrix of buttons arranged in rows and columns. The chip configures column pins as outputs (driven low) and row pins as inputs with internal pull-up resistors. When a key is pressed, it creates an electrical connection between a row and column, pulling the row input low and alerting the chip to scan the matrix. The scanner then systematically drives one column at a time while reading all row inputs to determine exactly which key was pressed. Each key press and release is automatically debounced using a 50 µs hardware filter and stored in a 10-event FIFO buffer along with whether it was a press or release. The chip generates an interrupt signal to notify your microcontroller that key events are ready to be read via I²C, allowing efficient event-driven operation without constant polling.

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The TCA8418 supports up to 80 keys in an 8×10 matrix configuration and can detect multiple simultaneous key presses accurately!

I²C communication​

The Inputronic Keyboard uses the I²C protocol to communicate with the microcontroller. It operates with a fixed I²C address of 0x34. Upon request, the chip responds with key event data from its FIFO buffer, including which key was pressed or released. The board supports I²C speeds up to 1 MHz (Fast Mode Plus), but you can use our library to interface with it more easily.

Key Features Explained​

Event FIFO​

The 10-byte FIFO stores key press and release events. Each event contains:

  • Key value (7 bits): Which key was pressed (1–80 for matrix keys, 97–114 for GPIO)
  • Press/Release flag (1 bit): Indicates whether the key was pressed (1) or released (0)

The FIFO automatically shifts as you read events, making it easy to process multiple key presses in sequence.

Hardware Debouncing​

Mechanical switches naturally "bounce" when pressed, creating multiple electrical signals for a single press. The TCA8418 includes integrated hardware debouncing with a 50 µs filter that eliminates these false signals, ensuring each key press is registered only once.

Keypad Lock/Unlock​

The chip includes a programmable security feature that requires a two-key sequence to unlock:

  • Program two specific unlock keys
  • Set the time window between key presses (0–7 seconds)
  • Optionally configure an interrupt mask timer for LCD backlight control

This feature is useful for implementing device security or preventing accidental key presses.

Power Efficiency​

The TCA8418 is designed for battery-powered applications:

  • Idle mode: 10 µA when waiting for key press
  • Active scan: 50–90 µA during key scanning
  • Sleep mode: 3 µA in deep sleep

The internal oscillator automatically turns off when no keys are pressed, minimizing power consumption.