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ZeroKeyUSB replaces mechanical buttons with five copper touch pads connected to a dedicated TS06 capacitive controller. The firmware polls this controller over SPI and translates gestures into navigation events.

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Hardware overview

ComponentPurpose
TS06Capacitive touch controller handling up to six channels
PadsLeft, Right, Up, Down, Center (gold-plated on PCB)
SPI busSERCOM1 @ 2 MHz
IRQ lineSignals when a pad changes state
The controller runs with its own baseline calibration and reports 8-bit touch intensity values, eliminating the need for analog tuning in firmware.

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Polling cycle

  1. The TS06 asserts the IRQ pin when a pad crosses the sensitivity threshold.
  2. The SAMD21 wakes (if sleeping) and performs an SPI transaction to read the status register.
  3. A 5-sample moving average filters noise and prevents accidental triggers.
  4. The debounced result is converted into one of the logical actions: LEFT, RIGHT, UP, DOWN, CENTER, or LONG_PRESS.
The loop runs every 8 ms at most, balancing responsiveness with low power consumption.

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Gesture mapping

GestureTriggerAction
Tap left/rightSingle pressNavigate between menu screens
Tap up/downSingle pressChange character or scroll items
Tap centerSingle pressConfirm selection / open item
Hold center>700 msApprove sensitive actions (import, erase, backup)
Hold right>700 msJump to next credential slot
Hold left>700 msReturn to previous menu level
Long presses require the pad to remain above the threshold for the duration; releasing early cancels the action.

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Debounce algorithm

if (raw_state != stable_state) {
  counter++;
  if (counter > 3) {
    stable_state = raw_state;
    emit_event(stable_state);
    counter = 0;
  }
} else {
  counter = 0;
}
  • The counter increments every poll when the raw state differs from the last stable state.
  • After four consecutive mismatches, the change is accepted.
  • Returning to the neutral state follows the same logic, avoiding rapid toggling.
This simple approach handles electrical noise and finger gliding without adding latency.

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Calibration and drift

  • On boot, the firmware issues CALIBRATE_ALL to the TS06 to set baseline capacitance values.
  • If ambient conditions change (humidity, temperature), a background task recalibrates idle pads after 5 seconds of no touch activity.
  • The controller tracks per-pad offsets, so neighboring touches do not interfere with each other.
No calibration data is stored in EEPROM; the TS06 handles persistence internally.

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Error handling

  • SPI timeouts fall back to a safe state and display TOUCH ERR on the OLED.
  • If the controller does not respond after three retries, the firmware disables menu navigation and instructs the user to reconnect power.
  • Diagnostic information is available through the serial TOUCHSTATUS command.
The result is a responsive, silent input system that matches the compact form factor of ZeroKeyUSB.
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