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The Microchip ATSAMD21G18 is the core of ZeroKeyUSB. It combines a 32-bit ARM Cortex-M0+ CPU, built-in USB controller, and enough peripherals to coordinate the display, touch inputs, and external EEPROM.

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Key specifications

FeatureValue
CPUARM Cortex-M0+ @ 48 MHz
Flash256 KB (firmware occupies ~60 KB)
SRAM32 KB
USBFull-speed device with HID + CDC composite support
GPIO38 general-purpose pins
Timers9 (TC/TCC) used for PWM, debouncing, and TOTP timing
ADC12-bit, used for IV entropy sampling
The firmware runs from internal flash and executes entirely from zero-wait-state memory, keeping latency low even while updating the OLED display.

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Clock configuration

  1. Internal 8 MHz oscillator feeds the Digital Frequency Locked Loop (DFLL).
  2. DFLL multiplies to 48 MHz for the CPU and synchronous peripherals.
  3. The generic clock controller divides the 48 MHz clock for:
    • 1 MHz I²C (SERCOM3) used by EEPROM and OLED
    • 2 MHz SERCOM1 for the touch controller SPI
    • 32 kHz reference for millisecond timing (via SysTick)
This configuration balances performance with low noise for the touch sensor.

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Peripheral mapping

PeripheralSERCOMFunction
SERCOM0USARTCDC serial channel (TX/RX on USB pads)
SERCOM1SPITouch controller (TS06)
SERCOM2I²CReserved/debug headers
SERCOM3I²CEEPROM (M24C64-W) + OLED display (SSD1306)
SERCOM4UnusedAvailable for future expansions
SERCOM5USBNative USB full-speed interface
The PORT multiplexer assigns each SERCOM to specific pins; see the KiCad design for exact pad numbers.

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Memory layout

  • Bootloader (8 KB) – UF2-compatible loader for factory flashing and community updates.
  • Application (240 KB max) – ZeroKeyUSB firmware; currently uses <30% of available flash.
  • EEPROM emulation is unused; all persistent data lives in the external M24C64-W.
  • SRAM buffers:
    • 512 bytes for OLED frame buffer
    • 128 bytes for USB HID reports
    • 96 bytes scratch space for AES blocks
The linker script reserves stack space for nested menu rendering and cryptographic routines.

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Power and sleep

  • The MCU runs in active mode while connected; consumption stays below 25 mA for the whole board.
  • After 60 seconds of inactivity the firmware dims the OLED and places the CPU into Standby while keeping USB active.
  • Touch or USB activity wakes the chip in under 3 ms.
This behavior ensures responsive interaction without exceeding USB current limits.

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Firmware responsibilities

  • Authenticating the Master PIN using AES routines.
  • Orchestrating the menu, display, and touch interactions.
  • Managing EEPROM read/write operations with wear-level tracking.
  • Generating USB HID reports and processing CDC commands.
  • Calculating TOTP codes using integer arithmetic (no floating point required).
The SAMD21 provides enough headroom to add features such as multiple keyboard layouts or additional security checks without hardware changes.
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