firmware: add README

2025-04-07 20:26:04 -04:00
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+# Inkclip Firmware
+
+This is the firmware implementation for Inkclip. It implements a USB 2.0 HID device with the following reports:
+
+- **Write Pattern:** Report ID = 0x01, Direction = Output (host to device), Size = 5,000 bytes.
+
+  Updates the pattern on the e-paper display of the device. The 5,000 bytes are interpreted as a 40,000-bit bit vector that represents a bitmap of the 200x200 pattern to be displayed. Each byte is the little-endian representation of 8 pixels on the display, from left to right, from top to bottom, in row-major order.
+
+  Namely, the `k`th least significant bit in the `n`th byte of the report represents the pixel value at coordinates `(x, y) = (n % 25 + k, n / 25)`.
+
+- **Serial Number Request:** Report ID = 0x02, Direction = Output (host to device), Size = 1 byte.
+
+  Sends a request for the device's unique serial number. The content of the 1-byte payload is ignored.
+
+- **Serial Number Response:** Report ID = 0x02, Direction = Input (device to host), Size = 16 bytes.
+
+  An ASCII representation of the device's unique serial number. Currently, this is implemented as the base-64 encoding of the 12-byte UID of the STM32 MCU.
+
+## Implementation Details
+
+- The USB device sets the maximum current to 100mA, which should be well above the active power draw of the actual hardware (9mA MCU + 8mA e-paper).
+- The SPI link between the MCU and the e-paper display is 20MHz, which is the maximum supported frequency listed on the display module's datasheet.
+- This firmware is implemented in no_std Rust with the [embassy](https://embassy.dev) framework and the [embassy-stm32](https://docs.embassy.dev/embassy-stm32/git/stm32f411ce/index.html) HAL.