initial import

docs/overview.txt

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+# pocket computer M4
+
+The PCM4 (pocket computer / M4) is a handheld computing device based
+around the Keyboard FeatherWing [1] and the Adafruit Feather M4 [2].
+The Feather M4 has 512K of flash memory, 192K of RAM, and 2M QSPI [3]
+flash.
+
+[1] https://www.tindie.com/products/arturo182/keyboard-featherwing-qwerty-keyboard-26-lcd/
+[2] https://learn.adafruit.com/adafruit-feather-m4-express-atsamd51?view=all
+[3] https://github.com/adafruit/Adafruit_QSPI
+
+It will have a scheme-based operating system.
+
+## Feather M4 pinouts
+
++ A0: reserved
++ A1: reserved
++ A2: reserved for RFM9X CS
++ A3: reserved for RFM9X RST
++ A4: reserved for airlift ESPBUSY
++ A5: reserved for airlift ESPRESET
++ D4: reserved for RFM9X IRQ
++ D5: SD card CS
++ D6: STMPE CS
++ D9: LCD CS
++ D10: LCD DC
++ D11: neopixel
+
+## Peripherals
+
++ Display: 320x240 touchscreen
+  + ILI9341 LCD controller
+  + STMPE610 touchscreen
++ Keyboard: BB Q10 keyboard + 4x tactile buttons + 5-way switch
+  + I2C address: 0x1F
++ Planned: DS3231 RTC
+  + I2C address: 0x66
++ Planned: RFM95 LoRa featherwing
++ Planned: ESP32 airlift
++ Planned: ATECC608A cryptoprocessor
+
+## Boot sequence
+
+1. Stage 1: UF2 bootloader [4][5]
+2. Stage 2: base hardware environment
+   + hardware init
+   + I2C/SPI interface
+   + SD card
+   + RTC core
+   + scheme core
+3. Load boot.scm from SD card.
+
+Stretch goal:
++ Holding down the center button in the joystick should bring up a menu
+  of scheme files on the SD card, allowing the user to select the boot
+  code.
+
+Note that each stage is as minimal as possible, so stage 2 should
+contain just enough to load the operating system from boot.scm,
+which should contain the actual operating system.
+
+[4] https://learn.adafruit.com/adafruit-feather-m0-express-designed-for-circuit-python-circuitpython/uf2-bootloader-details
+[5] https://github.com/Microsoft/uf2