Wiring

Adafruit Feather ESP32-S3 with the Waveshare 4.2” e-Paper Module (SKU 13353, 400×300, b/w). The pin numbers are also constants in src/board.py.

Display (SPI)

Display	Feather	GPIO	Function
VCC	3V	–	3.3 V supply
GND	GND	–	Ground
DIN	MO	35	SPI MOSI (data to the display)
CLK	SCK	36	SPI clock
CS	D10	10	Chip select, active low
DC	D9	9	Low = command, high = data
RST	D6	6	Reset, active low
BUSY	D5	5	Display busy (input)
PWR	D11	11	Supply switch (only newer modules with a 9-pin cable)

MISO is not needed; the display sends no data back.

Already in use

Function	GPIO	Note
I²C SDA (MAX17048)	3	Fuel gauge at address 0x36
I²C SCL (MAX17048)	4
I²C power	7	Must be high for the fuel gauge to run
Onboard LED (D13)	13	Left free
USB detection (A5)	8	Voltage divider, see below

A0–A4 (GPIO 18, 17, 16, 15, 14) are still free, for buttons for instance.

USB detection

A voltage divider tells the board directly whether USB power is present:

USB ──[68 kΩ]───┬── A5 (GPIO 8)
                │
GND ──[100 kΩ]──┘

It brings the 5 V of the Feather’s “USB” pin down to about 3.0 V (3.1 V at 5.25 V), within the ADC’s range (up to about 3.1 V with 11 dB attenuation) and safely above the level the pin needs to count as high (about 2.5 V): plugging in USB then wakes the board from deep sleep at once. The board counts it as USB power from 4.0 V before the divider. Current only flows while USB is plugged in, and then from USB, so it does not drain the battery. A5 is on ADC1, which works alongside the WLAN.

Enable it with USB_SENSE_PIN = 8 in src/config.py; other resistors need USB_SENSE_DIVIDER. A 100 kΩ/100 kΩ divider (ratio 2) measures fine, but at 2.5 V it is too close to the threshold to wake the board reliably. http://epaper.local/battery shows the measured USB voltage (usb_volts) for checking. Without the divider the board guesses USB power from the fuel gauge (charging, or full and stable); that takes a few minutes, and the battery recovering after a long awake phase can read as charging.