ESP32-C3

The ESP32-C3 is the brains of the Xteink X4. It's an ultra low power chip from Espressif and is really popular in IoT devices. The only downside is it doesn't have any PSRAM and only has 400 KB of SRAM. However, the Xteink X4 has 16 MB of flash for a ton of storage.

eInk Display

The Xteink X4 uses an SSD1677 controller with a 4.26" 800x480 GDEQ0426T82 display. It is available as a built-in monochrome eInk display with the CircuitPython firmware. It is connected via SPI:

• SCLK: GPIO8 (board.EPD_SCK)
• MOSI: GPIO10 (board.EPD_MOSI)
• CS: GPIO21 (board.EPD_CS)
• DC: GPIO4 (board.EPD_DC)
• RST: GPIO5 (board.EPD_RST)
• BUSY: GPIO6 (board.EPD_BUSY)

Buttons

There are 7 buttons available on the Xteink X4: the power button, up button, down button, right button, left button, confirm button and back button.

There is a bit of a twist though: only the power button is connected directly to a GPIO pin (GPIO3 / board.BUTTON). The rest of the six buttons are read with a resistor ladder that are connected to two ADC pins (GPIO1 / board.BUTTON_ADC_1 and GPIO2 / board.BUTTON_ADC_1). Each button reads a different set of analog values when pressed and that's how they are read in software. The CircuitPython helper library takes care of reading the two ADC inputs.

microSD Card

On the right side of the eReader is a microSD card. It uses the same SPI bus as the display and uses GPIO12 (board.SD_CS) as its CS pin and GPIO7 for MISO (board.SD_MISO).

Battery Monitoring

You can monitor the battery voltage with GPIO0 (board.A0). That pin is connected to the battery with a voltage divider. GPIO20 (board.D20) also detects if the battery is charging via USB.