E-ink Display Driver

The display driver handles initialisation, full/partial refresh, and front-light PWM brightness control.

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Library: GxEPD2

GxEPD2 is the most widely used e-ink library for Arduino/ESP32. It supports the SSD1683 controller used by the GDEY042T81-FL02.

GxEPD2 was designed for Arduino, but works under ESP-IDF via the arduino-esp32 component or the esp32-arduino-lib-builder.

Simplest approach: use PlatformIO with the Arduino framework for this project, then add ESP-IDF components for I2S and FATFS. PlatformIO handles the toolchain for you.

Alternatively, port to native ESP-IDF using Waveshare's e-Paper ESP-IDF library as a reference.

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Initialisation

#include <GxEPD2_BW.h>
#include <Adafruit_GFX.h>

// GDEY042T81-FL02 with SSD1683
GxEPD2_BW<GxEPD2_420_GDEY042T81, GxEPD2_420_GDEY042T81::HEIGHT>
    display(
        GxEPD2_420_GDEY042T81(
            /*CS=*/   10,   // GPIO10
            /*DC=*/    5,   // GPIO5
            /*RST=*/   4,   // GPIO4
            /*BUSY=*/  3    // GPIO3
        )
    );

void display_init(void) {
    display.init(115200, true, 15);  // serial speed, initial=true, reset duration=15ms
    display.setFullWindow();
    display.firstPage();
    do {
        display.fillScreen(GxEPD_WHITE);
    } while (display.nextPage());
}

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Full vs. Partial Refresh

Mode	Duration	Ghosting	Use when
Full refresh	~2 s	None	Time has changed, menu entered, first draw
Partial refresh	~0.3 s	Accumulates	Clock digits update every minute

Run a full refresh at least every 10 partial refreshes to clear ghosting:

static int partial_count = 0;

void display_update(bool force_full) {
    if (force_full || partial_count >= 10) {
        display.setFullWindow();
        partial_count = 0;
    } else {
        display.setPartialWindow(0, 0, 200, 80);  // update digits area only
        partial_count++;
    }

    display.firstPage();
    do {
        draw_clock_face();
    } while (display.nextPage());
}

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Drawing Text

GxEPD2 uses the Adafruit GFX API for drawing:

#include <Fonts/FreeSansBold48pt7b.h>  // large font for time

void draw_clock_face(void) {
    display.fillScreen(GxEPD_WHITE);

    // Time (large, centred)
    display.setFont(&FreeSansBold48pt7b);
    display.setTextColor(GxEPD_BLACK);
    display.setCursor(40, 160);

    char time_str[6];
    snprintf(time_str, sizeof(time_str), "%02d:%02d",
        current_time.tm_hour, current_time.tm_min);
    display.print(time_str);

    // Date (smaller, below)
    display.setFont(NULL);  // back to built-in small font
    display.setTextSize(2);
    display.setCursor(100, 200);
    char date_str[16];
    strftime(date_str, sizeof(date_str), "%a %d %b", &current_time);
    display.print(date_str);
}

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Front Light PWM

The GDEY042T81-FL02's front light is controlled by a PWM signal on GPIO2. Use the ESP32 LEDC peripheral:

#include "driver/ledc.h"

void frontlight_init(void) {
    ledc_timer_config_t timer = {
        .speed_mode      = LEDC_LOW_SPEED_MODE,
        .timer_num       = LEDC_TIMER_0,
        .duty_resolution = LEDC_TIMER_8_BIT,   // 0–255
        .freq_hz         = 5000,
        .clk_cfg         = LEDC_AUTO_CLK,
    };
    ledc_timer_config(&timer);

    ledc_channel_config_t channel = {
        .speed_mode = LEDC_LOW_SPEED_MODE,
        .channel    = LEDC_CHANNEL_0,
        .timer_sel  = LEDC_TIMER_0,
        .gpio_num   = GPIO_NUM_2,
        .duty       = 128,    // 50% brightness
        .hpoint     = 0,
    };
    ledc_channel_config(&channel);
}

void frontlight_set(uint8_t brightness) {
    ledc_set_duty(LEDC_LOW_SPEED_MODE, LEDC_CHANNEL_0, brightness);
    ledc_update_duty(LEDC_LOW_SPEED_MODE, LEDC_CHANNEL_0);
}

Call frontlight_set(0) to turn the light off, frontlight_set(255) for maximum.

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Memory Note

GxEPD2 allocates ~15 KB for the 400×300 1bpp frame buffer. This requires PSRAM on the ESP32-S3 (DevKitC-1 N16R8 has 8 MB; WROOM-2 N32R16 has 16 MB — both are sufficient). If you see a heap_caps_malloc failed panic, PSRAM is not enabled — check sdkconfig under Component config → ESP PSRAM.