Shelf label on an Arduino Uno

// Shelf label: a 2.9 inch e-paper panel on an SSD1680, driven with GxEPD2,
// as the price tag on a shop shelf.
//
//   e-paper   SCL (CLK) -> 13   SDA (DIN) -> 11   CS -> 10   DC -> 9
//             RES (RST) -> 8    BUSY -> 7         VCC -> 3.3 V   GND -> GND
//   button    pin 2 to GND (the pin's pull-up holds it high)
//
// The label is drawn with a full refresh, about three seconds with BUSY
// high, and then the controller sleeps: e-paper keeps the picture with no
// power, which is why shelf labels run for years on a coin cell. The button
// marks the item down with a partial refresh of the price alone.
//
// The panel is 296 x 128 pixels; GxEPD2 draws it in pages that fit this
// board's RAM.

#include <GxEPD2_BW.h>

#define MAX_DISPLAY_BUFFER_SIZE 800
#define MAX_HEIGHT(EPD) (EPD::HEIGHT <= MAX_DISPLAY_BUFFER_SIZE / (EPD::WIDTH / 8) ? \
                         EPD::HEIGHT : MAX_DISPLAY_BUFFER_SIZE / (EPD::WIDTH / 8))

GxEPD2_BW<GxEPD2_290_BS, MAX_HEIGHT(GxEPD2_290_BS)>
    display(GxEPD2_290_BS(10, 9, 8, 7));  // CS, DC, RES, BUSY

const int BUTTON = 2;
bool sale = false;

void price() {
  display.fillRect(150, 40, 146, 60, GxEPD_WHITE);
  display.setTextColor(GxEPD_BLACK);
  display.setTextSize(4);
  display.setCursor(156, 52);
  display.print(sale ? "$2.99" : "$4.99");
}

void drawLabel() {
  display.setFullWindow();
  display.firstPage();
  do {
    display.fillScreen(GxEPD_WHITE);
    display.fillRect(0, 0, 296, 28, GxEPD_BLACK);
    display.setTextColor(GxEPD_WHITE);
    display.setTextSize(2);
    display.setCursor(8, 6);
    display.print("FRESH BREAD");
    display.setTextColor(GxEPD_BLACK);
    display.setTextSize(2);
    display.setCursor(8, 52);
    display.print("Sourdough");
    display.setCursor(8, 76);
    display.print("800 g");
    display.drawFastHLine(8, 108, 280, GxEPD_BLACK);
    price();
  } while (display.nextPage());
}

void drawPrice() {
  display.setPartialWindow(150, 40, 146, 60);
  display.firstPage();
  do {
    price();
  } while (display.nextPage());
}

void setup() {
  Serial.begin(9600);
  pinMode(BUTTON, INPUT_PULLUP);
  display.init(115200);
  display.setRotation(1);
  drawLabel();
  display.hibernate();
  Serial.println("label drawn");
}

void loop() {
  if (digitalRead(BUTTON) == LOW) {
    sale = !sale;
    drawPrice();
    display.hibernate();
    Serial.println(sale ? "marked down" : "full price");
    delay(300);
  }
}