GxEPD2: drawing on an e-paper display

// E-paper name badge: a 2.13 inch black and white e-paper panel on an
// SSD1680, driven with GxEPD2, as a conference or club badge.
//
//   e-paper   SCL -> 13   SDA -> 11   CS -> 10   DC -> 9   RES -> 8   BUSY -> 7
//             VCC -> 3.3 V   GND -> GND
//   button    pin 2 to GND (the pin's pull-up holds it high)
//
// The badge draws its name once with a full refresh, which takes the panel
// about two seconds while BUSY is high. The button swaps the bottom line
// between the role and a contact line with a partial refresh, a fraction of
// a second, and then the controller goes back to deep sleep. E-paper keeps
// its picture with no power at all, so a real badge draws and then sleeps
// until the button is pressed again.
//
// The panel is 250 x 122 pixels, bigger than this board's 2 KB of RAM, so
// GxEPD2 draws it in pages of 50 rows: the do { } while (nextPage()) loop
// runs once for each page.

#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_213_BN, MAX_HEIGHT(GxEPD2_213_BN)>
    display(GxEPD2_213_BN(10, 9, 8, 7));  // CS, DC, RES, BUSY

const int BUTTON = 2;
bool contact = false;

void bottomLine() {
  display.fillRect(0, 96, 250, 26, GxEPD_WHITE);
  display.setTextColor(GxEPD_BLACK);
  display.setTextSize(2);
  display.setCursor(12, 102);
  display.print(contact ? "ADA@CLUB.ORG" : "ELECTRONICS CLUB");
}

void drawBadge() {
  display.setFullWindow();
  display.firstPage();
  do {
    display.fillScreen(GxEPD_WHITE);
    // The band across the top, white letters on black.
    display.fillRect(0, 0, 250, 24, GxEPD_BLACK);
    display.setTextColor(GxEPD_WHITE);
    display.setTextSize(2);
    display.setCursor(18, 5);
    display.print("HELLO, MY NAME IS");
    // The name, as big as it fits.
    display.setTextColor(GxEPD_BLACK);
    display.setTextSize(4);
    display.setCursor(30, 44);
    display.print("ADA");
    display.drawFastHLine(10, 88, 230, GxEPD_BLACK);
    bottomLine();
  } while (display.nextPage());
}

void drawBottom() {
  display.setPartialWindow(0, 96, 250, 26);
  display.firstPage();
  do {
    bottomLine();
  } while (display.nextPage());
}

void setup() {
  pinMode(BUTTON, INPUT_PULLUP);
  display.init(115200);
  display.setRotation(1);
  drawBadge();
  display.hibernate();
}

void loop() {
  if (digitalRead(BUTTON) == LOW) {
    contact = !contact;
    drawBottom();
    display.hibernate();
    while (digitalRead(BUTTON) == LOW) delay(10);
  }
  delay(20);
}