Add ssd1306 digital clock drawn using Cairo

ssd1306-cairo-clock.c

@@ -0,0 +1,126 @@
+// Draw a digital clock using Cairo on SSD1306 LCD in a Raspberry PI
+// $ cc ssd1306-cairo-clock.c `pkg-config --cflags --libs cairo` && ./a.out
+// Originally based on https://github.com/pive/rpi1306i2c/blob/master/rpi1306i2c.hpp
+
+#include <assert.h>
+#include <fcntl.h>
+#include <cairo.h>
+#include <linux/i2c-dev.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <time.h>
+#include <unistd.h>
+
+#define MEMORYMODE          0x20
+#define COLUMNADDR          0x21
+#define PAGEADDR            0x22
+#define SETSTARTLINE        0x40
+#define SETCONTRAST         0x81
+#define CHARGEPUMP          0x8D
+#define SEGREMAP            0xA0
+#define DISPLAYALLON_RESUME 0xA4
+#define DISPLAYALLON        0xA5
+#define NORMALDISPLAY       0xA6
+#define SETMULTIPLEX        0xA8
+#define DISPLAYOFF          0xAE
+#define DISPLAYON           0xAF
+#define SETPAGE             0xB0
+#define COMSCANINC          0xC0
+#define COMSCANDEC          0xC8
+#define SETDISPLAYOFFSET    0xD3
+#define SETDISPLAYCLOCKDIV  0xD5
+#define SETPRECHARGE        0xD9
+#define SETCOMPINS          0xDA
+#define SETVCOMDETECT       0xDB
+
+const uint8_t ssd1306_128x32_init_seq[] = {
+  0x80, DISPLAYOFF,
+  0x80, SETDISPLAYCLOCKDIV, 0x80, 0x80,
+  0x80, SETMULTIPLEX, 0x80, 0x1F,
+  0x80, SETDISPLAYOFFSET, 0x80, 0x00,
+  0x80, SETSTARTLINE | 0x00,
+  0x80, CHARGEPUMP, 0x80, 0x14,
+  0x80, SEGREMAP,
+  0x80, COMSCANDEC,
+  0x80, SETCOMPINS, 0x80, 0x12,
+  0x80, SETCONTRAST, 0x80, 0x1,//0x7F, brightness
+  0x80, SETPRECHARGE, 0x80, 0x22,
+  0x80, SETVCOMDETECT, 0x80, 0x20,
+  0x80, MEMORYMODE,
+  0x80, DISPLAYALLON_RESUME,
+  0x80, NORMALDISPLAY,
+  0x80, COLUMNADDR, 0x80, 0, 0x80, 128 - 1,
+  0x80, PAGEADDR, 0x80, 0, 0x80, (32 >> 3) - 1,
+  0x80, DISPLAYON,
+};
+
+static void writeBuffer(int deviceNode, const uint8_t *buffer, unsigned size) {
+  if (write(deviceNode, buffer, size) != size) {
+    printf("Could not write on the device");
+    abort();
+  }
+}
+
+static void debug(uint8_t *buffer, unsigned stride) {
+  for (unsigned i = 0; i < 32; i++) {
+    for (unsigned j = 0; j < 16; ++j) {
+      for (unsigned k = 0; k < 8; ++k) {
+        printf(buffer[i * stride + j] & 1 << k ? "*" : " ");
+      }
+    }
+    printf("|\n");
+  }
+}
+
+int main() {  
+  const int deviceNode = open("/dev/i2c-1", O_RDWR);
+  const uint8_t addr = 0x3C;
+  if (deviceNode < 0 || ioctl(deviceNode, I2C_SLAVE, addr) < 0) {
+    printf("Could not open\n");
+    abort();
+  }
+
+  writeBuffer(deviceNode, ssd1306_128x32_init_seq, sizeof(ssd1306_128x32_init_seq));
+
+  const int stride = cairo_format_stride_for_width(CAIRO_FORMAT_A1, 128);
+  assert(stride == 16);
+  uint8_t data[512];
+  cairo_surface_t *cairo_surface = cairo_image_surface_create_for_data(
+    data, CAIRO_FORMAT_A1, 128, 32, stride
+  );
+  cairo_t *cr = cairo_create(cairo_surface);
+  cairo_select_font_face(cr, "monospace", CAIRO_FONT_SLANT_NORMAL, CAIRO_FONT_WEIGHT_NORMAL);
+  cairo_set_font_size(cr, 42);
+  cairo_set_source_rgb(cr, 1, 1, 1);
+
+  uint8_t buffer[513] = {SETSTARTLINE};
+  while (1) {
+    const time_t now = time(0);
+    struct tm *tm = localtime(&now);
+    char str[8] = {};
+    snprintf(str, 6, "%02d%c%02d", tm->tm_hour, now % 2 ? ':' : ' ', tm->tm_min);
+
+    memset(data, 0, stride * 32);
+    cairo_move_to(cr, 0, 32);
+    cairo_show_text(cr, str);
+    debug(data, stride);
+
+    for (unsigned i = 0; i < sizeof(buffer) - 1; ++i) {
+      unsigned x = 127 - (i % 128);
+      unsigned y = (i / 128) * 8;
+      #define P(j) ((unsigned) !!(data[x / 8 + (y + j) * 16] & (1 << (7 - (x % 8)))) << j)
+      buffer[i + 1] = P(0) | P(1) | P(2) | P(3) | P(4) | P(5) | P(6) | P(7);
+      #undef P
+    }
+    writeBuffer(deviceNode, buffer, sizeof(buffer));
+    usleep(1000000);
+  }
+
+  cairo_surface_destroy(cairo_surface);
+  cairo_destroy(cr);
+  close(deviceNode);
+  return 0;
+}