diff --git a/mcu/main.c b/mcu/main.c
index 98c45bd..aafa7c8 100644
--- a/mcu/main.c
+++ b/mcu/main.c
@@ -7,20 +7,39 @@
 const uint PIN_DIGIT_BASE = 18;
 const uint PIN_SEGMENT_BASE = 2;
 
-const uint DIGITS = 4;
+const uint N_DIGITS = 4;
+
+uint8_t digit_states[4] = { 0b11001100, 0b11111111, 0b10101010, 0b00101001 };
+
+void write_digit(uint8_t digit) {
+  const uint mask = (0xffffffff << PIN_SEGMENT_BASE) ^ (0xffffffff << PIN_SEGMENT_BASE + 8);
+  gpio_put_masked(mask, (~digit & 0xff) << PIN_SEGMENT_BASE);
+  gpio_put_masked(mask, digit << PIN_SEGMENT_BASE);
+}
 
 int main() {
   bi_decl(bi_program_description("qCLK driver."));
 
   stdio_init_all();
 
-  gpio_init(LED_PIN);
-  gpio_set_dir(LED_PIN, GPIO_OUT);
+  for (uint i = PIN_DIGIT_BASE; i < PIN_DIGIT_BASE + N_DIGITS; i++) {
+    gpio_init(i);
+    gpio_set_dir(i, GPIO_OUT);
+    gpio_put(i, 0);
+  }
+  for (uint i = PIN_SEGMENT_BASE; i < PIN_SEGMENT_BASE + 8; i++) {
+    gpio_init(i);
+    gpio_set_dir(i, GPIO_OUT);
+  }
+  write_digit(0);
+
   while (1) {
-    gpio_put(LED_PIN, 0);
-    sleep_ms(250);
-    gpio_put(LED_PIN, 1);
-    puts("Hello World\n");
-    sleep_ms(1000);
+    for (uint i = 0; i < N_DIGITS; i++) {
+      uint prev = i == 0 ? N_DIGITS - 1 : i - 1;
+      gpio_put(PIN_DIGIT_BASE + prev, 0);
+      write_digit(digit_states[i]);
+      gpio_put(PIN_DIGIT_BASE + i, 1);
+      sleep_ms(1);
+    }
   }
 }