Add timer.{h, c}

src/timer.c

@@ -0,0 +1,22 @@
+#include "rcc.h"
+#include "timer.h"
+
+void tim4_init(void) {
+  // Enable timer
+  RCC->APB1ENR |= RCC_APB1ENR_TIM4_ENABLE;
+
+  // Reset timer
+  TIM4->CR1 = 0x0000;
+  TIM4->CR2 = 0x0000;
+
+  // Set prescaler
+  // f_clk = 48MHz -> /48000 = 1KHz counting frequency = 1ms
+  TIM4->PSC = (uint16_t) 48000 - 1;
+
+  // Set ARR to maximum value to get 1ms between updates
+  TIM4->ARR = (uint16_t) 0xFFFF;
+}
+
+void tim4_start(void) {
+  TIM4->CR1 |= TIM4_ENABLE;
+}