Matrix: Systick, please

src/matrix/matrix.c

@@ -67,6 +67,10 @@ int main(void)
 	rcc_periph_clock_enable(RCC_GPIOB);
 	rcc_periph_clock_enable(RCC_GPIOC);
 
+	systick_set_clocksource(STK_CSR_CLKSOURCE_AHB);
+	systick_set_reload(71999);
+	systick_counter_enable();
+
 	// PC13 = BluePill LED
 	gpio_set_mode(GPIOC, GPIO_MODE_OUTPUT_50_MHZ, GPIO_CNF_OUTPUT_PUSHPULL, GPIO13);