Add basic thermal printer control

src/system.c

@@ -0,0 +1,32 @@
+#include <stdint.h>
+#include "stm32f103x6.h"
+
+void SystemInit(void)
+{
+    // Activate HSE and wait for it to be ready
+    RCC->CR = RCC_CR_HSEON;
+    while(!(RCC->CR & RCC_CR_HSERDY));
+
+    RCC->CFGR = RCC_CFGR_SW_0;
+    while((RCC->CFGR & RCC_CFGR_SWS) != RCC_CFGR_SWS_0);
+
+    FLASH->ACR |= FLASH_ACR_LATENCY_1;
+
+    // Set PLL to x9 (-> 72MHz system clock)
+    RCC->CFGR |= RCC_CFGR_PLLMULL9 | RCC_CFGR_PLLSRC | RCC_CFGR_PPRE1_2;
+
+    // Activate PLL and wait
+    RCC->CR |= RCC_CR_PLLON;
+    while(!(RCC->CR & RCC_CR_PLLRDY));
+
+    // Select PLL as clock source
+    RCC->CFGR = (RCC->CFGR & ~RCC_CFGR_SW) | RCC_CFGR_SW_1;
+
+    // Resulting clocks:
+    // SYSCLK, AHB, APB2 72 Mhz
+    // APB1, ADC 36 MHz
+
+    // Disable all interrupts
+    RCC->CIR = 0x00000000;
+}
+