systick.c

#include <stdint.h>
#include "systick.h"

#define STK_CSR_ADDR 	0xE000E010U
#define STK_RVR_ADDR	0xE000E014U // SysTick reload value register (STK_RVR)
#define STK_CVR_ADDR	0xE000E018U // SysTick current value register (STK_CVR)
#define STK_CALIB_ADDR	0xE000E01CU

// reset values, missing values denote that reset value is kept unknown
#define STK_CSR_RV		0x00000004U
// #define STK_RVR_RV		16000000U 	/*24 bit timer, set countdown to approx 160000000 rvr value = 2 seconds*/  
#define STK_CVR_RV		0x00000000U /* clear current value register */   
#define STK_CALIB_RV 	0x00000000U
// 80000000 -> 1 sec 

// pointers to volatile data
extern volatile uint32_t* STK_CSR = (uint32_t*)STK_CSR_ADDR;
extern volatile uint32_t* STK_RVR = (uint32_t*)STK_RVR_ADDR; 
extern volatile uint32_t* STK_CVR = (uint32_t*)STK_CVR_ADDR;
extern volatile uint32_t* STK_CALIB = (uint32_t*)STK_CALIB_ADDR;
extern uint8_t systick_configured = 0;

void configure_systick(uint32_t reload_value, uint8_t enable_int) {
	if (systick_configured) return;

	systick_configured = 1; 
	*STK_RVR 	= reload_value;
	*STK_CVR 	= STK_CVR_RV;		
	*STK_CALIB	= STK_CALIB_RV;	
	*STK_CSR 	= STK_CSR_RV; 	
	// configure, ENABLE CSR, rest of the values are default, clock: system
	// 0b11
	// __asm volatile("BKPT #0");
	if (enable_int != 0) {
		*STK_CSR	|= 1 << 1;	// enable interrupt
	}

	*STK_CSR 	|= 1 << 0; // and enable systick finally
	// __asm volatile("BKPT #1");	
}