Read the SysTick_Wait subroutine and answer the following questions. 1) What val

Question

Read the SysTick_Wait subroutine and answer the following questions.

1) What values should be passed into the subroutine through R0 if we want a time delay of 25ms, 40ms and 100ms, respectively?

2) If the core clock was changed to 16 MHz, What values should be passed into the subroutine through R0 if we want a time delay of 25ms, 40ms and 100ms, respectively?

3) What is the maximum time delay that can be provided by the subroutine?

; Time delay using busy wait.

; Input: R0 delay parameter in units of the core clock 80 MHz(12.5 ns each tick)

; Output: none

SysTick_Wait

SUB R0, R0, #1 ; delay-1

LDR R1, =NVIC_ST_RELOAD_R

STR R0, [R1] ; time to wait

LDR R1, =NVIC_ST_CURRENT_R

STR R0, [R1] ; any value written to CURRENT clears

LDR R1, =NVIC_ST_CTRL_R

SysTick_Wait_loop

LDR R0, [R1] ; read status

ANDS R0, R0, #0x00010000 ; bit 16 is COUNT flag

BEQ SysTick_Wait_loop ; repeat until flag set

BX LR

Explanation / Answer

#define NVIC_ST_CTRL_R(*((volatile uint32_t *)0xE000E010))
#define NVIC_ST_RELOAD_R(*((volatile uint32_t *)0xE000E014))
#define NVIC_ST_CURRENT_R(*((volatile uint32_t *)0xE000E018))
void SysTick_Init(void){
NVIC_ST_CTRL_R = 0; // 1) disable SysTick during setup
NVIC_ST_RELOAD_R = 0x00FFFFFF; // 2) maximum reload value
NVIC_ST_CURRENT_R = 0; // 3) any write to CURRENT clears it
NVIC_ST_CTRL_R = 0x00000005; // 4) enable SysTick with core clock }
// The delay parameter is in units of the 80 MHz core clock(12.5 ns)
void SysTick_Wait(uint32_t delay){
NVIC_ST_RELOAD_R = delay-1; // number of counts
NVIC_ST_CURRENT_R = 0; // any value written to CURRENT clears
while((NVIC_ST_CTRL_R&0x00010000)==0){ // wait for flag } }
// Call this routine to wait for delay*10ms
void SysTick_Wait10ms(uint32_t delay){
unsigned long i; for(i=0; i<delay; i++){ SysTick_Wait(800000); // wait 10ms }

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