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SysTick Interrupt Example

Basic timer included in all

Cortex-M4 devices.

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Interrupt Summary

Determine the name of the interrupthandler (from vector table) & interruptindex (from derivative.h)

Create interrupt handler function Create interrupt setup function

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Interrupt handler

Use whatever name is given in thevector table.

Should clear the interrupt flag (if it

exists) check hardware details. Carry out whatever function is needed.

May need to do some setup for the

next interrupt e.g. schedule timer.

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Interrupt Setup

Initialise hardware Do whatever setup is required e.g. set tick

rate etc.

Enable interrupt signal (unmask it) inhardware

Enable interrupt signal (unmask it) inNVIC

NVIC_EnableIrq(int interruptIndex)

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IRQ Handler or ISR

The interrupt handler resembles a subroutineor C function.

Its address must be placed in the appropriateentry of the vector tableso that it may be

calledwhen triggered.

The Cortex-M family uses a interrupttechnique that allows the direct use of C

functionsas interrupt handlers. The correctregisters are saved and restored

by hardware (matches ARM-ABI used by C).

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SysTick Timer

This is a basic timer that is included inall Cortex-M3 & M4 devices.

Real-time Operating Systems (RTOSs)

require at least a basic timer that isable to generate regular interrupts.

Inclusion in the processor core allows

the creation of chip independent RTOS.A good (simple) example of a interrupt

source.

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Hardware

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24-bit down counter0

1

Referenceclock

Core clock

Reload register

SysTick->VAL

SysTick->LOAD

CLKSOURCE_MASK TICKINT_MASK

SysTick->CTRL

ENABLE_MASK

Control

CPU

Interrupt Request

Timer automatically re-loadswhen it reaches zero.

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Software Routine

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uint32_tSysTick_Config(uint32_tticks) {

if((ticks - 1) > SysTick_LOAD_RELOAD_Msk) {

/* Reload value impossible */return(1);

}/* Set reload register */SysTick->LOAD = ticks - 1;

/* Set Priority for Systick Interrupt */NVIC_SetPriority (SysTick_IRQn, (1CTRL =

SysTick_CTRL_CLKSOURCE_Msk | // Use system core clockSysTick_CTRL_TICKINT_Msk | // Enable interruptsSysTick_CTRL_ENABLE_Msk; // Enable timer

/* Function successful */return(0);}

Code to Set period (interrupt rate)

Enable interrupts

Enable timer

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Vector Table

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...

voidSysTick_Handler(void) WEAK_DEFAULT_HANDLER;...

VectorTable const__vector_table = {

/* omitted rows */

SVC_Handler, /* System Service Call via SVC instruction */

DebugMon_Handler, /* Debug Monitor*/

0, /* Reserved */PendSV_Handler, /* Pendable request for system service */

SysTick_Handler, /* System Tick Timer */

/* External Interrupts */

DMA0_IRQHandler, /* DMA channel 0 interrupt */

DMA1_IRQHandler, /* DMA channel 1 interrupt */

/* omitted rows */

};

Vector table entry for

SysTick Timer

The name of the actual interrupt handler functionmust agree exactly with the vector table entry

It must also have "C" linkage.

Weakly assigns to thedefault handler

Can be overridden.

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SysTick Handler

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/*

* This function is used for the

* System Timer interrupt handler.

*/

voidSysTick_Handler(void) {

...

}

Name agrees withVector Table entry

Code to

Handle Tick