InterruptsAn interrupt is any service request that causes the

CPU to stop its current execution stream and to execute an instruction stream that services the

interrupt

When the CPU finishes servicing the interrupt, it returns to the original execution stream at the point

where it left off.

Interrupts can be requested from any of the following sources:

•hardware devices

•software interrupts (programmed interrupt requests (PIRs))

•exceptions, such as page faults

Overview of PIC Interrupts• An interrupt is an event which forces a call to

a subroutine which is usually referred to as an interrupt service routine

• Typical sources of interrupts on the PIC include a positive or negative transition on the RB.0/INT input, a change on any of the inputs RB4 - RB7 or a timer / counter overflow from a value of FFH to 00H.

Overview of PIC Interrupts

• the processor saves the return address on the stack and program control is redirected to the interrupt service routine

• aside from the return address no registers are saved

• However the user may save such important registers as W and STATUS

Overview of PIC Interrupts

• On interrupt, program flow is directed to program location 004H

• As a reset directs program flow to 000H

• On interrupt, a flag bit associated with the type of interrupt is set by the processor

Overview of PIC Interrupts

• On interrupt, a flag bit associated with the type of interrupt is set by the processor

• flag bits may be used to determine the source of the interrupt and perform the appropriate action

Overview of PIC Interrupts

• a program using interrupts is usually structured as follows

ORG 000H ; a reset redirects program to this point

GOTO MAIN

ORG 004H ; an interrupt redirects the program to here

GOTO INT_SERV

Overview of PIC Interrupts

• The user may control the sources of interrupts. For example

BSF INTCON, INTE ; enable interrupts on RB0/INT BSF INTCON, RBIE ; enable change in RB4 - RB7

interrupt BSF INTCON, TOIE ; enable timer interrupt

• Any of these may be used alone

Overview of PIC Interrupts

• At any time in the program, the user may turn any of the sources off by clearing these mask bits

BCF INTCON, INTE BCF INTCON, RBIE

BCF INTCON, TOIE

• several sources may be enabled, depending on your application

Overview of PIC Interrupts

• you are ready to accept interrupts. Set the appropriate mask bit (INTE, RBIE or TOIE) and set GIE

• When no further interrupts are desired, either clear GIE or the mask bits or both

Overview of PIC Interrupts

• If an event then occurs, a flag bit assocociated with that interrupt is automatically set

• allows you to determine the source of the interrupt

Overview of PIC Interrupts

• On interrupt, the processor clears the GIE bit , inhibiting any further interrupts

• The bit is set on execution of the return from interrupts (RETFIE)

• programmer need not perform this book keeping

Overview of PIC Interrupts

• Note that the occurrence of an interrupt event sets the flag bit

• It is the user's responsibility to clear the flag bit

• Failure to clear the flag bit is interpreted by the processor as "a previous interrupt occurred "

PIC Timers

• Available in all PICs

• 14+bit may generate interrupts on timer overflow

• Some 8 bits some 16 bits some have prescalers

PIC Timers

• Can use external pin as clock in / clock out (ie for counting events)

• Warning: Some Peripherals shares timer resources.

PIC Timers

• The device has three readable and writeable hardware timers that can increment automatically each instruction cycle (if no prescaler is used). All timers can cause an interrupt on overflow, and then restart from zero

PIC Timers / Timer 0

• 8 bit timer/counter with prescaler

• Readable and writeable

• 8-bit software programmable prescaler

• Internal or external clock set

• Interrupt on overflow from 0xFF to 0x00

• Edge Select for external clock

PIC Timers / Timer 1

• 16-bit timer/counter with prescaler

• Readable and writeable

• 1, 2, 4, 8 programmable prescaler

• Internal or external clock select

• External clock can be syn. or asyn.

• Interrupt on overflow

• Second crystal permitted

PIC Timers / Timer 2

• 8-bit timer/counter with prescaler and postscaler

• Readable and writeable

• 1,4 or 16 programmable prescaler

• 4-bit programmable postscaler

• Interrupt on overflow

• Output to port pin

PIC Timers

• Timer 0

timer/counter with prescale

• Timer 1

timer/counter with prescale

• Timer 2

timer only with prescale and postscale

• Watch Dog Timer (discussed later)

Timers

• TIMER0 is an 8-bit timer with an eight bit prescaler, which can make the timer run 2 to 256 times slower than normal

• TIMER1 is a 16-bit timer (two 8-bit registers) with a 1:1 to 1:8 prescaler and some other features. Used by given C code to generate soft timer and sound

• TIMER2 is an 8-bit timer with 1:1 to 1:16 prescaler and a 1:1 to 1:16 postscaler It also has a period register.Used by given C code for PWM motor control