Interrupts

1
Interrupts

• How to do 2 things at the same time

2
Need for interrupts

• Computers often have to deal with asynchronous
  events.
• That is to say events that occur times that are
  unpredictable relative to the rest of your
  program.
• Whilst a computer is busy running a program
  something unexpected may occur
• A packet may arrive on a communications line
• A key may be pressed
• A clock tic occurs

3
Alternative- Polling

• One alternative is to have your program running a
  loop checking for input/output events. This is
  called Polling.
• It is what all the examples so far have done.
• Polling wastes processor resources checking for
  events that rarely happen
• Polling makes it hard to get the computer to do
  any useful work.

4
Idea of interrupts

• Main program
• repeat
• Do this
• Do that
• Try something else
• For ages

• Interrupt handler
• Key pressed
• read input port
• store result
• return

press
5

• Interrupts occur between two instructions.
• Control is transferred by the hardware to an
  interrupt location.
• The interrupt routine does its stuff
• It then returns to the following instruction in
  the main program.

6

• An interrupt is a procedure called by the
  hardware of the computer rather than by your
  program.

7
Interrupt control registers
8
Intcon
FLAGS
ENABLES
global interupt enable
INT pin interrupt TMR0 overflow interrupt
INT pin interrupt TMR0 overflow interrupt
9
what happens

• When an interrupt is serviced
• The GIE is cleared to disable any further
  interrupt
• The return address is pushed onto the stack
• The PC is loaded with 0004h
• Once in the Interrupt Service Routine, the
  source(s) of
• the interrupt can be determined by polling the
  interrupt
• flag bits. The interrupt flag bit(s) must be
  cleared in
• software before re-enabling interrupts to avoid
  GP2/
• INT recursive interrupts.

10
Interrupts on the PIC

• Context Saving During Interrupts
• During an interrupt, only the return PC value is
  saved
• on the stack. Typically, users may wish to save
  key
• registers during an interrupt, e.g., W register
  and
• STATUS register. This must be implemented in
• software.

11
timing of interrupt
12
Vectors
memory

• Reset vector, where we start on powerup
• Interrupt vector where we go on an interrupt

Goto main
0 1 2 3 4
Goto isr
13
Sample start of program

• ORG 0x000 processor reset
  vector
• goto Init go to beginning of
  program
• ORG 0x004 Interrupt vector
  location
• goto isr go to interrupt
  routine

14
Save context

• Registers that are used by the interrupt routine
  must always be saved. Program counter saved
  automatically but there are some you must save
• W reg
• STATUS reg 3
• PCLATH reg 10
• FSR reg 4

15
Save registers

• Isr
• Interrupt Vector - Interrupt Sources Used 1.
  TIMER0 Overflow
• 2.
  GP3 Pin-Change
• movwf WTEMP Save current W register
• movf STATUS,w
• clrf STATUS Force to page0
• movwf STATUSTEMP Save STATUS in STATUSTEMP
• movf PCLATH,w
• movwf PCLATHTEMP Save PCLATH
• movf FSR,w
• movwf FSRTEMP Save FSR
• BANK1 select bank 1

16
Determine source

• We next need to inspect the interrupt flags to
  see what device caused the interrupt
• TOIF Timer Overflow Interrupt Flag bit 2 of
  INTCON
• GPIF Gpio interrupt flag bit 0 of intcon

17
Check what is enabled

• We only need to test these flags if they are
  enabled.
• This means we must first check the relevant
  interrupt enable bits

18
Interrupt Source Checks

• 
  
• 
  
• Timer0InterruptCheck
• btfss INTCON,TOIE Is T0IE Set?
• goto Next1 No
• Yes
• btfsc INTCON,TOIF Is TOIF Set?
• goto Timer0Interrupt Yes
• Next1
• GPIFInterruptCheck
• btfss INTCON,GPIE Is GPIE Set?
• goto Next2 No
• btfsc INTCON,GPIF Yes
• Is GPIF Set?
• goto GPIFInterrupt Yes

19
Interrupt Source Code

• Timer0Interrupt
• call Display Update LED Array
• bcf INTCON,T0IF Clear TMR0 Interrupt Flag
• this prevents interrupt
• from occuring again
• spuriously
• goto EndIsr

20
Restore old registers

• Finally we must restore any saved registers
• There is a particular problem with this because
  of the fact that our saving code may change the
  flags in the status register
• This requires care to get round

21
Cleanup code

• EndIsr
• clrf STATUS Select Bank0
• movf FSRTEMP,w
• movwf FSR Restore FSR
• movf PCLATHTEMP,w
• movwf PCLATH Restore PCLATH
• movf STATUSTEMP,w
• movwf STATUS Restore STATUS
• swapf WTEMP,f
• swapf WTEMP,w Restore W without
  corrupting STATUS bits
• retfie Return from interrupt

22
Swapf

• This is defined to do the following
• SWAPF f,d Swap halves f
• f(03)lt-gtf(47)-gtd
• It does not alter the z flag of the status
  register
• swapf WTEMP,f swap halves of wtemp
• swapf WTEMP,w swap again and store in W
• Net result is that W contains original form of
  wtemp