1. PIC18 Interrupt Programming
“Explain the interrupts, timer interrupts, external hardware interrupts, serial communication interrupts and the interrupts priority”

2. Objectives Contra and compare interrupts versus pooling
Explain the purpose of the ISR
List all the major interrupts of the PIC18
Explain the purpose of the IVT
Enable and disable PIC18 interrupts
Program the PIC18 interrupt using assembly language

3. Interrupt
Whenever any device needs the microcontroller’s service the device notifies it by sending an interrupt signal. Upon receiving an interrupt signal, the microcontroller stops whatever it is doing and serve the device. The program associated with the interrupt is called ISR (interrupt service routine) or interrupt handler.
Each device can get the attention of the microcontroller based on the priority assign to it.
Can ignore a device request for service

4. Polling
The microcontroller continuously monitors the status of a given device; when the status condition met, it performs the service. After that, it moves on to monitor the next device until each one is service.
Cannot assign priority because it checks all devices in a round-robin fashion.
Cannot ignore a devices for service

5. Interrupt Service Routine (ISR)
ROM Location (hex)
Power-on-Reset
0000
High Priority Interrupt
0008 (Default upon power-on reset)
Low Priority Interrupt
0018
Interrupt Vector Table for the PIC18
Fixed Location in Memory

6. Step in Executing an Interrupt
It finishes the instruction it is executing and saves the address of the next instruction (program counter) on the stack
It jumps to a fixed location in memory (interrupt vector table (IVT)). The IVT directs the microcontroller to the address of the ISR
The microcontroller gets the address of the ISR from the IVT and jumps to it. It start to execute the interrupt service subroutine until it reaches the last instruction of the subroutine - RETFIE (Return from Interrupt Exit)
Upon executing the RETFIE instruction, the microcontroller returns to the place where it was interrupted

7. Sources of Interrupt Each Timers
3 interrupts for external hardware. Pin RB0 (INT0), RB1 (INT1) and RB2 (INT2)
2 interrupts for serial communication (Receive and Transmit)
The PORTB-Change interrupt (RB4-RB7)
The ADC
The CCP

8. Enable and Disable an Interrupt
BSF INTCON,GIE
BCF INTCON,GIE

9. Timer Interrupts Interrupt Flag Bit Register Enable Bit Timer0 TMR0IF
INTCON
TMR0IE
Timer1
TMR1IF
PIR1
TMR1IE
PIE1
Timer2
TMR2IF
TMR3IE
Timer3
TMR3IF
PIR3
PIE2
Timer Interrupt Flag Bits and Associated Registers
INTCON Register with Timer0 Interrupt Enable and Interrupt Flag

10. Program 11-1 (pg 430) Timer0 Interrupt ORG 0000H GOTO MAIN ORG 0008H
BTFSS INTCON,TMR0IF
RETFIE
GOTO T0_ISR
ORG 00100H
MAIN BCF TRISB,5
CLRF TRISD SETF TRISC MOVLW 0x08 MOVWF T0CON
MOVLW 0xFF MOVWF TMR0H
MOVLW 0xF2 MOVWF TMR0L
BCF INTCON,TMR0IF
BSF T0CON,TMR0ON BSF INTCON,TMR0IE BSF INTCON,GIE
OVER MOVFF PORTC,PORTD
BRA OVER
T0_ISR
ORG 200H
MOVLW 0xFF MOVWF TMR0H MOVLW 0xF2 MOVWF TMR0L BTG PORTB,5 BCF INTCON,TMR0IF
RETFIE
END
Timer0 Interrupt

11. Revisit

12. Please see Program 11-2 (pg 432) and Program 11-3 (pg 433)

13. External Hardware Interrupts
Flag Bit
Register
Enable Bit
INT0(RB0)
INT0IF
INTCON
INT0IE
INT1 (RB1)
INT1IF
INTCON3
INT1IE
INT2 (RB2)
INT2IF
INT2IE
Hardware Interrupt Flag Bits and Associated Registers
Positive-edge-triggered interrupt
PIC18 External Hardware Interrupt Pins

14. Program 11-4 (pg 440) Hardware Interrupt ORG 0000H GOTO MAIN ORG 0008H
BTFSS INTCON,INT0IF
RETFIE
GOTO T0_ISR
ORG 00100H
MAIN BCF TRISB,7
BSF TRISB, INT0
CLRF TRISD
SETF TRISC
BSF INTCON,INT0IE
BSF INTCON,GIE
OVER MOVFF PORTC,PORTD
BRA OVER
T0_ISR
ORG 200H
BTG PORTB,7 BCF INTCON,INT0IF
RETFIE
END
BCF INTCON2,INTEDG1
Hardware Interrupt
Negative-edge-trigerred

15. Please see Program 11-5 (pg 442)

16. Serial Communication Interrupts
Flag Bit
Register
Enable Bit
TXIF (Trasmit)
TXIF
PIR1
TXIE
PIE1
RCIF (Receive)
RCIF
RCIE
Serial Port Interrupt Flag Bits and Associated Registers
PIE1 Register Bits Holding TXIE and RCIE

17. Program 11-6 (pg 446) Serial Port Interrupt
ORG 0000H
GOTO MAIN
ORG 0008H
BTFSC PIR1,TXIF
BRA TX_ISR
RETFIE
ORG 00100H
MAIN SETF TRISD MOVLW 0x20
MOVWF TXSTA
MOVLW D'15'
MOVWF SPBRG
BCF TRISC, TX
BSF RCSTA, SPEN BSF PIE1,TXIE
BSF INTCON,PEIE
BSF INTCON,GIE
OVER BRA OVER
END
Serial Port Interrupt
ORG 0040H
TX_ISR
MOVWFF PORTD,TXREG
RETFIE
Enable peripheral Interrupt

18. Please see Program 11-7 (pg 446)

19. PORTB-Change Interrupt
INTCON Register
* Use in Keypad Interfacing
* Status: H-L or L-H

20. Differences Between External Hardware and PORTB-Change Interrupt
This (RB0-RB2) interrupts has its own pin and is independent each other
Use s all four (RB4-RB7) and considered to be a single interrupt even though it can use up to four pins
Has it own flag (INTxIF) and independent each other
Single flag only (RBIF)
Can be programmed to trigger on the negative or positive edge
Cause an interrupt either pin changes status from HIGH-LOW or LOW-HIGH

21. Please see Program 11-8 (pg 451) and Program 11-9 (pg 452)

22. Interrupt Priority Interrupt ROM Location (hex) Power-on-Reset 0000
High Priority Interrupt
0008 (Default upon power-on reset)
Low Priority Interrupt
0018

23. Interrupt Priority (Cont’d)
Upon Power-on Reset

24. IPR1 Peripheral Interrupt Priority Register

25. Please see Program 11-10 (pg 459) and Program 11-11 (pg 460)

26. “Never interrupt your enemy when he/she is making a mistake“
End Of Chapter
“Never interrupt your enemy when he/she is making a mistake“

27. *****Assignment
1) Compare and differentiate the features of the Timer 0, Timer1, and Timer2. 2) Write a program using Timer0 to generate a 1 kHz square wave frequency on PORTB RA5. Assuming XTAL=10kHz 3) Write a program using Timer0 to generate a 1 kHz and a 2 kHz square wave frequency on PORTB RB0 and PORTB RB4 respectively. PORTB RB7 will be used as a switch to select the frequencies. If RB7 = 0, a 1 kHz square wave will be generated and if RB7 = 1, a 2 kHz square wave will be generated.