Introduction to Micro Controllers & Embedded System Design Interrupt

1. Introduction to Micro Controllers&Embedded System DesignInterrupt Department of Electrical & Computer Engineering Missouri University of Science & Technology hurson@mst.edu A.R. Hurson

2. An interrupt is the occurrence of an event out of normal sequence of machine’s operation. • It causes a temporary suspension of a program while the event is serviced by another program known as interrupt handlers or interrupt service routine, very similar to a subroutine call. A.R. Hurson

3. In microcontroller, interrupt allows the system to respond asynchronously to an event and deal with the event. • An interrupt driven system gives the illusion of doing many things simultaneously. A.R. Hurson

4. Time Main Program Main Main Main ISR ISR A.R. Hurson

5. There are five interrupt sources in 8051: Two external, two timers, and a serial port interrupt. • Note: it is possible that more than one interrupt happen simultaneously or an interrupt occurs while another interrupt being serviced. A.R. Hurson

6. Timer CONtrolregister: A.R. Hurson

7. Timer CONtrolregister: A.R. Hurson

8. Serial port control register A.R. Hurson

9. Interrupt flag bits A.R. Hurson

10. Enabling and disabling Interrupts • Each interrupt source can be individually enabled or disabled through the bit addressable special function register IE (Interrupt Enable) at address 0A8H. • Note: There is a global enable/disable bit that can be cleared to disable all interrupts or set to turn on interrupts. A.R. Hurson

11. Byte address Byte address 90 8D 8C 8B 8A 89 88 87 83 82 81 80 P1 TH1 TH0 TL1 TL0 TMOD TCON PCON DPH DDL SP P0 FF F0 E0 D0 B8 B0 A8 A0 99 98 B ACC PSW IP P3 IE P2 SBUF SCON A.R. Hurson

12. Interrupt bits A.R. Hurson

13. Note: Two bits must be set to enable any interrupt: The individual interrupt bit and the global interrupt bit. • Question: do the above solutions have exactly the same effect? Example: SETB ET1 ; Enable timer1 interrupt bit SETB EA ; Enable global interrupt bit Alternatively, we can write: MOV IE, #10001000B A.R. Hurson

14. Interrupt Priority • Each interrupt source is individually programmed to one of two priority levels via bit addressable special function register IP (Interrupt Priority) at address 0B8H. A.R. Hurson

15. Interrupt priority bits A.R. Hurson

16. Order of handling several interrupts: • ISR is interrupted if a higher priority interrupt occurs while servicing a lower priority interrupt. • A high-priority interrupt cannot be interrupted. • The main program can always be interrupted regardless of the priority of the interrupt. A.R. Hurson

17. Order of handling several interrupts: • If two interrupts of different priorities happen simultaneously, the higher priority interrupt will be serviced first. • If two interrupts of the same priority occurs simultaneously, a fixed polling sequence determines which is serviced first, as follows. External0, Timer0, External1, Timer1, Serial Port A.R. Hurson

18. Processing Interrupts: In case of an interrupt the system follows the following steps: • The current instruction completes execution, • The content of PC is saved on the stack, • The current interrupt status is saved internally, • The interrupts are blocked at the level of interrupt, • The PC is loaded with the vector address of the ISR, • The ISR executes • ISR takes action in response to the interrupt • ISR finishes with a RETI instruction • Execution of main program continues from where it left off. A.R. Hurson

19. Interrupt vector • When an interrupt is accepted the value loaded into the PC is called the interrupt vector which is the address of the start of ISR routine for the interrupting source. • When vectoring to an interrupt, the flag that initiated the interrupt is automatically cleared by hardware, except RI and TI flags which trigger the serial port interrupt. A.R. Hurson

20. Interrupt vector • Note: System reserves eight bytes for each interrupt. A.R. Hurson

21. Example: • ORG 0000H ; Reset entry point • LJMP MAIN •  •  •  • ORG 0030H ; Main program entry point • MAIN:  •  •  A.R. Hurson

22. Graphical representation FFFF Main program 0030 002F Reset and Interrupt Entry points A.R. Hurson LJMP MAIN 0000

23. Example: Assume we have just one interrupt source (say Timer0) • ORG 0000H ; Reset entry point • LJMP MAIN • ORG 000BH ; Timer0 entry point • TOISR:  ; Timer0 ISR begins •  •  • RETI ; Return to main program • MAIN:  : Main program •  •  A.R. Hurson

24. Example: If an ISR is longer than 8 bytes, then its code must be somewhere else: • ORG 0000H ; Reset entry point • LJMP MAIN • ORG 000BH • LJMP TOISR; • ORG 0030H • MAIN:  : Main program •  •  • TOISR:  ; Timer0 ISR begins •  •  • RETI ; Return to main program A.R. Hurson

25. Example: Write a program using Timer0 and interrupt to create a 10Khz square wave on P1.0 A.R. Hurson

26. 0000 ORG 0000H 0000 020030 LJMP MAIN 000B ORG 000BH 000B B290 T0ISR: CPT P1.0 000D 32 RETI A, R5 0030 ORG 0030H 0030 758902 MAIN: MOV TMOD, #02H ; Timer0, Mode2 0033 758CCE MOV TH0, #-50 ; 50 s delay 0036 D28C SETB TR0 ; Start Timer 0038 75A882 MOV IE, #82H ; Enable Timer0 ; interrupt 003B 80FE SJMP $ A.R. Hurson

27. Serial Port interrupts • Serial port interrupts trigger when either the transmit interrupt flag (TI) or the receive interrupt flag (RI) is set. • A transmit interrupt occurs when transmission of the previous character written to SBUF has finished. • A receive interrupt occurs when a character has been completely received and is waiting in SBUF to be read. • Serial port interrupts are slightly different from other types of interrupts in the sense that the serial port interrupt is cleared by software rather than hardware. A.R. Hurson