Interrupt programming with 8051 microcontroller

Interrupt Programming with 8051 Prepared and Presented by – Rajvir Singh

Introduction to Interrupts • An interrupt is an external or internal event that interrupts the microcontroller to inform it that a device needs its service. • A set of program instructions written to service an interrupt is called the Interrupt Service Routine • 8051 has six different sources of interrupts External: Power-up reset, INT0, INT1 Internal: Timer0, Timer1, Serial Port

Interrupts vs Polling • There are two methods of writing software by which microcontroller can serve devices: Interrupts and Polling Interrupt: whenever any device needs service, it notifies the 8051 by sending an interrupt signal Polling: 8051 continuously monitors the status of a device, until some pre-determined condition is met, and then serves the device

Interrupt Polling • Microcontroller is free to execute any other programming task • Microcontroller cannot perform any task other than monitoring the device status Which technique, interrupt or polling, avoids tying down the microcontroller? Including reset, how many interrupts do we have in the 8051?

Example of Polling ORG 0H MOV TMOD, #2 MOV TH0, #-10 SETB TR0 BACK:JNB TF0, BACK CLR TR0 CLR TF0

Interrupt Service Routine • When microcontroller receives an interrupt signal from any of the six interrupt sources it executes a call to interrupt service routine • For every interrupt, there must be an interrupt service routine • The interrupt service routine for every interrupt must be located at a fixed location in program memory, called interrupt vector.

Interrupt Vector Table for 8051 Interrupts Interrupt Source ROM Location Pin Reset 0000H 9 INT0 0003H 12(P3.2) Timer0 000BH INT1 0013H 13(P3.3) Timer1 001BH Serial Port 0023H

Redirecting 8051 from Interrupt Vector Table at Power-Up ORG 0H LJMP MAIN ORG 30H MAIN: …………. …………. END Bypass all the interrupt vector locations

Why do we put a LJMP instruction at address 0? In the 8051, what memory area is assigned to a interrupt vector table? The 8051 programmer cannot change the memory space assigned to interrupt vector table(T/F) How many bytes of address space in the interrupt vector table is assigned to the timer 0 interrupt? How many bytes of address space in the interrupt vector table is assigned to the reset interrupt, and why? To put the entire Interrupt Service Routine in the space provided in interrupt vector table, it must be no more than _____ bytes in size

Enabling and Disabling Interrupt mechanism in 8051 • Upon reset, all interrupts are disabled • The interrupts must be enabled by software, only then 8051 will respond to them • A register called IE( Interrupt Enable ) is responsible for enabling and disabling the interrupts • Upon reset, all bits of IE register are 0

IE Register EA 0, Disables all interrupts 1, each interrupt is individually enabled or disabled by setting or clearing its enable bit. ES enables or disables serial port interrupt ET1/ET0 enables or disables timer 1/0 interrupt EX1/EX0 enables or disables external interrupt 1/0 EA -- -- ES ET1 EX1 ET0 EX0 D0D7  Write instructions to  enable serial interrupt and timer 0 interrupt  Disable all interrupts

How 8051 services an interrupt request ? • 8051 finishes the instruction it is currently executing, and saves the contents of Program Counter on the stack (address of next instruction) • It jumps to the interrupt vector location corresponding to the interrupt source • Executes the interrupt service routine, until it encounters RETI instruction • Returns back to the place where it was interrupted, by popping the contents of stack on PC, and starts execution at that address

ORG 0000H LJMP MAIN ;-- ISR for timer0 to generate square wave ORG 000BH CPL P2.1 RETI ;--main program for initialization ORG 0030H MAIN: MOV TMOD, #2 ;timer 0 in auto reload mode MOV P0, #0FFH MOV TH0, #92H MOV IE, #82H SETB TR0 BACK: MOV A, P0 MOV P1, A SJMP BACK END A program to continuously read data from P0 and display it on P1 & simultaneously creating a square wave of 200us on P2.1 using timer 0.

Programming Timer Interrupts • If timer interrupt bit in IE register is enabled, whenever the timer rolls over, TF flag is SET, and the 8051 is interrupted. • The interrupt service routine for timer can be placed at – interrupt vector location if it is small enough, or – elsewhere by using proper redirection at interrupt vector location

Programming External Hardware Interrupts • 8051 has the following external interrupts – RESET – INT0 – INT1 • RESET is used for power-on reset • Therefore there are two external interrupts INT0 & INT1, that can be used by external hardware(devices) to interrupt 8051

How interrupts are activated? • There are two activation levels for the external hardware interrupts – Level triggered – Edge triggered • Level triggered interrupts is the default mode upon RESET of the 8051

Level Triggered Interrupt • INT0 and INT1 are normally held high • If a low-level signal is applied to them, it triggers the interrupt Vcc P1.3 Vcc INT0 LED

P1.3 Vcc INT0 LED Device Normally High Interrupt Activated How 8051 knows that an interrupt is activated? If the hardware interrupts are enabled in the IE register,8051 samples the INT 0/1 pin for a low level signal once each machine cycle. If during sampling 8051 senses the signal on INT0/1 to be low, the interrupt is activated

What is the minimum duration for which INT 0/1 pin must be held low for interrupt to be activated?  The pin must be held low until the start of the execution of Interrupt Service Routine  The minimum duration is 4 machine cycles. How can we make sure that a single interrupt is not interpreted as multiple interrupts?  The low-level on INT0/1 pin be brought back to high before the execution of RETI instruction in Interrupt Service Routine

Remember that….. • There are two activation levels for the external hardware interrupts – Level triggered – Edge triggered • Level triggered interrupts is the default mode upon RESET of the 8051

Edge Triggered Interrupts • We must program the bits of TCON register to make interrupts edge-triggered • When a high-to-low signal is applied to INT0/1 pin, the 8051 will be interrupted P1.3 Vcc INT0 LED Device High-to-low transition

IT0 IE0 IT1 IE1 TR0 TF0 TR1 TF1 D0 D7 TCON.0 TCON.2 0, INT0 becomes level-triggered interrupt 1, INT0 becomes edge-triggered interrupt 0, INT1 becomes level-triggered interrupt 1, INT1 becomes edge-triggered interrupt To make INT0 edge-triggered the instruction is SETB TCON.0 To make INT1 edge-triggered the instruction is SETB TCON.2 Role of TCON Register

How Edge-triggered Interrupts are activated and serviced by 8051? • The falling edge on INT0/1 pins is latched by 8051, and held by the TCON register • TCON.1(IE0) AND TCON.3(IE1) bits are used by 8051 to keep track of edge-triggered interrupts only • When edge(high-to-low) transition takes place on INT0/1 pin, 8051 does the following- – Sets high IE0/1 bit in the TCON register – Jumps to the interrupt vector location – Executes the interrupt service routine to be continued…

IT0 IE0 IT1 IE1 TR0 TF0 TR1 TF1 D0 D7 TCON.1 TCON.3 Set by CPU when external interrupt edge on INT0 pin is detected Role of TCON Register Set by CPU when external interrupt edge on INT1 pin is detected IE0/1 are also called Interrupt-in-service flags When set to 1, indicates 8051 is executing an interrupt service routine

• At the end of service routine, RETI instruction is executed – This clears the IE0/1 interrupt-in-service flag What is the role of RETI instruction… In edge-triggered interrupts Timer interrupts Can we use RET instruction in Interrupt service routine?

Programming Serial Communication Interrupts

Serial Communication using Polling A program to transfer character ‘A’ serially at 4800 baud, continuously ORG 0H MOV TMOD, #20H ;timer 1, mode 2 MOV TH1, #-6 ;set baud rate to 4800 MOV SCON, #50H ;serial mode 1 SETB TR1 ;start timer 1 AGAIN: MOV SBUF, #’A’ ;send character ‘A’ for ;transmission HERE: JNB TI, HERE ;wait until TI flag is set CLR TI ;clear TI to transmit next char SJMP AGAIN

Serial Communication Interrupt Flags • TI(Transmit Interrupt): is raised when last bit, i.e. the stop bit, is transferred • RI(Receive Interrupt): is raised when the entire frame of data, including the stop bit is received • In polling method, 8051 just waits for the TI or RI flag to be raised, does nothing else SM0 SM1 SM2 REN TB8 RB8 TI RITI RI SCON Register

Using Serial Interrupt with 8051 • In 8051 there is only one interrupt for both serial data transmission and reception • When TI or RI is raised by serial data transfer, 8051 is interrupted, and jumps to interrupt vector location 0023H to execute the interrupt service routine(ISR) • In ISR, 8051 must examine TI and RI flags to determine which one caused the interrupt

A program to i) continuously read data from P1 and send it to P2 ii) receive data from serial port and send it to P0 ORG 0H LJMP MAIN ORG 0023H LJMP SERIAL MAIN: MOV P1, #0FFH MOV TMOD, #20H MOV TH1, #0FDH MOV SCON, #050H MOV IE, #10010000B SETB TR1 BACK: MOV A, P1 MOV P2, A SJMP BACK ORG 100H SERIAL: JB TI, TRANS MOV A, SBUF MOV P0, A CLR RI RETI TRANS: CLR TI RETI END

Interrupt Priority in 8051 What happens if two interrupts are activated at the same time? Which of these two interrupts is serviced first? – The interrupt which has the highest priority is serviced first – By default, 8051 assigns a priority level to all interrupts upon RESET

8051 Interrupt Priority upon RESET Highest to Lowest Priority External Interrupt 0 INT0 Timer Interrupt 0 TF0 External Interrupt 1 INT1 Timer Interrupt 1 TF1 Serial Communication RI + TI

• They are latched and kept internally by 8051 • Then 8051 polls all interrupts according to the default priority levels • If any interrupt is activated, it is serviced in that sequence. • Therefore IE0(INT0) is serviced first, then TF0(timer0), and finally IE1(INT1) What happens if interrupts INT0, TF0, INT1 are activated at the same time? Assume default priority levels and edge-triggered external interrupts.

Interrupt Priority Register PS Serial Port Priority bit PT1 Timer1 interrupt priority bit PX1 External interrupt 1 priority bit PT0 Timer0 interrupt priority bit PX0 External interrupt 0 priority bit -- -- PT2 PS PT1 PX1 PT0 PX0PX0 IP Register D0D7 Priority bit = 1, assigns high priority 0, assigns low priority

Assume that after RESET, the Interrupt priority is set by the instruction MOV IP, 00001100B Discuss the sequence in which the interrupts are serviced. What happens if the 8051 is executing the ISR of an interrupt and another higher priority interrupt is activated? Upon RESET all interrupts have the same priority.(T/F)

Interrupt programming with 8051 microcontroller

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Interrupt programming with 8051 microcontroller