timer and interrupts

8/8/2019 Timer and Interrupts

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Timer Programming and InterruptsTimer Programming and Interrupts

EE-342 Embedded System


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Timer ProgrammingContrast and compare interrupt versus polling

Interrupt Handling

Interrupts of the 8051

Purpose of the interrupt vector table

Enable or disable interrupts

Timers using interrupts

Interrupt priority

In this Lecture .In this Lecture .


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Timers Programming (1/10)

The 8051 has two timers: Timer 0 and Timer 1

They can be used either as timers to generate a time delay or

as counter to count events happening outside themicrocontroller

Both Timer 0 and Timer 1 are 16 bits wide

They are accessed as two separate registers, low byte and

high byte. (TL0 & TH0 for Time 0 and TL1 & TH1 for

timer 1)


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Timers Programming (2/10)

TH0 TL0

D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0

There are three Special Function Registers for timer settings

1. Timer Registers (Timer 0 & Timer 1) store the starting values

of the Timer 0 & Timer 1. Each timer is 16-bit register which is

split into two bytes (THx & TLx).

Timer 0 register (16 Bit)

TH1 TL1

D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0

Timer 1 register (16 Bit)


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Ti r r r i (3/ )

(MSB) (LSB)

TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0

2. Timer Control Register (TCON) use to turn ON/OFF of the timers

and timer interrupt control. It is a 8-bit register and bit-addressable,

and only the upper 4-bit refers to timer control.

Timer ON/OFF control bits

TR1 Timer 1 run control bit. Set = Timer ON and Clear = Timer OFF

TF1 Timer flag which is set when the Timer 1 rolls over from FFFFH to 0000H

Cleared the bit in software !.

TR0 Timer 0 run control bit. Set = Timer ON and Clear = Timer OFF

TF0 Timer flag which is set when Timer 0 rolls over from FFFFH to 0000H.

Cleared the bit in software !

IE1 and IT1 Set the trigger mode of external interrupt 1

IE0 and IT0 Set the trigger mode of external interrupt 0

Trigger mode of external interrupt control bits


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Ti r r r i (4/ )

(MSB) (LSB)

GATE C/T M1 M0 GATE C/T M1 M0

3. Timer Mode Register (TMOD) use to set the various timer operation

modes. It is a 8-bit register and bit-addressable.

Timer 1

GATE Gating control when set. The timer/counter is enable only while the INTx

pin is high and the TRx control pin is set. When cleared, the timer is enabledwhenever the TRx control bit is set.

C/T Timer or counter selected. Cleared for timer operation (input from internal

system clock) and Set for counter operation (input from Tx input pin)

M1 & M0 Mode bits

Timer 0


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Ti r / C u t r M d : 3 Bit C u t r u i Ti r


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Ti r / C u t r M d : 6 Bit C u t r u i Ti r

TL1

(8 Bit )

Timer value range from 0000H to FFFFH in TL1 TH1


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Ti r / C u t r M d : 8 Bit Aut r l d


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Example 8Example 8--11

Solution:

Convert the values from hex to binary:

(a) TMOD = 0000 0001, mode 1 of Timer 0 is selected

(b)TMOD = 0010 0000, mode 2 of Timer 1 is selected

(c) TMOD = 0001 0010, mode 2 of Timer 0, and mode 1 of Timer 1are

selected

Indicate which mode and which timer are selected for each of the following.

(a) MOV TMOD, #01H

(b) MOV TMOD, #20H

(c) MOV TMOD, #12H

Ti r r r i (6/10)


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Solution:

(1)

MOV TMOD, #01H ; Timer 0 and mode 1 set, C/T = 0 to use XTAL

; clock source, Gate = 0 to use software ON/OFF

MOV TL0, #0ABH ; TL0 =ABH

MOV TH0, #0F0H ; TH0 =F0H

SETB TR0 ; Start Timer 0

1. Write instructions to do the followings:

a. Set Timer 0 in mode 1, use 8051 XTAL for the clock source,

instructions to start and stop the timer,

b. Set value F0ABH to Timer 0.

c. Start Timer 02. Determine the time forTimer 0 rolling over if XTAL = 12 MHz.

Ti r r r i (7/10)


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Ti r r r i (8/10)

Solution:

(2)

Counts for Timer rolling over = Counts from F0ABH to FFFFH

plus rolling over to 0

Timer clock cycles = (FFFFH F0ABH + 1) = 0F55H

= 3925 in decimal

@ Time for Timer 0 rolls over = 3925 x 1 Qs = 3925 Qs #

Time for 1 Timer clock = 1 machine cycle = s1s12x

10x12

16

Q!

Assume 12 MHz clock :

THx TLx # of count to set TFx

FFH FFH 1FFH FEH 2

65536 10 F0ABH


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Solution:The period of the square wave, T = 1/50 Hz = 20 ms

of it for the high and low portions of the pulse = 10 ms

10 ms/ 1Qs = 10,000 timer cycles are needed for each pulse.

Timer 1 value to be set = 65536 10000 = 55536 in decimal = D8F0H

i.e. TH1 = D8H and TL1 = F0H ( need 16 bit Counter, so take mode 1)

MOV TMOD, #10H ; Timer 1, mode 1

AGAIN: MOV TL1, #0F0H ; TL1 =F0H

MOV TH1, #0D8H ; TH1 =D8H


BACK: JNB TF1, BACK ; Stay until timer rolls over

CLR TR1 ; Stop Timer 1

CPL P1.3 ; Complement P1.3 to set Hi, Low

CLR TF1 ; Clear Timer flagby software

SJMP AGAIN

Assume XTAL = 12 MHz, write a program to generate a square wave of 50 Hz

frequency on pin P1.3 by using timer 1 as time control.

Ti r r r i (9/10)

20ms

65535 10000 + 1

Ie. Toggle P1.3 every 10ms


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; Time Delay subroutine

MOV TMOD, #10H ; Timer 1, mode 1

MOV R3, #200

AGAIN: MOV TL1, #08H ; TL1 =08H

MOV TH1, #01H ; TH1 =01H


BACK: JNB TF1, BACK ; Stay until timer rolls over

CLR TR1 ; Stop Timer 1

CLR TF1 ; Clear Timer flag

DJNZ R3, AGAIN ; Loop until R 3 = 0

RET

Examine the following subroutine and find the time delay for it. (12 MHz clock)

Ti r r r i (10/10)

Solution :

Timer value = 0108H = 264 in decimal, counts to rolls over = 65536 264 = 65272 in decimal

Timer cycle of Timer = 65272

Total machine cycles of the subroutine = [2 + 1 + (5 + 65272 + 4) x 200 + 2] = 13056205

Total delay time 13056205 x 1Qs = 13056.2 ms

Machine cycles

2

1

2

2

1

2

1

1

2

2


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A single microcontroller always connects to serve

several peripheral devices through its I/O ports

There are two ways for the peripheral devices to

request service from microcontroller

Polling

Interrupt

Interrup

t & PollingInterru

pt & Po

lling


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Microcontroller continuously monitors the status of a

certainnumber of devices in sequence

Services to a device if preset condition metAfter the service, the microcontroller will move on to

monitor the status of another device until all devices

are serviced

The operation described above is called polling

Programmed I/O (Polling)Programmed I/O (Polling)


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Whenever any device needs the service, it notifies the

microcontroller by sending it aninterrupt request (IR)

signal while the microcontroller is doing other work.

The microcontroller suspends its work to service the

device at once.

Note that each IR is associated with aninterrupt

service routine (ISR)

Interrupt I/O (Interrupt)Interrupt I/O (Interrupt)


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Comparison between Interrupt and PollingComparison between Interrupt and Polling

Interrupt Polling

Method Devices notify MCU

by sending it an

interrupt signalswhile the MCU is

doing another work

MCU continuously

monitors devices to

determine whetherthey need service

Response time Faster Slower

Need of MCU

time

Less More

Priority setting Yes No


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St p i H dli I t rrupt R qu t

The current instruction will be finished

The PC is saved on the stack

The current interrupt status is saved internally

The PC is loaded with the vector address of the ISR

from the interrupt vector table (Jump to execute ISR)

The ISR is executed and will be finished with a RETIinstruction (return from interrupt)

Return to main program by popping the PC from the

stack

When an interrupt activates and is accepted by the MCU, the main

program is interrupted. The following actions occurs:


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Pr r Ex cuti with I t rrupt

Main Main Main

ISR ISR

stack

PC saved

on stack

PC popped

from stack

The program that deals with an interrupt is called an interrupt

service routine (ISR)

ISR executes in response to the interrupt and generally performs I/O

operation to a device


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There are 6 interrupts in the 8051

Types of Interrupt in the 8051Types of Interrupt in the 8051

Reset when the reset pin is activated, the 8051 will reset allregisters and ports and jumps to address location 0000H starting upexecution.

2 external interrupts Hardware external interrupts (INT0 andINT1) at pins 12 & 13 are used to receive interrupt signals fromexternal devices.

2 timer interrupts They are Timer 0 and Timer 1 which will giveout interrupt signal when the timers count to zero.

1 serial port interrupt - It is used for data reception andtransmission during serial communication.

Apart from the Reset, only the 5 interrupts are available to the user


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I t rrupt V ct r Tabl f r th 8051

Priority Interrupt Flag ROM location Pin

1 Reset RST 0000H 9

2External 0(INT0)

IE0 0003H P3.2 (12)

3 Timer 0 TF0 000BH ---

4

External 1

(INT1) IE1 0013H P3.3 (13)

5 Timer 1 TF1 001BH ---

6 Serial port RI or TI 0023H ---

interrupt vector table holds the addresses of ISR


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Enabling an Disabling an Interr t

Upon reset, all interrupts are disabledThe interrupts must be enabled by software

A register called IE (interrupt enable) register, which is bit-

addressable, is responsible for enabling and disabling the interrupts

Bit IE. 7 must be set high to allow the rest of register to take effect

EA = 1 ; Global enable interrupt

EA = 0 ; Global disable interrupt

IE.7 IE.6 IE.5 IE.4 IE.3 IE.2 IE.1 IE.0

EA -- ET2 ES ET1 EX1 ET0 EX0

IE (interrupt enable) register


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E abli and i ablingan Int rrupt

Bit Symbol Description (1=Enable, 0=Disable)

IE.7 EA Global Enable/disable

IE.6 -- Not implemented, reserved for future use

IE.5 ET2 Not use for 8051 (8052 only)

IE.4 ES Enable/disable serial port interrupt

IE.3 ET1 Enable/disable timer 1 interrupt

IE.2 EX1 Enable/disable external interrupt 1

IE.1 ET0 Enable/disable timer 0 interrupt

IE.0 EX0 Enable/disable external interrupt 0


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Solution:

(a) MOV IE, #10010110B ; enable serial, Timer 0, EX1 interrupts

or

SETB IE.7 ; EA=1, Global enable

SETB IE.4 ; enable serial interrupt

SETB IE.1 ; enable Timer 0 interruptSETB IE.2 ; enable EX1 interrupt

(b) CLR IE.1 ; disable Timer 0 interrupt

(c) CLR IE.7 ; disable all interrupts

Write instructions to

(a) Enable serial interrupt, Timer 0 interrupt and external interrupt 1, and

(b) Disable Timer 0 interrupt only, then

(c) Disable all the interrupt with a single instruction


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Ti r Int rrupt

TF0 Timer 0 Interrupt Vector

1 Jumps to 000BH

When timer rolls over, its timer flag (TF) is set.

If the timer interrupt in the IE register is enable, whenever the TF isset, the microcontroller is interrupted and jumps to the interruptvector table to service the ISR.

With timer interrupt is enabled, microcontroller can do other thingsand no need to monitor the TF for rolling over.

TF1 Timer 1 Interrupt Vector

1 Jumps to 001BH


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Write a program to generate a squarewave of 50 Hz on pin P1.3. This is similarto Example 8.3 except that it uses aninterrupt for Timer 1.

Exampl 8Exampl 8--66

Solution:ORG 0000HLJMP MAIN ;by-pass interrupt vector table

; ISR for Timer 1 to generate square waveORG 001BHLJMP ISR_T1 ; jump to ISR_T1

; Main Program for initializationORG 30H

MAIN: MOV TMOD, #10H ; Timer 1, mode 1MOV TL1, #0F0H ; TL1 =F0HMOV TH1, #0D8H ; TH1 =D8HMOV IE, #88H ; enable Timer 1 interruptSETB TR1 ; Start Timer 1

HERE: SJMP HERE; Timer 1 ISRISR_T1: CPL P1.3 ; Complement P1.3 to set Hi, Low

MOV TL1, #0F0H ; reload timer valueMOV TH1, #0D8H ;SETB TR1 ; Start Timer 1RETIEND

Timer 1 value to be set = 65536 10000 = 55536 in decimal = D8F0H

Write a program to generate a square wave of 50 Hz on pin P1.3. This is similar

to Example 8.3 except that it uses an interrupt for Timer 1.

Example 8.3 (use TIMER)


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Interr t Priority

Table 8.1: 8051 Interrupt Priority Upon Reset

External Interrupt 0 (INT0)

Timer Interrupt 0 (TF0)

External Interrupt 1 (INT1)Timer Interrupt 1 (TF1)

Serial Communication (RI+TI)

Upon reset, the priorities of interrupt source are assignedfrom top to bottom as in the following Table 8 , i.e. ifINT0 and INT1 are activated at the same time, INT0 isfirst responded.

Highest

Priority

Lowest

Priority

INT0 > TF0 > INT1 > TF1 > SERIAL(RI+TI)


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Setting Interr t Priority it t e IP register

IP.7 IP.6 IP.5 IP.4 IP.3 IP.2 IP.1 IP.0

-- -- PT2 PS PT1 PX1 PT0 PX0

The sequence of Table 8 can be changed by assigning ahigher priority to any one of the interrupts.

It is done by setting high at the corresponding bit in theIP (interrupt priority) register.

IE (interrupt priority) register (Bit-addressable)

Priority bit = 1 (assign high priority) Priority bit = 0 (assign low priority)

PT2, Timer 2 (8052 only), PT1 & PT0 -- Timer 1 & Timer 0 interrupts

PX1 & PX0 External interrupts 1 & 0, PS Serial port interrupt


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Setting Interrupt Pri rit with the IP regi ter

Example 8.7 :

(a) Program the IP register to assign the highest priority to INT1,

(b) Discuss what happens if INT0, INT1 and TF0 are activated at the

same time.

Solution :

(a) MOV IP, #00000100B ; set IP.2=1 INT1 has the highest priority

or SETB IP.2

(b) Priority of interrupt will be changed to INT1 > INT0 > TF0The 8051 will services INT1 first and then INT0 and TF0.

(As the INT0 and TF0 bits in IP register are 0, their priorities follow thesequence in Table 8.1)


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Setting Interrupt Pri rit with the IP regi ter

Example 8.8 :

The IP register is set by the instruction MOV IP, #00001100B after

reset. Discuss the sequence in which the interrupts are serviced.

Solution :

MOV IP, #00001100B instruction sets INT1 & TF1 to a higher

priority level compared with the rest of the interrupts.

Priority of interrupt will then beINT1 > TF1 > INT0 > TF0 > Serial

POWER UP DEFAULT PRIORITY: INT0 > TF0 > INT1 > TF1 > SERIAL(RI+TI)


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Interrupt insidean interrupt

Higher-priority interrupt can interrupt a low-priority interrupt.

An interrupt cannot be interrupted by a low-priority interrupt

No low priority interrupt can get the immediate attention of the CPU

until the 8051 has finished servicing the high-priority interrupts

Main Main Main

ISR ISR

ISR

ISR

Low ISR is

interrupted by a

higher priority ISR

higher ISRlower Lower ISR cannotinterrupt a higher

priority ISR


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Example 8.9

Assume that the INT1 pinis connected to a switchthat is normally high.Whenever it goes low, itshould turn on an LED. The LED is connected toP1.3 and is normally off.Whenit is turned onit

should stay on for a fraction of a second. As longas the switch is pressed, the LED should stay on.

to LED

80515v

0v

P1.3INT1 1=ON

0=OFF


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Example 8.9 - Solution

ORG 0000HLJMP MAIN ;by-pass interrupt vector table

; ISR for INT1ORG 0013HLJMP ISR_INT1 ; jump to ISR_INT1

; Main ProgramORG 30H

MAIN: MOV IE,#10000100B ;enable external INT 1 + GLOBALHERE: SJMP HERE ;stay here until get interrupted

; INT1 ISRISR_INT1: SETB P1.3 ;turn on LED

MOV R3,#255BACK: DJNZ R3, BACK ;keep LED on for a while

CLR P1.3 ;turn off LEDRETI

END


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1. How many timers do we have in the 8051_____ ?(8052 _____?)

2. The timers of the 8051 are ___-bit and are designated as ____ and_______.

3. The registers ofTimer 0 are accessed as ______ and ______.

4. The registers ofTimer 1 are accessed as ______ and ______.

5. The TMOD register is ____-bit register.

6. Find the TMOD value for both Timer 0 and Timer 1, mode 2, softwarestart/stop (gate=0), with driving by internal clock.

7. Find the frequency and period used by the timer if the crystal attachedto the 8051 has the following values.

(a) XTAL = 11.0592MHz (b) XTAL = 12MHz

(c) XTAL = 16MHz (d) XTAL = 20MHz

(e) XTAL = 24MHz (f) XTAL = 30MHz

Review QuestionsReview Questions


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8. Find the value (in hex) loaded into TH in each of the following.

(a) MOV TH1, #-120 (b)MOV TH1, #-67

(c) MOV TH1, #-222 (d)MOV TH0, #-104

9. What address in the interrupt vector table is assigned to Time 0?

10.T

o put the en

ti

rein

terrupt servi

ce routin

ein

thein

terrupt vector table,it must be no more than _____ bytes in size.

11.Why do we put an LJMP instruction at address 0?

12.What register keeps track ofinterrupt priority in the 8051? Is it a bit-addressable register?

13.If the value of IP register

is 09H, arra

nge the pr

ior

ity of the 5

interrupts(exclude Reset) of 8051 in descending order.

14.Assume that the IP register contains all 0s. Explain what happens if bothINT0 and INT1 are activated at the same time.

Review QuestionsReview Questions


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The 8051Microcontroller and Embedded

Systems - Using Assembly and C,Mazidi

Chapter 9 P.239

P.255Chapter 11 P.317 P.339

Read referenceRead reference


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EE341 Embedded System

Dept. of EE, NITR

Timer Programming and InterruptsTimer Programming and Interrupts

timer and interrupts

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