6 Working With Time Interrupts Counters and Timers

7/30/2019 6 Working With Time Interrupts Counters and Timers

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Working with time: interrupts,

counters and timers

Chapter Six

Dr. Gheith Abandah 1


2/27

Outline

Interrupts

Interrupt Structures

Interrupt Demonstration Program

Dealing with Multiple Interrupt Sources

Context Saving

Counters and Timers

Watchdog Timer

Sleep Mode



3/27

General Interrupt Structure



4/27

The 16F84A Interrupt Sources


1. External interrupt. It shares a pin with Port B,

bit 0. It is edge triggered.

2. Timer 0 overflow. It occurs when the timers

8-bit counter overflows.

3. Port B interrupt on change. This interrupts

when a change is detected on any of the

higher 4 bits of Port B.

4. EEPROM write complete.


5/27

The 16F84A Interrupt Structure


INTCON:

SFR 0Bh


6/27

PIC Response to an Interrupt



7/27

Interrupt Demonstration 1 Page 1

#include p16f84A.inc

;

;Port A all output

;Port B: Bit 0 = Interrupt input;

org 00 ;Reset start

goto start

org 04 ;Int Service Routine start

goto Int_Routine



8/27


9/27

Interrupt Demonstration 1 Page 3

wait

movlw 0a ;set up initial port output values

movwf porta

movlw 15

movwf portagoto wait

;

org 0080

Int_Routine ;Int Service Routine continues heremovlw 00

movwf porta

bcf intcon,intf ;clear INTF - Important

retfie

end Dr. Gheith Abandah 9


10/27

Multiple Interrupts Identifying

the Sourceinterrupt

btfsc intcon,0 ;test RBIF

goto portb_int ;Port B Change routine

btfsc intcon,1 ;test INTF

goto ext_int ;external interrupt routinebtfsc intcon,2 ;test T0IF

goto timer_int ;timer overflow routine

btfsc eecon1,4 ;test EEPROM write complete flag

goto eeprom_int ;EEPROM write complete routine



11/27

Bad Effects of Interrupt Service

Routines

movf plo,0

addwf qlo,0 ;What happens on an interrupt here?

movwf rlo

btfsc status,0

Int_Routine

bcf status,0 ;clear the Carry flagmovlw 0ff ;change W reg value

bcf intcon,intf

retfie

end



12/27

Solution: Context Saving

PUSH movwf w_temp ;Copy W to W_TEMP register

swapf status,0 ;Swap status into W

movwf status_temp ;Save status

...

Actual ISR goes here...

POP swapf status_temp,0 ;Swap nibbles into W

movwf status ;Move W into STATUS register

swapf w_temp,1 ;Swap nibbles in W_TEMPswapf w_temp,0 ;Swap nibbles in W_TEMP into W

Clear interrupt flag(s) here

retfie



13/27

Critical Regions

In critical regions, we cannot allow the

intrusion of an interrupt.

Critical regions generally include all time-

sensitive activity and any calculation where

the ISR makes use of the result.

Disable, or mask, the interrupts for their

duration, by manipulating the enable bits in

the INTCON register.



14/27

Counters and Timers

A digital counter is

usually made of flip

flops and counts up or

down. When the triggering

event is a constant

frequency clock, we get

a timer.



15/27

Timer Applications

(a) Measure the time

between two events

(b) Measure the time

between two pulses(c) Measure a pulse

duration

Use polling or interrupts



16/27

PIC 16F84A Timer 0 Module



17/27

PIC 16F84A Timer 0 Module



18/27

Option Register

T0CS: Clock source select T0SE: Source edge select

PSA: Prescaler assignment bit

PS2:PS0: Prescaler rate select



19/27

Counter Demonstration Page 1

;

;Demos Timer 0 as counter, using ping-pong hardware

;

;Clock freq 800kHz approx (RC osc.)

;Port A 4 right paddle (ip) Counter input.; 2 "out of play" led (op)

;Port B 7-0 "play" leds (all op)

;

list p=16F84A#include p16f84A.inc

;



20/27


;

org 00

; Initialise

bsf status,rp0 ;select memory bank 1

movlw B'00011000'movwf trisa ;set port A to above pattern

movlw 00

movwf trisb ;all port B bits output

movlw B'00101000' ;set up T00 for external input,; +ve edge, and no prescale

movwf TMR0

bcf status,rp0 ;select bank 0

;



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;

movlw 04 ;switch on a led to show power is on

movwf porta

loop movf TMR0,0 ;Continuously display T0 on Port B

movwf portbgoto loop

end



22/27

Hardware-Generated Delays

In software-generated delays, the CPU is tied

up during the delay time.

Can use Timer 0 for delays.

The timer overflow interrupt is used to inform

the CPU when the delay time is up.

Example:

Clock = 800 KHz Fosc/4 = 200 KHz T = 5 s

Preselect = 8 for 125 count 125 x 8 x 5 s = 5.00 ms

Need to set T0 at 256-125 = 131 to get overflow after 5 ms



23/27

HW Delay Demonstration Page 1

...

;Initialise

org 0010

start bsf status,5 ;select memory bank 1

movlw B'00011000'movwf trisa ;set port A to above pattern

movlw 00

movwf trisb ;all port B bits output

movlw B'00000010' ;set up T0 for internal input,; prescale by 8

movwf option_reg

bcf status,5 ;select bank 0

...



24/27

HW Delay Demonstration Page 2

...

;introduces delay of 5ms approx

delay5

movlw D'131' ;preload T0, so that 125 cycles,

; each of 40us, occur to overflowmovwf TMR0

del1 btfss intcon,2 ;test for Timer Overflow flag

goto del1 ;loop if not set

bcf intcon,2 ;clear Timer Overflow flagreturn



25/27

The Watchdog Timer

A big danger with any computer-based system

is that the software fails in some way and that

the system locks up or becomes unresponsive.

The WDT continually counts up. If it ever

overflows, it forces the microcontroller into

reset.

If the instruction clrwdt is not executed every

(18 ms * prescaler rate), overflow occurs.



26/27

Sleep Mode

When the sleep instruction is executed, the

PIC halts execution and enters power saving

mode.

It awakes at:

External reset (MCLR)

WDT wake-up

Occurrence of an enabled interrupt



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Summary Interrupts and counter/timers are important

hardware features of almost all microcontrollers.

They both carry a number of important hardware

and software concepts, which must be understood.

The basic techniques of using interrupts and

counter/timers have been introduced in this

chapter. There is considerably increased

sophistication in their use in more advancedapplications.


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6 Working With Time Interrupts Counters and Timers

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