Working with time: interrupts,counters and timers

Chapter Six

Dr. Gheith Abandah 1

Outline

• Interrupts– Interrupt Structures– Interrupt Demonstration Program– Dealing with Multiple Interrupt Sources– Context Saving

• Counters and Timers• Watchdog Timer• Sleep Mode

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General Interrupt Structure

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The 16F84A Interrupt Sources

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1. External interrupt. It shares a pin with Port B, bit 0. It is edge triggered.

2. Timer 0 overflow. It occurs when the timer’s 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.

The 16F84A Interrupt Structure

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INTCON:SFR 0Bh

PIC Response to an Interrupt

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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

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Interrupt Demonstration 1 – Page 2

start

bsf status,rp0 ;select bank 1

movlw 01

movwf trisb ;bits 1-7 output, bit 0 input

movlw 00

movwf trisa ;porta bits all output

bcf status,rp0 ;select bank 0

bsf intcon,inte ;enable external interrupt

bsf intcon,gie ;enable global int

;Remove semi-colon from following instruction to change

;interrupt edge

; bsf option_reg,intedgDr. Gheith Abandah 8

Interrupt Demonstration 1 – Page 3

wait

movlw 0a ;set up initial port output values

movwf porta

movlw 15

movwf porta

goto wait

;

org 0080

Int_Routine ;Int Service Routine continues here

movlw 00

movwf porta

bcf intcon,intf ;clear INTF - Important

retfie

endDr. Gheith Abandah 9

Multiple Interrupts – Identifying the Source

interrupt

btfsc intcon,0 ;test RBIF

goto portb_int ;Port B Change routine

btfsc intcon,1 ;test INTF

goto ext_int ;external interrupt routine

btfsc 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

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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 flag

movlw 0ff ;change W reg value

bcf intcon,intf

retfie

end

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Solution: Context SavingPUSH 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_TEMP

swapf w_temp,0 ;Swap nibbles in W_TEMP into W

Clear interrupt flag(s) hereretfie

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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.

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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.

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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

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PIC 16F84A Timer 0 Module

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PIC 16F84A Timer 0 Module

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Option Register

• T0CS: Clock source select• T0SE: Source edge select• PSA: Prescaler assignment bit• PS2:PS0: Prescaler rate select

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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

;

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Counter Demonstration – Page 2;

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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Counter Demonstration – Page 3;

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

movwf porta

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

movwf portb

goto loop

end

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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 μs– Preselect = 8 for 125 count → 125 x 8 x 5 μs = 5.00 ms– Need to set T0 at 256-125 = 131 to get overflow after 5 ms

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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

...

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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 overflow

movwf TMR0

del1 btfss intcon,2 ;test for Timer Overflow flag

goto del1 ;loop if not set

bcf intcon,2 ;clear Timer Overflow flag

return

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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.

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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 advanced applications.

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