Working with time: interrupts, counters and timers Chapter Six Dr. Gheith Abandah1

Outline Interrupts – Interrupt Structures – Interrupt Demonstration Program – Dealing with Multiple Interrupt Sources – Context Saving Counters and Timers Watchdog Timer Sleep Mode Dr. Gheith Abandah2

General Interrupt Structure Dr. Gheith Abandah3

The 16F84A Interrupt Sources Dr. Gheith Abandah4 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.

The 16F84A Interrupt Structure Dr. Gheith Abandah5 INTCON: SFR 0Bh

PIC Response to an Interrupt Dr. Gheith Abandah6

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 Dr. Gheith Abandah7

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,intedg Dr. Gheith Abandah8

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 end Dr. Gheith Abandah9

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 Dr. Gheith Abandah10

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 Dr. Gheith Abandah11

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_TEMP swapf w_temp,0 ;Swap nibbles in W_TEMP into W Clear interrupt flag(s) here retfie Dr. Gheith Abandah12

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. Dr. Gheith Abandah13

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. Dr. Gheith Abandah14

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 Dr. Gheith Abandah15

PIC 16F84A Timer 0 Module Dr. Gheith Abandah16

PIC 16F84A Timer 0 Module Dr. Gheith Abandah17

Option Register T0CS: Clock source select T0SE: Source edge select PSA: Prescaler assignment bit PS2:PS0: Prescaler rate select Dr. Gheith Abandah18

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 ; Dr. Gheith Abandah19

Counter Demonstration – Page 2 ; org 00 ; Initialise bsf status,rp0 ;select memory bank 1 movlw B' ' movwf trisa ;set port A to above pattern movlw 00 movwf trisb ;all port B bits output movlw B' ' ;set up T00 for external input, ; +ve edge, and no prescale movwf TMR0 bcf status,rp0 ;select bank 0 ; Dr. Gheith Abandah20

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 Dr. Gheith Abandah21

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 = 131 to get overflow after 5 ms Dr. Gheith Abandah22

HW Delay Demonstration – Page 1... ;Initialise org 0010 start bsf status,5 ;select memory bank 1 movlw B' ' movwf trisa ;set port A to above pattern movlw 00 movwf trisb ;all port B bits output movlw B' ' ;set up T0 for internal input, ; prescale by 8 movwf option_reg bcf status,5 ;select bank 0... Dr. Gheith Abandah23

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 Dr. Gheith Abandah24

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. Dr. Gheith Abandah25

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 Dr. Gheith Abandah26

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. Dr. Gheith Abandah27