Interrupts, Counter and Timers

Interrupts (1)

Interrupt-driven I/O uses the processor’s interrupt system to “interrupt” normal program flow to allow execution of an I/O service routine

An interrupt is like a hardware-initiated subroutine call

2

Interrupts (2)Advantages

Immediate response to I/O service request

Normal execution continues until it is known that I/O service is needed

DisadvantagesCoding complexity for interrupt service routines

Extra hardware neededProcessor’s interrupt system I/O device must generate an interrupt request3

Basic Steps for an Interrupt (1)An interrupt cycle begins at the next fetch cycle if: Interrupts are enable (GIE = 1) Interrupt request is active (I=1)

Program counter (PC) is saved on the stackPC is loaded with address of interrupt service routine (ISR)

Different schemes for determining this address

Fixed location 0x0004 on the PIC processor

4

Basic Steps for an Interrupt (2)

Interrupt service routine executesSaves any registers that will be altered so that normal program flow is not disturbed

Performs input and/or output operations to clear the interrupt request

Restores saved registersReturns

Normal execution resumes5

Program Flow for an Interrupt

6

Example Pseudo code for an ISR

ISR: save register(s)if (IN_RDY == 1)

input dataif (OUT_RDY == 1)

output datarestore register(s)return

7

Example Error if Registers are not Saved before Executing ISR

8

16F84 INTCON Register

9

16F84 Interrupt Structure

10

Outline of 16F84 Interrupt Structure

11

16F84 Timer/Counter

12

16F84 OPTION Register

13

Relationship between TMR0 and Prescaler

14

Timing Diagram of TMRO Interrupt

15

Determining Timer SettingsConsider the “wait for 500ms” for LED

problem…Fosc = 4 MHz, internal timer can be driven

by Fosc/4 = 1MHzTo have a 500ms interrupt on overflow

interval, desired scale = 500ms/1μs = 500,000

Maximum scale provided by Timer0 is 256*256=65,536

Probably we can set up an interrupt interval shorter than 500ms, say M ms, then toggle the LED after every N interrupts, where M*N = 500. The possible value of M has constraints. It is helpful to

express the needed scale in terms of its prime factors, then allocate these among the prescaler, PR and the TMR0 register.

let’s try M=4 and N=125 for this example.16

Initialize the Timer0Desired scale for 4ms timing is

4ms/1μs = 4,000. Play with the math we have

32*125 =4,000In order to get an exact 4 ms timing, we’ll need

to set the timer as follows:PS2 PS1 PS0 =100 ; set prescaler to 32TMR0 = 256-125 =131 ; initialize TMR0T0IF = 0 ; clear timer0 interrupt

flagTOIE =1 ; enable timer0 interrupt

17

Timer0 Initialization CodeInitTmr0

bcf STATUS, RP1 ;select bank 1bsf STATUS, RP0movlw B’11010100’ ; set up OPTION, prescaler and timer0 resourcesmovwf OPTIONbcf STATUS, RP0 ; select bank 0movlw D’131’movwf TMR0 ; initialize TMR0 bcf INTCON, T0IF ; clear Timer0 interrupt flagbsf INTCON, T0IE ; enable Timer0 interrupt sourcebsf INTCON, GIE ; enable global interruptsreturn

18

“Wait for 500ms” --- Use TimerSince our Timer is set to overflow every 4ms, waiting 4ms

really means waiting until the timer overflows. We will write an interrupt service routine to handle the timer overflow in a future class. Now we assume that in the interrupt service routine (ISR), BLNKCNT is decremented by 1 each time an overflow occurs.

Waiting 500ms means waiting timer overflow 125 times, which is counted by BLNKCNT.

Five00Msmovlw 124 ; initialize BLNKCNT to 124, why???movwf BLNKCNT ; BLNKCNT is a variable

wait500btfss BLNKCNT, 7 ; wait for 00000000 to 11111111 changegoto wait500 ; wait if notreturn ; and return if yes

19

Put it All Together - The Mainline Program

Mainlinecall Initial ; Initialize PortBcall InitTmr0 ; Initialize Timer0

MainLoopcall Blink ; Blink LEDcall Five00ms ; Insert 500ms delaygoto MainLoop

Note: BLNKCNT will be decremented in the interrupt service routine which we will talk about in the future!20

Watchdog TimerFree running counter with its own oscillatorCan be enabled or disabled through special

directives in the programUsually used to reset the controller if the

program gets stuck or works incorrectlyProgrammer needs to be careful when using

it so that it does not cause problems

21