Timer and Interupt.

Referring to the code as given, modify the value of TH0 and TL0. Then, discuss the observation. Modify the code by changing the involved port number and discuss the observation.

ORG 0 ; reset vector

JMP main ; jump to the main program

ORG 3 ; external 0 interrupt vector

JMP ext0ISR ; jump to the external 0 ISR

ORG 0BH ; timer 0 interrupt vector

JMP timer0ISR ; jump to timer 0 ISR

ORG 30H ; main program starts here

main:

SETB IT0 ; set external 0 interrupt as edge-activated

SETB EX0 ; enable external 0 interrupt

CLR P0.7 ; enable DAC WR line

MOV TMOD, #2 ; set timer 0 as 8-bit auto-reload interval timer

MOV TH0, #-50 ; | put -50 into timer 0 high-byte - this reload value, with system clock of 12 MHz, will result ;in a timer 0 overflow every 50 us

MOV TL0, #-50 ; | put the same value in the low byte to ensure the ;timer starts counting from ; | 236 (256 - 50) rather than 0

SETB TR0 ; start timer 0

SETB ET0 ; enable timer 0 interrupt

SETB EA ; set the global interrupt enable bit

JMP $ ; jump back to the same line (ie; do nothing)

; end of main program

; timer 0 ISR - simply starts an ADC conversion

timer0ISR:

CLR P3.6 ; clear ADC WR line

SETB P3.6 ; then set it - this results in the required ;positive edge to start a conversion

RETI ; return from interrupt

; external 0 ISR - responds to the ADC conversion complete interrupt

ext0ISR:

CLR P3.7 ; clear the ADC RD line - this enables the ;data lines

MOV P1, P2 ; take the data from the ADC on P2 and send ;it to the DAC data lines on P1

SETB P3.7 ; disable the ADC data lines by setting RD

RETI ; return from interrupt