Hey what im tryin to do is set up a timer to overflow every2 ms so this code will sample an ecg wave at 500Hz. Cant get my head around how to do this and any help would be greatly appreciated. Thanks Code: $MOD52 rd1 equ P0.0 ;Read signal P0.0 wr1 equ P0.1 ;Write signal P0.1 cs equ P0.2 ;Chip Select P0.2 intr equ P0.3 ;INTR signal P0.3 adc_port EQU P2 ;ADC data pins P2 adc_val EQU 30H ;ADC read value stored here org 0H start: ;Start of Program acall conv ;Start ADC conversion acall read ;Read converted value mov P3,adc_val ;Move the value to Port 3 sjmp start ;Do it again conv: ;Start of Conversion clr cs ;Make CS low clr wr1 ;Make WR1 Low nop setb wr1 ;Make WR High setb cs ;Make CS high wait: jb intr,wait ;Wait for INTR signal ret ;Conversion done read: ;Read ADC value clr cs ;Make CS Low clr rd1 ;Make RD1 Low mov a,adc_port ;Read the converted value mov adc_val,a ;Store it in local variable setb rd1 ;Make RD1 High setb cs ;Make CS High ret ;Reading done END
Timers are usually pretty complicated, loaded with features. For the most simple of uses, you will need to calculate how many clock cycles constitute two milliseconds. Depending on the processor, this might not be period*frequency. The 18F PICs, for example, operate at 1/4th the oscillator frequency. For a 32MHz oscillator, then, the PIC will operate at 8MHz. 0.002*8MHz is 16000 cycles. Choose a 16-bit timer, and you will need to load it with 0xFFFF - 16000 = 49535. Then it will overflow in 16000 counts, or 2 ms. Simple idea with polled interrupts: 0) make sure the interrupt for this timer is not active. It should reset this way if your program isn't doing anything else you've not mentioned. If it is doing other things, be sure you're not using it for something else, or choose some other timer. 1) stop the timer. Load 2ms_count into the timer. Clear the interrupt flag. Start the timer. (check your datasheet for the exact order of operations.) 2) wait for overflow flag to be set. If not set, goto 2. 3) start ADC conversion. This usually takes some time, maybe you can get away with a small nop-loop. 4) read the value and do whatever with it. 5) goto 1.
