I am working on my first project using a 4MHz PIC16F84. The whole purpose of the project is to set portb pin from HIGH to LOW for a prolong period (as much as 30 minutes) before tuning back to HIGH upon detection of activity at porta pins.
My idea is to cascade a series of fully loaded counters (I am using 4
8-bit file registers, i.e. count1 to count4, in my experiment) so as to generate the required lengthy delay. The idea is simple but it doesn't work out as expected however.What's strange about the counting process is count3 will only count from 0xFF to 0xF5 before reseting itself to 0xFF. That is where my trouble is about. It has been confirmed both on simulator and my hardware circuit. If I load the counters with smaller initial values, say 0x0F, the problem disappears. Of course, smaller values doesn't generate long delay. Can somebody give me some idea about why the PIC behaves differently from what the programme logic states?
Below is the problematic code that I've been experimenting. Thanks for any help.
;;;;;;;;;;;;;;;;; PROBLEMATIC PIC CODE ;;;;;;;;;;;;;;; list p=16F84 radix hex porta equ 0x05 portb equ 0x06 count1 equ 0x0c count2 equ 0x0d count3 equ 0x0e count4 equ 0x0f
org 0x000 ; processor reset vector goto start start movlw 0xff tris porta ; configure porta as input port movlw 0x00 tris portb ; configure portb as output port clrf portb ; set portb LOW scan btfss porta, 0 call timer btfss porta, 2 call timer btfss porta, 3 call timer goto scan
timer bsf portb, 0 ; set portb HIGH
; initialise count1 to count4 movlw 0xff movwf count1 movlw 0xff movwf count2 movlw 0xff movwf count3 movlw 0xff movwf count4 delay decfsz count1, f goto delay movlw 0xff movwf count1 decfsz count2, f goto delay movlw 0xff movwf count2 decfsz count3, f ; THIS GUY NEVER COME TO ZERO BUT 0xF5, WHY??? goto delay movlw 0xff movwf count3 decfsz count4, f goto delay bcf portb, 0 ; set portb LOW return
end