microcontroller - Maximum attainable delay with Micro controller

I am designing a microcontroller based delay circuit to implement delays of 2 hours, 1 hour, 45 minutes, and 30 minutes. The circuit will automatically turn on off a relay after this time period has elapsed.

I am stuck with a narrow selection of microcontrollers available locally in market:

• 89C51


• 89C52


• 89S51


• 89S52


• 89C2051



• PIC 16C71


• PIC 16F84

I have checked the datasheets of these microcontrollers but there is no information about the maximum delay they can produce.

What is the maximum delay that can be produced with these microcontrollers?

Answer

The delay can be as long as you want. If a timer won't give you the delay you need, simply increment a register, or several registers, each time it overflows. Here is a simple program using Timer0 that illustrates the technique:

        #include    "P16F88.INC"

#define LED 0


    errorlevel -302     ;suppress "not in bank 0" message

#define RB0 0
#define INIT_COUNT 0    ;gives (255 - 199) * 17.36 = 972 us between interrupts


    cblock  0x20
    tick_counter
    temp    

    endc

    ;reset vector
    org     0
    nop
    goto    main

    ;interrupt vector
    org     4
    banksel INTCON

    bcf     INTCON,TMR0IF   ;clear Timer0 interrupt flag
    movlw   0x00            ;re-initialise count
    movwf   TMR0
    decf    tick_counter,f
    retfie

main:
    banksel OPTION_REG
    movlw   b'00000111'     ;prescaler 1/128
    movwf   OPTION_REG      ;giving 7.3728 Mz/128 = 57600 Hz (period = 17.36 us)

    banksel TMR0
    movlw   0x00            ;initialise timer count value
    movwf   TMR0
    bsf     INTCON,GIE      ;enable global interrupt
    bsf     INTCON,TMR0IE   ;enable Timer0 interrupt
    banksel TRISB
    bcf     TRISB,LED   ;RB0 is LED output
    banksel 0

mainloop:

    goto    mainloop

    bsf     PORTB,LED
    call    dly
    bcf     PORTB,LED
    call    dly 
    goto    mainloop

dly:
    movlw   20

    movwf   tick_counter
dly1:
    movf    tick_counter,f
    skpz
    goto    dly1
    return

    end