Specifications

Chapter 2 - Microcontroller PIC16F84

External interrupt on RB0/INT pin is triggered by rising signal edge (if bit INTEDG=1 in OPTION<6>

INTCON register. INTF bit (INTCON<1>) must be reset in interrupt routine, so that interrupt wouldn't

occur again while going back to the main program. This is an important part of the program which

programmer must not forget, or program will constantly go into interrupt routine. Interrupt can be

turned off by resetting INTE control bit (INTCON<4>).

Interrupt during a TMR0 counter overflow

Overflow of TMR0 counter (from FFh to 00h) will set T0IF (INTCON<2>) bit. This is very important

interrupt because many real problems can be solved using this interrupt. One of the examples is time

measurement. If we know how much time counter needs in order to complete one cycle from 00h to

FFh, then a number of interrupts multiplied by that amount of time will yield the total of elapsed

time. In interrupt routine some variable would be incremented in RAM memory, value of that variable

multiplied by the amount of time the counter needs to count through a whole cycle, would yield total

elapsed time. Interrupt can be turned on/off by setting/resetting T0IE (INTCON<5>) bit.

Interrupt during a change on pins 4, 5, 6 and 7 of port B

Change of input signal on PORTB <7:4> sets RBIF (INTCON<0>) bit. Four pins RB7, RB6, RB5 and

RB4 of port B, can trigger an interrupt which occurs when status on them changes from logic one to

logic zero, or vice versa. For pins to be sensitive to this change, they must be defined as input. If any

one of them is defined as output, interrupt will not be generated at the change of status. If they are

defined as input, their current state is compared to the old value which was stored at the last reading

from port B. Interrupt can be turned on/off by setting/resetting RBIE bit in INTCON register.

Interrupt upon finishing write-subroutine to EEPROM

This interrupt is of practical nature only. Since writing to one EEPROM location takes about 10ms

(which is a long time in the notion of a microcontroller), it doesn't pay off to a microcontroller to wait

for writing to end. Thus interrupt mechanism is added which allows the microcontroller to continue

executing the main program, while writing in EEPROM is being done in the background. When writing

is completed, interrupt informs the microcontroller that writing has ended. EEIF bit, through which

this informing is done, is found in EECON1 register. Occurrence of an interrupt can be disabled by

resetting the EEIE bit in INTCON register.

Interrupt initialization

In order to use an interrupt mechanism of a microcontroller, some preparatory tasks need to be

performed. These procedures are in short called "initialization". By initialization we define to what

interrupts the microcontroller will respond, and which ones it will ignore. If we do not set the bit that

allows a certain interrupt, program will not execute an interrupt subprogram. Through this we can

obtain control over interrupt occurrence, which is very useful.

The above example shows initialization of external interrupt on RB0 pin of a microcontroller. Where

we see one being set, that means that interrupt is enabled. Occurrence of other interrupts is not

allowed, and all interrupts together are disallowed until GIE bit is keeping to one.

The following example shows a typical way of handling interrupts. PIC16F84 has only one location

where the address of an interrupt subprogram is stored. This means that first we need to detect

which interrupt is at hand (if more than one interrupt source is available), and then we can execute

that part of a program which refers to that interrupt.

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