IocInterrupts

Interrupt On Change interrupts induce a Flag whenever a PORT Pin changes state from low-to-high, and/or from high-to-low. Some PICs have generous IOCs while many limit them to PORTB/C.

Older PICs use a generic Read Only IOC Interrupt Flag (IOCIF). Individual PORT bits in the IOCB/C Registers, and IOCIE in the INTCONx Register must be Enabled (Set, = 1). The PIC18FxxK22 is a popular example.

More recent PIC versions allow virtually any Input pin to be configured as an IOC, with polarity control. In fact, the PIC18FxxK40 will allow an IOC interrupt, with Rising Edge AND/OR Falling Edge trigger on EVERY PORT PIN (including MCLR when configured as an Input)! This means you can generate an IOCIF when the PORT goes Positive, and generate an IF when it goes Negative selectively; and you can assign BOTH Rising AND Falling.

IOC is similar to INT0/1/2 in that it is triggered by a change in the logic state of the PORT Pin. The PIC18FxxK50 allows control over Rising/Falling edge trigger over the INTx Interrupts, but any change in logic state on the IOC pins will trigger an IOC interrupt.

Without exception, the Global Interrupt Enable (GIE) bit of the INTCON Register must be set (= 1), and usually the Peripheral Interrupt Enable (PIE) bit must also be set (= 1). The Interrupt On Change Interrupt Enable (IOCIE) bit must be set, along with the individual PORT Pin Interrupt Enable (IOCB, IOCC).
For the PIC18FxxK40, you must set the associated IOCxN bit (IOCAN, IOCBN, IOCCN…) for a Negative Edge trigger and/or the IOCxP bit for a Positive Edge trigger.

For older PICs, the IOCIF merely indicates one of the Enabled IOC PORT Pins have changed logic state.

Within the Interrupt Handler you must read the PORTs to determine which of the Enabled pins triggered the Interrupt. Even if you only have one IOC pin Enabled, the method of clearing the IF is often by reading the PORT.

When a global IOCIF is implemented along with individual PORT IOC flags, the global IF is often Read Only. The method of clearing this Flag is to clear the individual IOC[PORT] Flag(s). The global IF indicates one of the individual IOCIFs are set. Clearing all individual IOCIFs clear the global IF.
With newer PICs, you will only get one single IOCAF (IOCBF, IOCCF…) type Flag, which is specific to the PORT Pin that changed logic state. Clearing that Flag is all that’s needed.

Whether using “ON INTERRUPT GOTO [Label]”, ASM Interrupts, or DT_INTs, older PICs require special attention when handling IOC Interrupts. (Newer PICs with IOCAF type Flags can be treated like any other Interrupt.)

The "IF PORTB.4 = 1" statement initiates a PORT Read, which is one of the requirements for clearing the RBIF (PORTB IOCIF). According to the Data Sheet, this alone should clear the RBIF. Some application Interrupt Handlers may require the RBIF to be cleared manually, INTCON0.0 = 0. Everything else is covered in the PBP3 Reference Manual and Data Sheet.

Notice we did not have to clear the RBIF in the Interrupt Handler (denoted as INTCON.0 = 0 in the “ON INTERRUPT” example). In the Interrupt Processor there is an option to automatically reset the flag, which this example uses yes.

Assembly Language Interrupts are covered in Section 6.2 (page 248) of the PBP3 Reference Manual. The first 3 lines of the Interrupt Handler “Alarm” save important Registers to temporary variables, declared with all your other Variables. They are Restored in 3 of the last 4 lines. RETFIE is the ASM version of RETURN (from Interrupt Handler)
Newer PIC MCUs incorporate automatic context saving. This would be covered in the Interrupt section of the Data Sheet. When Context Restore is available, the MOVx commands storing and restoring context to the W, STATUS, and BSR registers would not be required; but RETFIE FAST would be used instead of RETFIE.

Example 4: DT_INTS on PIC16F1619

;Includes:
include     "DT_INTS-14a.bas"
include     "ReEnterPBP.bas"

;Interrupt Processor:
ASM
INT_LIST  macro      ;IntSource,  Label,     Type, ResetFlag? 
        INT_Handler   IOC_INT,    _Alarm,    PBP,  yes   ;IOCB.4 
    endm
    INT_CREATE                      ;Creates the interrupt processor
ENDASM

;Special Function Registers:
INTCON = %11001000		    ;.0 = IOCIF, .3 = IOCIE
TRISB.4 = 1                         ;RB4 = Input
IOCBP.4 = 1                         ;IOC Interrupt when RB4 goes from low-to-high

;Variables:
wsave   VAR BYTE    $20     SYSTEM  ;location for W if in bank0
;wsave  VAR BYTE    $70	    SYSTEM  ;alternate save location for W
				    ;if using $70, comment wsave1-3

;Labels:
Main:
@ INT_ENABLE IOC_INT
Do
   PAUSE 100
LOOP

Alarm:
HIGH LED
@ INT_RETURN

Using DT_INTS-14 is almost identical to -18 but with the addition of the variable wsave. If you open DT_INTS-14.bas in MicroCode Studio, lines 22 & 23 have 2 variables commented out with instructions to “Place a copy of these variables in your Main program; The compiler will tell you which lines to un-comment; Do Not un-comment these lines.” By pasting both lines into your code, the compiler will give you an error for the wrong one. Since both versions use the same Variable name, you must test them one at a time by commenting out one and trying to compile. In this case, the PIC16F1619 needs the $20 version.

While in DT_INTS-14.bas, lines 30 - 32 list wsave1_2_3 with instructions, “IF any of these three lines cause an error ?? Comment them out to fix the problem; Which variables are needed, depends on the Chip you are using”. When you leave all 3 uncommented and try to compile, you will get an error for wsave3. Simply comment that line out and save this version of DT_INTS-14.bas as something different, like DT_INTS-14a.bas. With these few exceptions, the -14 version will perform the same as the -18 version.

Another good resource covering Assembly Language programming for PIC18 MCUs:
"Applying PIC18 Microcontrollers", by Barry B. Brey (Copyright 2008 Pearson Education, Inc., ISBN 0-13-088546-0).