question on "ON INTERRUPT" in PicBasic Pro

ON INTERRUPT

    ON INTERRUPT GOTO Label

ON INTERRUPT allows the handling of microcontroller interrupts by a PICBASIC PRO™ subroutine.

There are 2 ways to handle interrupts using the PICBASIC PRO™ Compiler. The first is to write an assembly language interrupt routine. This is the way to handle interrupts with the shortest latency and lowest overhead. This method is discussed under advanced topics in a later section.

The second method is to write a PICBASIC PRO™ interrupt handler. This looks just like a PICBASIC PRO™ subroutine but ends with a RESUME.

When an interrupt occurs, it is flagged. As soon as the current PICBASIC PRO™ statement=s execution is complete, the program jumps to the BASIC interrupt handler at Label. Once the interrupt handler is complete, a RESUME statement sends the program back to where it was when the interrupt occurred, picking up where it left off.

DISABLE and ENABLE allow different sections of a PICBASIC PRO™ program to execute without the possibility of being interrupted. The most notable place to use DISABLE is right before the actual interrupt handler. Or the interrupt handler may be placed before the ON INTERRUPT statement as the interrupt flag is not checked before the first ON INTERRUPT in a program.

Latency is the time it takes from the time of the actual interrupt to the time the interrupt handler is entered. Since PICBASIC PRO™ statements are not re-entrant (i.e. you cannot execute another PICBASIC PRO™ statement while one is being executed), there can be considerable latency before the interrupt routine is entered.

PBP will not enter the BASIC interrupt handler until it has finished executing the current statement. If the statement is a PAUSE or SERIN, it could be quite a while before the interrupt is acknowledged. The program must be designed with this latency in mind. If it is unacceptable and the interrupts must be handled more quickly, an assembly language interrupt routine must be used.

Overhead is another issue. ON INTERRUPT will add an instruction after every statement to check whether or not an interrupt has occurred. DISABLE turns off the addition of this instruction. ENABLE turns it back on again. Usually the additional instruction will not be much of a problem, but long programs in small microcontrollers could suffer.

More than one ON INTERRUPT may be used in a program.

	ON INTERRUPT GOTO myint	' Interrupt handler is myint
	INTCON = %10010000	' Enable RB0 interrupt

	. . .

	DISABLE		' Disable interrupts in handler
	myint: 	led = 1		' Turn on LED when interrupted
	RESUME		' Return to main program
	ENABLE		' Enable interrupts after handler


To turn off interrupts permanently (or until needed again) once ON INTERRUPT has been used, set INTCON to $80:

        INTCON = $80

BUTTON

BUTTON Pin,Down,Delay,Rate,BVar,Action,Label

Read Pin and optionally performs debounce and auto-repeat. Pin is automatically made an input. Pin may be a constant, 0-15, or a variable that contains a number 0-15 (e.g. B0) or a pin name (e.g. PORTA.0).
------------------------------------------------------------------------------------
Down: State of pin when button is pressed (0..1).

Delay: Cycle count before auto-repeat starts (0..255). If 0, no debounce or auto-repeat is performed. If 255, debounce, but no auto-repeat, is performed.

Rate: Auto-repeat rate (0..255).

BVar: Byte-sized variable used internally for delay/repeat countdown. It must be initialized to 0 prior to use and not used elsewhere in the program.

Action: State of button to act on (0 if not pressed, 1 if pressed).

Label: Execution resumes at this label if Action is true.
------------------------------------------------------------------------------------

example:
	' Goto notpressed if button not pressed on Pin2
	BUTTON PORTB.2,0,100,10,B2,0,notpressed

BUTTON needs to be used within a loop for auto-repeat to work properly.

BUTTON accomplishes debounce by delaying program execution for a period of milliseconds to wait for the contacts to settle down. The default debounce delay is 10ms. To change the debounce to another value, use DEFINE:

        > Set button debounce delay to 50ms

        DEFINE BUTTON_PAUSE 50

Be sure that BUTTON_PAUSE is all in upper case.

In general, it is easier to simply read the state of the pin in an IF..THEN than to use the BUTTON command as follows:

        If PORTB.2 = 1 Then notpressed

Example program:

INCLUDE "modedefs.bas" 	' Include serial modes
SO 	Con 	0 	' Define serial output pin
Bpin 	Con 	2 	' Define Button input pin
B0 	Var 	Byte

	B0 = 0 		' Zero Button working buffer

loop: BUTTON Bpin,1,10,5,B0,0,notp 	' Check button(skip if not pressed)
	Serout SO,N2400,["Press",13,10] 	' Indicate button pressed

notp: Serout SO,N2400,[#B0,13,10] 	' Show working variable
	
	Pause 100 	' Wait a little
	Goto loop 	' Do it forever

Delay: Cycle count before auto-repeat starts (0..255). If 0, no debounce or auto-repeat is performed. If 255, debounce, but no auto-repeat, is performed.

Rate: Auto-repeat rate (0..255).

Action: State of button to act on (0 if not pressed, 1 if pressed).