Interrupt 101

[Charles Linquis]

And most people don't realize it, but you can have an interrupt on *ANY* pin - sort of. This is handy when you have a switch that simply must be read and it is on port C, for example.
Just run a high-speed interrupt on TMR0 and read whatever pins you want inside the ISR. I typically run a 1mSec interrupt.

A small example of an "interrupt header"

 
  PreloadH      VAR BYTE BANKA SYSTEM
  PreloadL       VAR BYTE BANKA SYSTEM
  MasterClock   VAR WORD BANKA SYSTEM
  ButtonFlag    var BYTE BANKA SYSTEM
 
 
  PreloadH= $D8
  PreloadL = $F7    ; 1 mSec@40Mhz
 
  T0CON = %10001000  ; Timer ON, no prescaler
 
 
'---------------------------------- --------------------------------  
 
        INCLUDE "DT_INTS-18.bas"         
 
'        INCLUDE "ReEnterPBP-18.bas"     ; Include if using PBP interrupts
 
ASM
INT_LIST  macro    ;IntSource,        Label,  Type, ResetFlag?
 
     INT_Handler    TMR0_INT,   MainTimer, ASM,  yes 
; INT_Handler    RX1_INT, GetChar,    PBP,  yes
 
endm
 
INT_CREATE               
 
ENDASM
 
Goto OverInt
 
 
'---[INT - interrupt handler]---------------------------------------------------
 
Asm                            
 
MainTimer
 
    movff    PreloadH,TMR0H  ; Preload depends on clk speed
    movff    PreloadL,TMR0L
     clrf     INTCON,2
 
     infsnz  MasterClock
     incf    MasterClock + 1
 
     btfss  PORTC,4
     bsf  ButtonFlag,0
 
@ INT_RETURN
 
ENDASM      
 
OverInt:
 
   ButtonFlag = 0
   MasterClock = 0
   INTCON =%11100000  ; just to be certain!
 
@ INT_ENABLE TMR0_INT  
 
 
Your program here

This particular code will increment MasterClock at a 1000Hz rate (1 ms / count). You can read it at any time, although you should either disable INTs when you test it, or only test that it is ABOVE (not equal to) a value, since it is a 16 bit var and can increment in the middle of a PBP read.

ButtonFlag.0 will be set any time after someone has pressed a button bringing PORTC.4 to GND. You can read it anytime, then clear it in your main code.

If you have more complicated things to do on an interrupt, you can write it in PBP.


So THERE! mister_e and pxidr84!

[amgen]

That's a good setup !
How about switch de-bounce (make sure its a good press)

I use;

if sw not_pressed then out
pause 10 ' or 20 or 50 ms
if sw not_pressed then out
good sw pressed stuff here

don

[Charles Linquis]

You often don't need a debounce if you read a switch in software. Hardware is so fast that it can see one push as a bunch of pushes, but software is generally a lot slower and you can use that to your advantage. In the example I gave above, the bit gets set as soon as the first contact is made. If the main loop reads the bit at that time, and then does something that takes 50 mSec, you can clear ButtonFlag at the end of that routine (rather than at the top). The switch will certainly have quit bouncing by then, so the bit won't get re-set by the bounce. This saves 50mSec of waiting.
If things really get "hairy", I do the debounce in ASM. I count the loops that the switch has been down (or up) and only generate a flag when that number of loops has been executed.

My programming style uses PAUSE sparingly, if at all. That is the only way you can get real performance out of a PIC and PBP.

[mister_e]

And most people don't realize it, but you can have an interrupt on *ANY* pin - sort of. ........
So THERE! mister_e and pxidr84!

Sure, I classify that as slow background process

amgen, you just need to read the switch all the time, keep a reading of the previous(s) and compare them. Depending of your timer rate, you may want to make sure it is hold down for X interrupt. set a flag to say a switch is ready, Once done, and the button being read, you wait Y interrupt before reading another and "publish" it.

Tons of way to do so. But for sure, you don't want any PAUSE in an ISR.

[amgen]

Yes, getting more accustomed to thinking in terms of COUNT rather than time..... although the count is always exactly related to your timing base. Right ? Also brings more into play, Events to do actions (like windows), and more use of GOSUB's.

don

[Hylan]

Okay, I've been looking at Daryl Taylors interrupt programs as recommended by Steve. I downloaded everything and tested the blinky blinky to make sure it worked. I changed the toggle pin to one on my board with an LED and it worked.

Now I'm trying to figure out how to incorporate this into my program and what terms I can change and what I can't and where it needs to be placed in the program. I don't have anything called a "main" loop for example.

So for example with this sample code:

 
LED1   VAR  PORTB.1
 
INCLUDE "DT_INTS-18.bas"     ; Base Interrupt System
INCLUDE "ReEnterPBP-18.bas"     ; Include if using PBP interrupts
 
ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
        INT_Handler    INT_INT,  _ToggleLED1,   PBP,  yes
    endm
    INT_CREATE               ; Creates the interrupt processor
ENDASM
 
@   INT_ENABLE   INT_INT     ; enable external (INT) interrupts
 
Main:
  PAUSE 1
GOTO Main
 
'---[INT - interrupt handler]---------------------------------------------------
ToggleLED1:
     TOGGLE LED1
@ INT_RETURN

I understand that the "include" statements go at the top when declaring stuff but I'm not sure where to put the "ASM" section
and the "INT" Interrupt handler commands

I tried a couple things and the blinky blinky always worked but I never got my part of the program to run so it seemed to only stay in the above portion.

Getting closer, I can smell it, but I can't quite see it or taste it.
Thank you,
Hylan

[mackrackit]

In the example you have posted place your code in the "Main" loop. To start try a PAUSE type blink in the Main loop to seehow ot works.

[Hylan]

Beautiful! I've got the interrupt working properly on the one pin. Now I can start playing with all the other layers and types. I'm sure I will have more questions but at least I've got a starting point.
Thank you!
Hylan

[Hylan]

I'm finally back looking at this again and would like to use the RX2 pin as the interrupt trigger when a serial string comes in on my 18F6520 chip.
I'm currently using a hserin command and just running it in my main loop to pick up new signals and 999/1000 it works fine.
I'm looking through pic manual but it's pretty Greek to me.
Does anyone have an example of how to configure this for the RX2 pin?
Thank you,
Hylan