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#int_rda and math routines

 
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ljbeng



Joined: 10 Feb 2004
Posts: 205

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#int_rda and math routines
PostPosted: Thu Jun 01, 2006 4:03 pm     Reply with quote
3.249

18F2620

In my int_rda routine, I do a short math function.

Code:
rposition = rposition * 225 / 256;


In my main routine, I am mostly just writing data to the lcd. I found that sometimes when this line ran:

Code:
printf(lcd_putc,"OpHr%06lu",hoursrun);


I know hoursrun = 0 but I would see a flicker on the lcd and I found that 225 was being displayed sometimes. So I changed 225 in my int_rda equation to 245. Guess what, 245 was displayed in flickers. Will the compiler use the same ram for math in an interrupt routine as it uses for the main loop routines?

Here is the listing for that area....
Code:
....................                rposition = rposition * 225 / 256;
2330:  MOVFF  1C5,84B
2334:  MOVFF  1C4,84A
2338:  MOVFF  1C3,849
233C:  MOVFF  1C2,848
2340:  MOVLB  8
2342:  CLRF   x4F
2344:  CLRF   x4E
2346:  CLRF   x4D
2348:  MOVLW  E1
234A:  MOVWF  x4C
234C:  MOVLB  0
234E:  RCALL  1D86
2350:  MOVFF  01,1C2
2354:  MOVFF  02,1C3
2358:  MOVFF  03,1C4
235C:  MOVLB  1
235E:  CLRF   xC5


225 (E1) is being written to x4C. If I search the listing for other places that x4C is used, it shows up in a lot of places in my main routine. How can I fix this?
Ttelmah
Guest
PostPosted: Fri Jun 02, 2006 4:24 am     Reply with quote
Unfortunately, without seeing a lot more of your code, it is not possible to see where the problem lies. There is a temporary 'scratch' are used for arithmetic, but this is automatically preserved by the interrupt handler. Things that might cause problems, are:
1) Enabling int_global inside a handler.
2) Accessing a 2byte, or 4byte value in 'main', which is updated in the handler.
3) Using your own int_global handler, which does not protect the scratch area.
4) Using the 'fast' keyword on an interrupt, while doing such operations.
5) A bug. Possible, but a lot of people have been using 3.249 for a while, and while there are problems in some places, I have not seen this one.

Best Wishes
rnielsen



Joined: 23 Sep 2003
Posts: 852
Location: Utah

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PostPosted: Fri Jun 02, 2006 8:58 am     Reply with quote
If, at all possible, do not do any math calculations inside an interrupt. They should be short...short...short. If you need to calculate or print something then simply grab the data, that is coming in, store it and then set a flag. In main() look for that flag and do your time intensive stuff there. If you try to do calculations or print inside your int_rda you could very well lose data while doing them.

Ronald
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