Meaning of #device HIGH_INTS=TRUE

[Mrinmoy Dey]

I am using PIC18F452 in CCS C compiler.
In the system configuration the line have to compile, #device HIGH_INTS=TRUE.
I want to know the meaning of this and why this line is being used?

[Ttelmah]

On the PIC18 chips, there are two hardware interrupt priorities.

HIGH_INTS=TRUE adds the code to allow these to be used.

It is in the manual, but is under the #INT_XXXX entry, for how to organise an interrupt handler:

[Torello]

Hi Ttelmah,
Creating one fast interrupt routine (just incrmenting int8) the CCS way will generate ~500 bytes of extra code on a 18LFxxx pic.
Wow, 500 bytes..??! It is pretty fast however. Running @4Mhz going in/out the running code takes ~12us for just incrementing an int8 in the handler.
But I want to create more of those fast handlers. Adding one extra can't be fast anymore according to CCS, and, indeed, then in/out jumps to 105us. For the same action. Ouch!

I'm looking for a DIY way to CCS code more fast i'rupt routines with no priority and also no interrupting any on-going interrupt. Fast and more then one. Did you go into that corner by any chance?

Best regards,
Edwin
_________________
Regards, Edwin. PCWHD v5.114

[Ttelmah]

You are misunderstanding slightly.

There are high priority interrupts, 'HIGH' and 'FAST'. Two separate but interconnected things.

HIGH_INTS=TRUE, allows both to be enabled.

FAST, is the high priority equivalent of the #INT_GLOBAL for the low priority interrupts. With this, _your code_ must save any registers that need to be saved. Very fast response, but if you don't save a register, disaster. Also the handler does no testing for which interrupt has triggered. Hence only one such interrupt. However if you have enabled one 'FAST' handler, there is nothing to stop you enabling the high priority bit for another interrupt, and having this too arrive at the same handler. Then you have to check what interrupt has actually triggered. Just like the #INT_GLOBAL handler. There is only one physical vector for the high priority interrupts and one for the low.

HIGH gives you high priority interrupts. Remember 'high priority' does not make the interrupt any faster. It still has to save the registers, and test what interrupt has happened. Exactly the same time as for the low priority interrupts (actually the low priority interrupts will then become very slightly slower since these will now use RETFIE, and the high interrupt will now use RETFIE 1 which saves a couple of instructions). The point about high priority interrupts is they can interrupt the low priority interrupt handlers.

FAST or #INT_GLOBAL are much faster (just direct calls to the routines), but require your code to then handle everything (just like assembler interrupt handlers). If you are using 'FAST' and not saving registers in your code, then you have a 'disaster waiting to happen'....

FAST will only add a handful of bytes to code size.
HIGH will add many dozens of bytes as a complete second set of code to save the registers and test the interrupt source has to be added.