16F to 18F context saving differences

[brittons]

Hi, I am trying to move my PicBasic Pro programs from 16F876 to 18F2525. I am unclear as to the difference in context saving/restoring with interrupts. In the PicBasic Pro manual it says that:

"If a 14-bit core PICmicro MCU has more than 2K of code space, an
interrupt stub is automatically added that saves the W, STATUS and
PCLATH registers into the variables wsave, ssave and psave, before
going to your interrupt handler. Storage for these variables must be
allocated in the BASIC program:"

wsave var byte $20 system
wsave1 var byte $a0 system ‘ If device has RAM in bank1
wsave2 var byte $120 system ‘ If device has RAM in bank2
wsave3 var byte $1a0 system ‘ If device has RAM in bank3
ssave var byte bank0 system
psave var byte bank0 system

My original intrrupt rountine, which seems to have worked fine for years had the following code in an included file containing my inerrupt rountines in assembly language.

;======= System variables: ======================
wsave VAR BYTE $70 system
ssave VAR BYTE bank0 system
psave VAR BYTE bank0 system

I had no context saving intering the routine as I believe PBP is doing this for me.

Upon exit I have the following:
intsrvx bcf STATUS,RP0 ;select Bank 0
bcf STATUS,RP1
movf psave, W ;restore the state of everything
movwf PCLATH
swapf ssave, W
movwf STATUS
swapf wsave, F
swapf wsave, W
retfie

The interrupts I am using are Timer1(16F876) and Serial Recieve. In my intrrupt rountine I am writing to EEProm too.

As I said I believe all is well here. My question is, can anyone recommend what will be required to do for the 18F2525 to provide the context save/restore to accomplish the same thing? Do I supply the context saving as well as restoring. Does anyone have any recommended methods?


Thanks for any help you can provide, Bob

[mister_e]

usually the datasheet give some example of it... sorry to sound like a broken record :(

Have a look to Darrel's instant interrupt code...

[sougata]

Hi,

Hope you are using high priority interrupts only. (That is no way of a low priority interrupt interrupted by a high priority). Then this should suffice:



retfie fast


18F series have a single level shadow register stack which saves your w,status and bsr automatically on interrupt.

[brittons]

Thanks for your reply, am I to understand that if I am using only high priority interrupts that I don't have to save and restore context at all? Only 'retfie fast'.

Should I avoid save/restore altogether. Is there a time when I should save/restore context or only restore? If so what are the predetermined variables to restore?

If you could point me to a datasheet were this is explained I would appreciate it. I have looked through the docs and started to make the transition to the 18F but have seen some times when I am viewing EEProm in the Microcode studio ICD that I get random address writes. It's kind of stange the value is written to zero then back to what it was on the next pass. I'm not sure if it's the ICD or my program as the value never remains changed. It only seems to happen when I have my interrupts enabled so I may be doing something wrong. I have run the program overnight without the ICD and the interrupts enabled and examined the EEProm data the next day and everything looks fine. I just want to make sure I understand what I need to do in interrupt handling with the 18F.

I have two interrupts, serial receive and a timer. I'm still very new to the 18F series and not to sure about the archetecture as yet so I have to check to see if they are considered high priority. In the 16F everything in my asm interrupt routine is in bank0. Is the scheme the same in the 18F?

Thanks again for your help ,Bob

[sougata]

If you could point me to a datasheet were this is explained I would appreciate it.

Hi,
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1510&stc=1&d=1175394335">

Could you post your code snippets so that we could discuss this stuff in detail. Also try using Darrel Taylor's Instant Interrupt. As you are already familiar with asm. A review of Darrel's Code would make things much clear. I will get back to this thread.