topic HS12 Interrupts with CALL in S12 / MagniV Microcontrollers

HS12 Interrupts with CALL

astro_goto — Wed, 12 May 2010 00:10:17 GMT

My application uses the Modulus downcounter to generate interrupts when the count reaches zero to update a real time clock. Calling a subroutine in ppage $22 hangs up the micro unless Modulus interrupts are disabled. The interrupt service routine is in a non-paged location. The same thing happens with serial communications to a LCD display. I've had to change serial operations to a polling mode to avoid those interrupts.

Why can't interrupts be used with the CALL function? The RTI instruction does not seem to restore the ppage value and this may be the reason. Can anyone shed some light on this problem?

The processor is a MC9S12DP512.

Re: HS12 Interrupts with CALL

kef — Wed, 12 May 2010 01:37:38 GMT

Do you exit from paged routine with RTS? CALL'ing routines from ISR is OK, provided you exit back to ISR with RTC instruction. Also if your ISR writes to and modifies PPAGE, then you should restore PPAGE before return from interrupt.

Re: HS12 Interrupts with CALL

astro_goto — Wed, 12 May 2010 05:26:53 GMT

Yes, my paged subroutine was indeed ended with RTC, and the interrupt ISR did not touch PPAGE, it only updated the real time clock variables in un-paged RAM. The serial transmissions to the LCD display also took place in un-paged Flash memory. Changing the serial operations from interrupt to polled and disabling the Modulus interrupts solved the problem.

Does anyone else use the CALL function with ongoing interrupts? I would be most interested in comparing systems.

Re: HS12 Interrupts with CALL

Lundin — Wed, 12 May 2010 20:10:58 GMT

I'm not certain what you are trying to do here... could you describe it with pseudo code? CALLing functions is harmless. CALL stacks the PPAGE value and it is an atomic instruction that can't be interrupted. RTC restores PPAGE and it can't be interrupted either. Obviously you can't CALL "my_isr_routine", but I don't think that's the case here...? Does the subroutine change the variables used by the interrupt? If so, the bug is very likely related to missing semaphores or a missing volatile keyword.

Re: HS12 Interrupts with CALL

astro_goto — Thu, 13 May 2010 03:40:03 GMT

My application is a telescope control sytem that uses the Modulus module to generate interrupts for a real time clock. It is setup with the following code:

;   modulus downcounter setup
   bset   MCCTL,#modmc+mcen     ;enable modulus mode
   bset   MCCTL,#mczi+mcpr1+mcpr0      ;enable modulus ints, set
;               prescaler to divide bus clock by 16
;               bus clock = PLLCLK/2 = 24MHz
   bset   MCCTL,#flmc     ;force load reg into mod count reg
   movw   #50000,MCCNT   ;set load reg for 30 interrupts/sec

The Modulus ISR is:

modulus_irq:    equ   *   ;modulus int service routine, 30/sec
   bset   MCFLG,mczf   ;clears the modulus interrupt flag by writing
;               a 1 to the mczf bit7, all other bits reserved
    dec   point5       ;0.5 sec timer
   bne   mi1
   movb   #15,point5   ;reload 0.5 sec timer
   jsr   clock       ;update date/time variables by 0.5 sec
   movb   #$FF,time   ;set time flag
mi1:   rti

The application program loaded in non-paged Flash $3F contains the statement:

call get_star_coord,$22 ;(uses only global variables)

The following code segment is loaded in ppage $22:

org $8000

get_star_coord:   equ   *    ;converts ascii coord to fp degrees
   std   temp_fp     ;d points to ascii seconds value
   jsr   ASCFLT         ;convert ascii seconds to fp in FPACC1
   ldx   #fp_60         ;constant 60 converts seconds to minutes
   jsr   GETFPAC2

................
   jsr   FLTDIV         ;convert seconds to fractional minutes
   jsr   UINT2FLT       ;convert degrees to fp in FPACC1
   jsr   PULFPAC2     ;retrieve fractional degrees on stack
   jsr   FLTADD       ;add fractional degrees
   rtc         ;return (get_star_coord,$22) with fp degrees in FPACC1

The subroutine is doing coordinate calculations in floating point using other subroutines in non-paged Flash but is being interrupted periodically by the Modulus system. There is not enough space in non-paged Flash to load this sub or others. I hope this makes clear what the program is doing. I'll be happy to explain further, if needed.

Re: HS12 Interrupts with CALL

astro_goto — Thu, 13 May 2010 09:09:24 GMT

OOPS!!!! Problem solved and my face is RED. Careful examination of the program code revealed an incorrect value in setting up the Modulus module. When corrected as shown in the above code snippet, the interrupt hang-up disappeared.

I'm truly sorry for the confusion I may have caused.