Why does the S12X COPCTL allow two writes?

[jlever]

Try this simple C program; add a breakpoint when at the assignment to ARMCOP.

void @near main (void) {
    COPCTL=0x40;
    COPCTL=0x43;
    ARMCOP=6;
    while(1) { }
}

If the software begins from an external reset COPCTL will contain the value 0x43 prior to setting ARMCOP=6.

However, after the ARMCOP reset, COPCTL will contain the value 0x40, and the assignment to ARMCOP will no longer cause a reset.

According to the datasheet describing the COPCTL fields

Writing CR[2:0] to “000” has no effect, but counts for the “write once” condition.

I expect when ARMCOP=6; is reached COPCTL would always contain the value 0x40.

Specifically tested on an MC9S12XS128 Mask 1M04M.

While this is a simple example, assigning ARMCOP an invalid value is used by the full software to trigger a reset on demand. COPCTL=0 was placed in an earlier module during development and was mistakenly not removed. causing the second reset request to fail.

[lama · Accepted solution]

Hi,

if you debug the MCU it is clear you are not in normal mode but in the special mode.

Write:

— Anytime in special modes

Moreover, I suggest you to look at https://community.nxp.com/docs/DOC-103737 

plus attached example.

Best regards,

Ladislav

[jlever]

Thank you, I attempted again using an oscilloscope on PORTA pin 1, and running from power up with the debugger disconnected.

I was able to confirm that COPCTL can only be written to once when the debugger is not connected.

void _at(near) main (void){
   unsigned short i,j;
   DDRA   = 1u << 1;
   
   /* High->Low->High pulse for the oscilloscope */
   PORTA  = 0xFFu;
   for(i=0;i<1000;i++){ (volatile void)__asm("NOP"); }
   PORTA^=0xFFu;
   for(i=0;i<4000;i++){ (volatile void)__asm("NOP"); }
   PORTA^=0xFFu;
   for(i=0;i<1000;i++) { (volatile void)__asm("NOP"); }
   PORTA^=0xFFu;
   
   COPCTL = 0x40u;
   COPCTL = 0x43u;
   ARMCOP = 6u;
   
   /* Create 10 edges (5 pulses) */
   for(j = 0u; j < 10; j++) 
   {
       for(i=0;i<1000;i++)
       {
          (volatile void)__asm("NOP");
       }
       PORTA^=0xFFu;
   }
   
   /* End with logic 1 */
   PORTA=0xFFu;
   
   while(1) {}
}