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[DPRG] HELP with 68HC11 stack problem

Subject: [DPRG] HELP with 68HC11 stack problem
From: Mike McCarty jmccarty at ssd.usa.alcatel.com
Date: Thu Jun 21 14:08:05 CDT 2001

I agree with the facts as you state them, and that the initial stack
address is suspicious, but I fail to see how it explains why his
program would work on a straight RESET but not on POWER ON RESET. If
his stack is incorrectly being set, then that is a fact which will not
be altered by the type of reset being used.

> I have read the other responses and would like to add my 2 cents
> worth.  
> 
> There are 4 types of stack mechanisms.  They are pre increment,
> post increment, pre decrement and post decrement.  Pre refers to
> whether the pointer is changed before the write to memory and
> post refers to after the write.  Then there is incrementing and
> decrementing which is self explanatory.
> 
> The 68HC11 uses a post decrement type of stack pointer.  The data
> byte is written then the pointer is decremented.  When the stack
> is set up you want the pointer at the last address of the RAM
> memory.  $4000 doesn't sound right.  It sounds like the beginning
> address of a memory space and there my not be memory chip there
> to write the first by on the stack to.  $3FFF sounds like a more
> reasonable pointer.
> 
> I agree there are a couple of other things to check.  Make sure
> you have the MODA and MODB pins set correctly.  This gets
> complicated when your program starts from internal memory.  Your
> program can turn on the external memory by writing to one of the
> configuration registers (I don't have the data here to give you
> the details).



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