For those with 6809 experience

osi.superboard osi.superboard at
Tue Mar 3 08:39:42 CST 2020

My assumption is, that the LDA address access is too late at the end of 
the instruction cycle, and the CPU already started the next instruction 
cycle internally. HALT will be acknowledged only during the instruction 
cycle following the LDA.
It would be interesting to measure, when the LDA pulls down the HALT 
compared to the last instruction cycle signal (LIC). Maybe using LDD 
will work better, because the HALT signal will be asserted one clock 
before the instruction cycle ends.


Am 29.02.2020 um 23:42 schrieb Jim Brain via cctalk:
> Looking at the datasheet for the 6809 (specifically, the 6809E that 
> needs incoming quadrature clock), I read that !HALT can be asserted 
> 200nS (for 1MHz part) before falling Q and the CPU will finish the 
> existing instruction and then go into a HALT state as long as the HALT 
> line is low during the falling edge of Q.
> That's the store from the datasheet, but when I am testing it, I see 
> that, even if I pull HALT low at the very beginning of the last cycle 
> of an instruction, the 6809 will not acknowledge the HALT until 
> executing the next instruction.
> My logic is watching for IO address $ff61.  When found, it drops Q
> so, to start the HALT condition, I need only:
> lda $ff61
> Not that the trigger is being performed by the code, so the current 
> instruction (the lda) should complete and then the CPU should go into 
> HiZ.  What I see is:
> lda $ff61
> lda $ff60 <- the next instruction
> executed, and THEN the CPU goes into HiZ.
> I can deal with this (Yes, I should just look at BS=BA=1, which tell 
> when to safely use the bus, but I don't have access to those signals 
> for this project), but I thought I'd see if this was known by all, or 
> if there is something I am missing.
> Jim

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