Re: CIA old/new?

From: Gerrit Heitsch <*gerrit_at_laosinh.s.bawue.de*> Date: Sun, 3 Jan 2021 21:31:54 +0100 Message-ID: <d5dc21bd-afe3-554a-9877-251db2f8fc44_at_laosinh.s.bawue.de> · gerrit_at_laosinh.s.bawue.de

On 1/3/21 9:15 PM, Francesco Messineo wrote:
> Hi all,
> I always thought the old (6526 I guess) and the new (8521) CIAs were identical,
> then I've seen this code:
> 
> testCIAVersion:
>      //; Set NMI vector
>      lda #<continue
>      sta $fffa
>      lda #>continue
>      sta $fffb
> 
>      lda #$81  //;also don't forget to set mask. Setting $01 to an
> appropriate value will also help :-) (Bitbreaker/Oxyron)
>      sta $dd0d
> 
>      //; Set timer to 5 cycles
>      lda #4
>      sta $dd04
>      lda #0
>      sta $dd05
> 
>      //; Clear the detection flag
>      sta oldCIA
> 
>      //; Fire a 1-shot timer
>      lda #%10011001
>      sta $dd0e
> 
>      //; This should be interrupted before the INC
>      //; only if it's a newer chip.
>      lda $dd0d
>      lda $dd0d
>      inc oldCIA
> 
>      jmp * //; just in case
> 
> So it seems the NMI triggered by some CIA revisions has an off-by-one
> cycle error?
> Can anyone explain what's the difference between the two CIAs?

NMOS vs. HMOS-II process and die shrink.

For a while the HMOS-II version was labeled '8521R0' before MOS went 
back to the '6526' marking. You can tell by the datecode (end of '86 or 
later = HMOS-II) and the '206A' or '216A' right of the datecode.

  Gerrit