
Analyzing the vintage 8008 processor from die photos: its unusual counters - reportingsjr
http://www.righto.com/2017/03/analyzing-vintage-8008-processor-from.html
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sehugg
The Atari 2600's TIA made use of these pseudorandom counters, too, for the
current horizontal raster position of sprites and other objects. As a result,
the programmer couldn't set the position directly -- you'd have to wait for
the counter to reach the desired value, then strobe a register that sets the
position register (to the opaque pseudorandom value). You could then nudge it
-8 or +7 pixels with yet another register.

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jonsen
A Johnson counter is often used in similar circumstances.

To count eight states

    
    
      0000 -> 0001 -> 0011 -> 0111 -> 1111 -> 1110 -> 1100 -> 1000 -> 0000
    

it would have to be a four bit register ABCD where A = B, B = C, C = D, and D
= NOT A

It would be one more flip-flop but the counting logic would be free. A flip-
flop has a NOT output. Decoding of the eight states are also simple:

    
    
      State 0000 is NOT A AND NOT D
      State 0001 is NOT C AND D
    

and so on. Notice the NOTs are free.

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burntrelish1273
Here's the user manual in pdf form:
[http://www.classiccmp.org/8008/8008UM.pdf](http://www.classiccmp.org/8008/8008UM.pdf)

