Picture 1 Figure: 1.

Picture 2 Figure: 2.

Content in ROM(U56)

   T0:  00
   T1:  00
   T2:  00
   T3:  04
   T4:  00
   T5:  00
   T6:  04
   T7:  00
   T8:  00
   T9:  04

(I have not written T0-T9 or the colon in the ROM file, I have included them to explain the T states)

Explanation of the figures:

Here, The user input is, UP signal for U55 counter, which is being used to read values from a ROM(U56). The CP2 signal is being used to increment the counter (U13).

If I keep increment UP for U55, for the two increments CP2 is zero, according to the ROM content.

As CP2(it goes to U13 and U13 only) is zero for the first three states (Start state, first increment , second increment for U55 counter), the counter output of U13 should not increment (intuitively).

But, as we can see from the figure-2, the output of U13 changes in third state, although CP2 remains zero until this point.

Please someone kindly bring some light into this matter. Perhaps I am confused with some concepts.

I am new to proteus designing. Thank you.


1 Answer 1


When the U55 outputs change from 1 to 2, the output of the prom will go to an undefined state before returning to 0. This undefined state is causing U13 to increment. You need to latch the output of the prom.

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