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Altera_Forum's avatar
Altera_Forum
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12 years ago

Strange - counter counts without clock

Something strange happened, and I have no idea for now, what to look for... Below you can see the part of the schematics (testing on DE2-115 board, clk frequency - 10MHz). Very straightforward design...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
12 years ago

When an internal node is connected to a pin, it does not necessarily take the same internal node.

In this case, 111 that feeds to CLKA may not be the same node that is connected out for observation.

You can imagine that there are 2 sets of NAND, NOR and INV internally. Timings are slightly different. There can be glitch driving to CLKA but not the one connected to oscilloscope.