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flipflops vs. latches and busses and bugs


From: Tuukka Toivonen
Subject: flipflops vs. latches and busses and bugs
Date: Wed, 6 Jun 2001 15:20:04 +0300 (EEST)

Now I understand why Electric "flipflops" behaved strangely. The reason
is that they are NOT flipflops at all but latches instead.

Flipflop is edge triggered, latch is level triggered. This is the custom I
see used in practically all places. Those items what are named
"flipflops" are really latches. Fortunately it is easy to construct a real
flipflop from two latches. But I do think that Electric is a small bug
here.

What comes to the "busses don't work with the simulator", the bug might
not be with busses actually. I got the same behaviour with wires. Also, to
my surprise, I got the busses working at least in some situation.

When I cut out the bus signals into separate wires, buffered them, and
then assembled back to bus, it seems to work in the simulator. In the
waveform display, I can set the value of the incoming bus to something and
the signals going in the outgoing bus are updated accordingly, just as it
should be. However, if I combine the signals of the outgoing bus into one
(with the "B"-key), the waveform display seems to show a wrong value
(althought it displays just the right value for incoming bus).

The incoming bus is 2 signals wide. The outgoing should show same values,
but instead I get the following:
0b00 -> 0b00
0b01 -> 0b01
0b10 -> 0b00    <- wrong
0b11 -> 0b11
0b10 -> 0b10
0b01 -> 0b00    <- wrong
0b11 -> 0b11
0b01 -> 0b01
The signals itself are correct, as I can see by double-clicking
the outgoing bus, but sometimes the bus shows incorrectly "0".
What's wrong?

The same happens with all number bases.




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