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Edge-triggered latches: Flip-Flops

T

Thread Starter

tony ennis

Joined Aug 25, 2012
3
This page: http://www.allaboutcircuits.com/vol_4/chpt_10/5.html

The following comes out of the blue:

It is important to note that the invalid state for the S-R flip-flop...
Click to expand...
I assume the invalid state is that Q and Q' are both 0. Only when writing this did I realize that this isn't the entire problem. When the clock pulse goes low, we can't say for sure what the next state of the flip-flop will be and thus the flip-flop goes insane.

This would manifest as a real effect if the set/reset were being controlled by a push-button. The push button might be held down for large amounts of time, causing the flip-flop to settle on some unexpected value for long enough for something unexpected to happen.

My feedback would be to introduce and explain the invalid state with more words. Of course, this is from a novice...

The S-R flip-flop has two flaws that must be dealt with. The first is the illogical result of Q and Q' having the same value when R and S are both 1. This happens only in the instant the clock enables the circuit. Thus if your circuit depends upon Q being the same as not-Q', you might see inexplicable behavior. Second and worse, this type of flip-flop has an inherent race condition that manifests when in this state and the clock disables the circuit. The values of Q and Q' will become logical but undependable. That is, Q could be 1 for one flip-flop but 0 for another flip-flop. In a cruel world, it would be possible for the same flip-flop to exhibit different behavior from day-to-day. This inconsistency is caused by real-world imperfections in the silicon and wiring. The NOR gate with the fastest silicon (or shortest connecting traces...) is going to produce a 1 first, and this will cause the other NOR gate to produce a 0.
Click to expand...
 
Last edited:
T

Thread Starter

tony ennis

Joined Aug 25, 2012
3
Nope. I was looking up up/down counters are realized I wasn't going to understand them until I really understood flip-flops. So I started at the beginning... I just thought it was a bit abrupt.

Who is the target audience for the page I linked?
 
Georacer

Georacer

Joined Nov 25, 2009
5,182
@OP,

You aren't entirely wrong about what you are trying to explain. Indeed, race conditions are a pain and cannot be explained easily.

It even doesn't have to do with the fastest transistor, but sometimes by the more potent. I wouldn't try to explain the ins and outs of a race condition.

You describe the repercussions of the race condition nicely, though.
 
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