This is a follow-up question to https://electronics.stackexchange.com/a/95195/13354, in which I was asking about a specific solution to the more general problem described here.
I'd like to create a logic function that toggles with the (inverted) clock when enabled and stays low when disabled, like so:
clk ena out
0 0 0
1 0 0
0 1 1
1 1 0
In addition to that, I'd like the out signal to be glitch-free, as shown in the following example waveform:
As shown in this image, potential glitches of the ena input can happen shortly after the clk edges; nevertheless, the out waveform should be glitch free.
According to this answer to aforementioned question, the circuit should be a clock gate rather than a self-resetting flip-flop. This is the proposed schematic:
simulate this circuit – Schematic created using CircuitLab
Unfortunately, this still causes glitches when ena goes low, because the Q output of the flip-flop is delayed with respect to clk (note that I ignored the common delay between clk and its inversion, which I think doesn't contribute to the issue):
Now my question: What circuit should I choose to have the out signal glitch-free?
Best Answer
I think, that in this case you should delay the inverted clock signal, using the even number of inverters between the CLK input of REG1 and the lower input of the AND gate.
Please note, that even in this case you may run into trouble caused by the metastability of REG1. To reduce this risk, I'd suggest to add yet one stage of synchronization of the ena signal (however of course it will slightly change the functionality, as ENA will be delayed).
simulate this circuit – Schematic created using CircuitLab
You can also try another circuits shown in the article linked in my response to your previous question, however you should keep in mind that the last and most promising one seems to be patented :-(.