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A flip flop is built from two back to back latches with opposite polarity clocks, which form a master slave topology.
The type of latch is irrelevant (JK, SR, D, T) to this constraint, but it is important that the transparency is controlled by some pin (call it clock or enable or whatever you like).
SR latches throw everyone for a loop because the most ...
19
One reason we clock flip flops so that there isn't any chaos when the outputs of flip flops are fed through some logic functions and back to their own inputs.
If a flip-flop's output is used to calculate its input, it behooves us to have orderly behavior: to prevent the flip-flop's state from changing until the output (and hence the input) is stable.
This ...
19
It is true that the latch will "wake up" in an unknown state. In the real world, given a little time, the latch will have a valid state with either Q=0 or Q=1.
You make the latch "get started" by setting one of the inputs (R or S) to be a 1 while the other input is a 0. This will force the latch into a known state, regardless of whatever the previous state ...
16
Instead of just poking at random values and asking what difference they make, stop and actually think about the circuit. Once you understand what each part does, you'll be able to find acceptable values for them yourself.
Q1 and Q2 are arranged with positive feedback so that they have two stable operating points. When Q1 is off, Q2 is off, which then does ...
14
What makes an inferred latch?
For combinatorial logic, the output of the circuit is a function of input only and should not contain any memory or internal state (latch).
In Verilog, a variable will keep its previous value if it is not assigned a value in an always block. A latch must be created to store this present value.
An incomplete if-else statement ...
13
Of course there's an external power supply. Take a look at a NOR gate chip (say, 74HC02) and you'll see it there clear as day:
Logic symbols purely show the logic flow of a circuit, not the power flow. Many schematic capture programs will take the power connections as read and automatically connect them to the right nets for you.
Internally a typical CMOS ...
13
The base to emitter voltage is pretty stable at 0.6V, you are not adding any feedback with the resistor.
You actually built a emitter follower or common collector. The main property of this circuit is that the emitter is pretty much at the same voltage as the base voltage.
If you are hinting at the non ideal behavior of the transistor, even the slight ...
11
They start out undefined, that is they could be set to either.
When you switch power on, assuming a real latch with no input signals, both gates will want to output high. However due to no two gates being exactly the same (and other real world effects), one will "win" the race to bring it's output high first, and set the others output to low. The same ...
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I have been thinking about this definition a lot today.
As others pointed out, the exact meanings will vary. On top of that, you will probably see more people get this wrong, even on this site, than right. I don't care what wikipedia says!
But in general:
A flip flop will change it's output state at most once per clock cycle.
A latch will change its ...
10
Latches are very tricky to use in FPGAs or CPLDs, so many people just avoid them completely. One of the reasons is that many FPGAs don't have a built in latch, so they are made out of logic gates - this can cause nasty timing issues.
Also you don't have any control over timing delays and race conditions when using a latch (unless there is a native element)
...
10
Because the electronic device reminded engineers of both this item:
and this one:
In the first case, the devices have two states: flipped (in contact with heel) and flopped (not in contact with heel). In normal use they alternate continuously between those two states at about 1Hz.
In the latter case, you can change the object's state from open (unlatched) ...
10
Flip-flops are single bit devices with two stable states. The outputs are typically Q and \$\mathsf{\small \overline{\text{Q}}} \$. There are several kinds, here are probably the most common:
SR flops have two inputs, S (Set) and R (Reset). As they name implies, asserting either of these either sets or resets the flip-flop. After the input is de-...
10
The analog-focused answers are correct, but there's also a digital way to look at it. The circuit you linked to is basically two inverters connected in a loop, not unlike an SRAM cell. Here's a simplified version to make this more clear:
simulate this circuit – Schematic created using CircuitLab
Homemade Circuit Designs added some resistors for ...
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When you power on a bi-stable element like a NOR SR latch (with inputs at 0) it will start at to some state. That is not necessaraiy a 'binary' state, the the outputs can for instance be halfway between 0 and 1.
Most bi-stable elements are designed with a positive feedback factor, which means that any difference between the two outoputs (or, same thing but ...
10
They are more-or-less equivalent but the timing is not identical, especially for data stable and runt pulses on E in the second circuit. There may be other differences you can find.
9
Currently the LED is lighting because your switch provides a direct path, it's not the transistors working. There is no means of supplying current to the bottom transistors base with the switch open (the emitter output will always be lower than the base, so it cannot provide current)
You need something like this (M1 can be a PNP transistor if desired):
...
9
In general, "flip-flop" and "gated latch" are synonyms. However, the term "flip-flop" is often used when referring to "edge-triggered flip-flop" (just because it is shorter). When people speak about "D flip-flop" the usually refer to "D edge-triggered flip-flop", however when you hear "JK flip-flop" it can refer to both "JK edge-triggered flip-flop" and "JK ...
9
Of course there is an external power supply (as you suspected). Here's a quad nor gate: -
Notice pin 7 and pin 14
The same sort of thing is done with op-amps - they don't necessarily show the power pins because they are assumed folk know they are there: -
But once you've learned about them you realize the basic op-amp has power pins (normally 7 and 4): -
8
The circuit shown does not have the transistors doing anything useful, or even turning on at all.
When the push-button is pressed, a circuit is completed via the connection between base and emitter, straight to the resistor. This turns on the LED. Relase the button, that circuit is broken, LED goes off.
The base-emitter junction does not see any voltage ...
8
I duplicated your circuit in Logisim (as an opportunity to do something in Logisim). There's nothing wrong with your circuit. There is something about Logisim I don't understand.
First off, the red lines are not lines in a high state; they are errors. One would expect this sort of error if two outputs were tied together. I did a bunch of breaking the ...
7
A rising-edge flip flop may be envisioned as two latches back to back, one of which is enabled shortly after the clock signal goes low and remains enabled until it goes high; the second is enabled shortly after the clock goes high and remains enabled until it goes low. Having a brief moment during which neither flip flop is enabled means that the output of ...
7
The two are different because the first doesn't show the internal structure of the SR latch, and it is hooked up in reverse of the second example. The first example is concerned entirely with the logic of the latch, while the second is concerned with the gate connectivity.
That is, in both cases, a high enable, a high set and a low reset will both cause Q ...
7
Each digital circuit can only work with a given amount of capacitance load which is given by the logical effort and its fan-out. If you connect too many blocks (latches in your case) at the output of the same rising edge detector, it would not work properly and would lead to lot of setup or hold violations after synthesis. The reason is you need to drive too ...
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Sequential logic designs constructed by using combinatorial logic and feedback generally make an assumption which would seem reasonable when using physical gates: that a gate's output will not change in response to a change in input, until sometime after the input has actually changed. There are some occasions where that assumption may not hold when using ...
6
This is one of those circuits that reminds me how clever some people can get with a handful of inexpensive bipolar transistors that can create a a boat-load of weird effects due to all the variables of transistor gain and parasitic effects, including the Miller capacitance of a few pF dominant on Vbc of the SCR pair.
I am going out on a limb here and ...
answered Oct 21 '12 at 19:34
Tony Stewart Sunnyskyguy EE75
97.4k2 gold badges35 silver badges140 bronze badges
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The point you're missing is the fact that once the Q output has gone high, the output of the gate that S is connected to is forced low, regardless of whether S remains high or subesquently goes low. In other words, once that happens, the output of that gate is no longer dependent on the value of S.
By symmetry, the same can be said of the R input. Once it ...
6
After having experimented a lot, and read some more, I found an answer to my question.
Simple switch model approach
After seing a demo on the switch models I finally understood the parameters and workings of the SPICE switch model, which I will explain below.
Switch model syntax
.model MODEL SW VT VH RON ROFF
SX N+ N- NC+ NC- MODEL <ON><OFF>
(...
6
http://digikey.com/product-detail/en/A8GS-S1305/SW1533-ND/4248837
Maybe try one of these. You could have a cap charged to 5V, a depletion-mode fet that is held off when your power is on and dumps the 5V charge through the coil when power is off? This would allow the switch to manually be reset even if power was lost and not purposefully reset.
edit: If it ...
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