My t flip flop is not working

Hi @mssong
Can you also show the waveforms between the NANDs?

Although the logic is similar for all NAND what is the main reason?

Why does it fail?

1. It is not an edge-sensitive CLK design, just state-sensitive, asynchronous ( the two inputs can be swapped)
2. There is a metastable condition a "race condition" with symmetrical {thresholds, source impedance, load capacitance and thus delays}.
3. There is a -ve feedback loop for just the output pair but a +ve feedback loop on the outside which together is the reason it oscillates or fails to toggle.

What is the main reason 1, 2, or 3? or a combination?

Why do ideal NAND gates appear to work when I introduce a larger delay on one?



But is this really working the way you thought it should?
I don't know your "specs" but it does toggle the output Q

Next with very fast low Rout gates but asymmetric delay with the slow path causing spikes on it's side..


So do you fix with analog values ? or logic?

Note below how the "D" FF has only negative feedback.



ANSWER?

The original circuit is a latch, not a JKFF. A second stage
wired up right, and you'll toggle,.

A latch is Asynchronous
A FlipFlop is Synchronous using edge sensing Clk

None shown are JK except below which with the internal complementary feedback requires an inverted clock to avoid metastables.



Final test.

Which of these two negative edge "D" FF's is better and why? (Tricky question assume ideal Gates, 0 delay, 0 Ohms out, infinite impedance in., identical threshold 2.5V.)

My answer IMHO neither is better and both can have metastable conditions if D changes during CLK unless the IC has defined setup and Hold times that are met from the spec.

sidun.av said:

Can you also show the waveforms between the NANDs?

Click to expand...

That makes a lot of waveforms to show (through a lot of effort in most simulators). The simulator featured in Tony's posts 3 & 5 is Falstad's animated interactive simulator. The flip-flops (made from multiple NAND gate arrangements) are built into the Circuits menu.

falstad.com/circuit

By watching the wire colors change (as the simulator runs) you can see which turn positive or negative. You can click data input wires high or low and then watch outputs change at the next clock transition. (Scope traces are optional though helpful. LED monitors are optional though helpful.)

Watching this simulator I learned the mystery as to how the toggle ff divides frequency by 2 because one or the other RS ff's reaches a cycle where it ignores a change of input while the other RS ff does change state at that moment. Hence the division by 2.

If you only want /2 toggle there exists a simpler
"dynamic DFF", faster than logic styles (NAND or
TINV), weak drive. This is what you use in RF
CMOS for prescaler front stages.

The dynamic FF uses almost 50% fewer FETs using tristate, T gates or transmission gates with the 2 stage inverted clocks that run faster than the parasitic decay. This is both smaller and faster than the static master-slave sampled FF and thus has no metastable condition but has a minimum clock interval.