Troubleshooting Gated D Latch Waveforms

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In summary, your D-Latch schematic appears to be correctly labeled, but your simulation output does not match. You may have a missed/screwed up inherited connection somewhere. You can check all your inherited connections then look for connections that do not make sense. Good luck!
  • #1
perplexabot
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Hello all. So I finished making a gated D latch. I used the Analog Environment to plot the waveforms. I have provided the schematic and the waveforms below.

The output is kind of weird, as you will see.

Thank you for reading.
EDIT: Been trying to figure out what the problem is for a while now, still don't know what it could be. Could it have to do with transistor sizing? I did the same circuit on my phone (using an app called EasyCircuit) and I believe the correct waveforms were achieved. I have no idea what may be causing the output to change state (in the case of the image shown, the output changes at the rising edge, as it should, but then changes on the falling edge too! [which it should not, because the input didn't change!]). What may be causing this peculiar event? I hate going to sleep like this, but it looks like I have no choice. If anyone has any suggestions, anything at all! Please send them my way. Thanks again. I will reply after I wake up (assuming I get replies.)

latch_schem.gif

waveforms.gif
 
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  • #2
The labels on your simulation output do not correspond to your schematic. Does INP = D and OUT = Q.

This is indeed strange. I suspect you have a missed/screwed up inherited connection (power/ground/substrate) somewhere.

Can you check all your inherited connections then look at every single node until you find one that doesn't make sense?

Good luck!
 
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  • #3
analogdesign said:
The labels on your simulation output do not correspond to your schematic. Does INP = D and OUT = Q.

This is indeed strange. I suspect you have a missed/screwed up inherited connection (power/ground/substrate) somewhere.

Can you check all your inherited connections then look at every single node until you find one that doesn't make sense?

Good luck!

The reason for that is because I created a symbol for the D-Latch and then used that in a new schematic. Sorry, I should have stated that. Here is a screenshot. Thank you for the reply and help.

EDIT1: I am not so sure what you mean by "inherited connections," but I double checked the vdd!'s and gnd!'s for the nand, not and nor gates. I was not able to find anything strange.
EDIT2: Triple checking nodes as you have suggested, I can see a vdd supply net connected to a node that is called vdd! and not vdd. Is this a problem? I am assuming this is as it should be.

PS: Anyone have a clue? Please this is really bugging me!

true_schem.gif
 
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  • #4
The problem is QN. It should be floating rather than grounded. You can either leave the QN node open or you can connect it to a very small capacitor.
 
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  • #5
analogdesign said:
The problem is QN. It should be floating rather than grounded. You can either leave the QN node open or you can connect it to a very small capacitor.

I just want to say, "I love you!"
 
  • #6
perplexabot said:
I just want to say, "I love you!"

Ha ha, no problem! In my experience 95% of all really weird simulation results are due to how we set up the test bench or have biased or connected the circuit.
 
  • #7
analogdesign said:
Ha ha, no problem! In my experience 95% of all really weird simulation results are due to how we set up the test bench or have biased or connected the circuit.

Thank you for your (wonderful) help and interesting information.
 

1. What is a gated D latch and how does it work?

A gated D latch is a digital electronic circuit used to store a single bit of data. It has two inputs - data (D) and enable (E) - and two outputs - Q (output) and Q' (inverted output). When the enable input is high, the value of the data input is stored in the latch and reflected on the output. When the enable input is low, the latch holds its previous value.

2. How do I troubleshoot issues with gated D latch waveforms?

The first step in troubleshooting gated D latch waveforms is to check all connections and make sure they are secure. Next, check the input signals to ensure they are in the correct state and timing. If the problem persists, check for any faulty components such as resistors or capacitors. If necessary, consult the datasheet for the specific latch being used for further troubleshooting steps.

3. Why are my gated D latch waveforms showing unexpected values?

There are a few possible reasons for this issue. It could be due to incorrect connections, faulty components, or incorrect timing of the input signals. It is also important to check for any noise or interference that could be affecting the signals. Double-checking all connections and using a logic analyzer or oscilloscope to monitor the signals can help identify the source of the problem.

4. How can I ensure the gated D latch is functioning correctly?

To ensure the gated D latch is functioning correctly, you can perform a functional test. This involves applying different input combinations and verifying that the output matches the expected values. Additionally, you can use simulation software to simulate the circuit and compare the results with the actual waveform. If there are any discrepancies, it may indicate a problem with the circuit or components.

5. Are there any common mistakes to avoid when troubleshooting gated D latch waveforms?

One common mistake to avoid is assuming that all problems are related to the latch itself. It is important to also check for any issues with the input signals, power supply, or other components in the circuit. Another mistake is not double-checking all connections, as a loose or incorrect connection can cause unexpected waveform behavior. It is also helpful to have a good understanding of the latch's datasheet and functionality to aid in troubleshooting.

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