Lifelong mental trauma as legacy
hardware26
joined 2 years ago
Can you read what u1 and q1 are in the first image? Q1 looks like a transistor switch and u1 might be a counter. It may be counting a clock or it might be a comparator checking whether a capacitor is charged or discharged to check time. Is there any components on the other side of the board? I would expect a capacitor or oscillator at least for timing.
What is the misunderstanding here?
If you are completely new I suggest watching a tutorial. Otherwise some basic tips could be
- if oscilloscope has multiple channels, make sure the channel you connected is the channel nel you see on the screen
- make sure you did the ground connection. You need to connect both signal and ground tips if the probe
- make sure voltage range is fine. If you use a large range in the screen, little amplitude you got may seem like a flat line.
- calibrate your probe, if oscilloscope has that option. But an uncalibrated probe doesn't explain flat line, it would explain distorted square wave, for example.
- Use another probe. They can get broken. If it is broken all you will see will be the noise the cable pics up like an antenna.
- separate the issue. Connect probe ends to a battery with k own voltage and see what you receive. If it doesn't work, you know either probe or oscilloscope is wrong.
List can get longer really, but I think a tutorial and these can go a long way for the beginning.
Nice and rare to see a fellow SV user. Did you come across to any relevant communities for digital design and verification, asic or fpga?
Problem you are having could be
- output impedance of the signal generator you use to generate the square wave. When you set it to low, output impedance of the signal generator builds a voltage divider together with the internal pull up, and the device ends up sensing a higher voltage than "low". This is something you can see if you have an oscilloscope, try to hook up a probe to the input and ensure whether you get what you set in the signal generator.
- some signal sources have no or limited capacity to drain current. And when you set it to low, this is exactly what it us supposed to do, drain current from the internal pullup.
As you mentioned in another comment, solution is a simple buffer. This could be an opamp, but even a simple nmos transistor should suffice (open drain as you said). But you need yo be careful with current ratings of the transistor, which you can easily calculate by dividing 5V by the pull up resistor. Send a message if you need help.
Before these, I suggest you yo use an oscilloscope or multimeter to measure the voltage when it is supposed to be low, and see that in fact the problem is that voltage at the input doesn't go "low".
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