Electronic – Does this design smell bad? Can I pull a pin simultaneously down and up


I am building a simple circuit using PT7C4511 PLL Clock Multiplier. This chip has an OE pin that stops the output when LOW. By default (with no external signal driving it) it stays HIGH because of a built-in pull-up (270K).

What I want to do is to turn on and off the chip's output using a 2-pin header. However, I want it to be working when the header is shorted, and stop the output otherwise. To achieve this, I have connected 40K pull-down to OE pin, strong enough to overcome the internal pull-up, but not too strong, so that when OE is shorted to VCC, the pin can easily go HIGH again.

The design

I showed this to a friend with more experience in EE and while he agreed that it should work, he wasn't too excited about the design. He could not point to a specific issue, but the whole thing "smelled" to him.

Is he right? Why?

Best Answer

Well, you can't really remove that 270K pull-up, so that just means you have to use a significantly smaller (stronger) pull-down. Also, as an on-chip resistor, the precise value of that pull-up is not going to be very well controlled and could vary by quite a bit. I would recommend going even smaller on the pull-down, perhaps 10k or even 4.7k or 1k.