Pull-Up Resistor Question

[danner123]

What would happen if I have a 5V TTL voltage pulled up through a resistor by 3.3V? Would the voltage stay 5V or become 3.3V?

Thanks!

 

[KlausST]

Hi,

this is more a pulldown than a pullup...


it depends on the driver. Read it´s specifications.
TTL levels are specified to be low when <0.7V and high when >2V (if i remember right).

So many TTL drivers don´t pull up to more than 3.5V - especially when there is a load to GND.
*******

To limit the voltage you could use
* two resistors as voltage divider, or
* a series resistor with a zener to GND (this is a clean solution).
..
There are 3V3 powered logic devices with 5V tolerant inputs.

Klaus

 

[Audioguru]

An ordinary old 74xxx TTL has an output low maximum voltage of 0.4V when it has a 16mA load and an output high minimum voltage of 2.4V when it has a load to ground of 0.4mA.

I can't remember how high a TTL output high can be but I have never seen one higher than 3.5V.

 

[betwixt]

If the question refers to TTL with an open collector output and pull-up, the TTL supply is not present at the output so it is irrelevant what it is. The ouput will pull up to any voltage within safe range at the top of the pull-up resistor, including 3.3V

Brian.

 

[chuckey]

"an open collector output and pull-up", surely an open collector has a load, I thought a pull up was for an input? Good debating point what with wired ORs/ ANDs?
Frank

 

[j33pn]

I thought a pull up was for an input?

Much like men and garbage, one chip's output is another chip's input.