Same resistor for pull up and down ?

[DJLinux]

specs: 5V TTL max 100 KHz. signals on breadboard.

Often I saw 10K for pull up and 470 ohms for pull down.

If you would have only one value of resistors which value would be a good choice for both ?

Or how to calculate pull up and down for 5V TTL stuff ?

Does the frequency of the signals are a part of the calculation ?

thank you

DJ

[cowana]

10k is standard for a pullup or pulldown.

470\$\Omega\$ sounds unusually low - are you sure that wasn't driving an LED in series to ground?

[edavid]

It depends on exactly what you mean by TTL.

A standard TTL gate input, like on a 7400, has an 0.8V maximum input low level, and sources 1.6mA of current.  So, the highest value pulldown you would want to use is 0.8/0.0016 = 500ohm.

If it's a different TTL family, or an oddball part, check the datasheet and recalculate.

P.S. This is why you try to avoid using pulldown resistors with TTL.

[DJLinux]

470\$\Omega\$ sounds unusually low - are you sure that wasn't driving an LED in series to ground?

You are right my fault :-)

So, the highest value pulldown you would want to use is 0.8/0.0016 = 500ohm.

pulldown = U / I

and how to calculate pullup for example if high pegel of the IC starts at 2.8 V - 5.0V ?

is it = 2.8V /  I (max or min current from datatsheet of the part)

Sorry for this stupid questions but it's a beginner forum section :-)

DJ

[edavid]

and how to calculate pullup for example if high pegel of the IC starts at 2.8 V - 5.0V ?

is it = 2.8V /  I (max or min current from datatsheet of the part)

The highest value pullup is: (minimum V_CC - minimum V_IH) / maximum I_IH

So for a standard TTL gate input:  (4.5V - 2.0V)/40uA = 62K

I hope you were asking about input pullups rather than output pullups, because that's the question I answered 

[Zero999]

pulldown = U / I

and how to calculate pullup for example if high pegel of the IC starts at 2.8 V - 5.0V ?

is it = 2.8V /  I (max or min current from datatsheet of the part)

Sorry for this stupid questions but it's a beginner forum section :-)

DJ

What's the part number of the IC?

Can you post a link to the data sheet?

If the part number is something like DM74xx or 74LSxx which are TTL then the answers are given above. If it's 74HCxx, 74HCTxx, or 74ACxx etc. then a much higher value can be used, limited by noise pick-up and the required speed.