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Topic: voltage divider vs logic level converter? (Read 2 times) previous topic - next topic

gerg


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Was it a problem because a voltage divider was used

Yes it slowed down the edges like I said.


Please keep in mind my background is not hardware. Sorry if this is a dumb question. How does a resister "slow down the edge"?

Grumpy_Mike

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How does a resister "slow down the edge"?

Not at all a dumb question but given that RuggedCircuits described how it does it in reply #4, well ....

Anyway it does it because of the combination of resistance and the capacitance in the tracks an on the input to the device. This is known as stray capacitance and is always present in every practical circuit. The combination of a series resistor and capacitor to ground forms a low pass filter. A perfect edge, that is a change from one logic level to another, consists of an infinite number of harmonics or frequencies. If those harmonics don't go all the way up to infinity but stop short (as all practical circuits will) then the edge can't be perfect and therefore the edge has a finite rise time. The resistor and stray capacitance form a filter that limits the number of harmonics in the edge more than it normally would be, and so the rise time is longer than it normally would be. The bigger the resistance or large the capacitance the slower the edge rises.
All digital components need the rise time on the edges to be a minimum value, this varies considerably so see the data sheet on the component you have in mind.
An SD card with it's socket is a particularly rich source of stray capacitance and cheap memory in SD cards sometimes do not have the specification you would hope for.

gerg

Okay. That makes sense.

I think you said it, but I want to clarify. In this case, the capacitance comes from circuits in the IC on both sides of the divider? Whereby the divider only adds to the harmonic? Correct me as need.

So it seems there was a some truth in what I had originally read.

Thanks.


Grumpy_Mike

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Whereby the divider only adds to the harmonic?

Not quite, there is capacitance all round but the resistor works with the capacitor to make a filter. For a discussion of simple RC filters see this page:-
http://www.thebox.myzen.co.uk/Tutorial/PWM.html
It is the reduction of the number of harmonics not the addition that makes the rise time slower.

gerg


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Whereby the divider only adds to the harmonic?

Not quite, there is capacitance all round but the resistor works with the capacitor to make a filter. For a discussion of simple RC filters see this page:-
http://www.thebox.myzen.co.uk/Tutorial/PWM.html
It is the reduction of the number of harmonics not the addition that makes the rise time slower.


Ohhh....

Thanks for the link. I'll check that out.

Again, thanks for everyone's patience and contributions.

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