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ringing and overshoot on data lines for screen.

Hey everyone!

I am working on a project that is using a microcontroller to drive a
TFT display. This is a new PCB so the layout is new and I am seeing
lots of overshoot and ringing on the data lines!!!! On our last PCB I
didn't see any of this.

What is the best way to correct this? Add capacitors to all these
lines? If so what would be a good starting value? The clock
frequency for the data is about 6 MHz

Thanks for the help.

-Mike
 
P

PN2222A

Jan 1, 1970
0
I am working on a project that is using a microcontroller to drive a
TFT display. This is a new PCB so the layout is new and I am seeing
lots of overshoot and ringing on the data lines!!!! On our last PCB I
didn't see any of this.

What is the best way to correct this? Add capacitors to all these
lines? If so what would be a good starting value? The clock
frequency for the data is about 6 MHz
You are seeing ringing on the lines because they are acting as poorly
terminated transmission lines. A correct termination will make this
much better.

As a start, try 100 ohms in series at the source end.
A (worse) alternative is 100 ohms in series with 50 pf at the
destination.

PN2222a
 
You are seeing ringing on the lines because they are acting as poorly
terminated transmission lines. A correct termination will make this
much better.

As a start, try 100 ohms in series at the source end.
A (worse) alternative is 100 ohms in series with 50 pf at the
destination.

Thanks for the information. The lines coming out of the
microcontroller are not floating and go directly to the screen.
Doesn't the screen terminate the signal?
 
Thanks for the information. The lines coming out of the
microcontroller are not floating and go directly to the screen.
Doesn't the screen terminate the signal?

The screen will look like a capacitor. Transmission lines are
terminated by a resistive load roughly equal to their characteristic
impedance, which tends to be in the range from 50R to 75R depending on
the width of track, the spacing to the underlying ground plane and the
dielectric constant of the spacing material(s).
 
The thought is that the ringing and the overshoot is caused by
reflections coming back from the screen? When I remove the screen
completely I am still seeing the overshoot and the ringing on the data
lines!! Do you guys still think this still is a reflection problem?

Thank you!

-Mike
 
E

Eeyore

Jan 1, 1970
0
Thanks for the information. The lines coming out of the
microcontroller are not floating and go directly to the screen.
Doesn't the screen terminate the signal?

NO.

Graham
 
E

Eeyore

Jan 1, 1970
0
The thought is that the ringing and the overshoot is caused by
reflections coming back from the screen? When I remove the screen
completely I am still seeing the overshoot and the ringing on the data
lines!!

Because it's an unterminated connection.

You need some series resistance.

Graham
 
Because it's an unterminated connection.

You need some series resistance.

Graham

Does the series resistance that I am going to add to the trace have
anything to do with the input impedance of the screen? Instead of
trying different values how do I know the exact resistor value to
add? If I just look at my layout program and it tells me the
impedance of the trace do I just add that value as an in series
resistor?

Don't I want to match the impedance of the transmission line with that
of the screen? In all of the above text the trace is considered but I
don't see any mention of the screen.

Thanks for clarifying! :)

-Mike
 
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