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Visitor
Visitor
6,299 Views
Registered: ‎12-16-2013

Line Simulation Clock Edge Has Non-Monotonic edges

Simulating the CDCE949 TI Clock Device to Zynq SoC.  I am seeing non-monotonic edges and I can't seem to smooth those out with different series resistance or impedance or distance.  Any suggestions?

Using the LVCMOS_33_F_8_PSMIO model for the Zynq receive end.  

I used some other devices as receivers in the model and they don't seem to have this problem.  

CDCE_to_Zynq_FS_EDGES.jpg
CDCE_to_Zynq_FS_EDGESSCHEM.jpg
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Professor
Professor
6,296 Views
Registered: ‎08-14-2007

The position of the bump suggests to me that making that 50 ohm resistor zero ohms instead would help.  Have you tried that?

-- Gabor
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Visitor
Visitor
6,286 Views
Registered: ‎12-16-2013

I've tried 0 ohms and 50 ohms and 75 ohms and 100 ohms with combinations of transmission line impedances as well....the only thing that smooths it out is about 500 ohms :-P -- that was just a silly attempt to see what happens with a completely stupid series resistance...

I attached the 0 ohm version jpg here

CDCE_to_Zynq_FS_EDGES0ohm.jpg
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Professor
Professor
6,284 Views
Registered: ‎08-14-2007

I'm trying to understand why the signal at the Zynq end looks like it's in the middle of a transmission line rather than the end.  With series termination at the source, I would assume you have no termination inside the Zynq?  What is the IO standard at the Zynq end?

-- Gabor
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Visitor
Visitor
6,282 Views
Registered: ‎12-16-2013

Using  LVCMOS33_F_8_PSMIO

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Professor
Professor
6,278 Views
Registered: ‎08-14-2007

That sounds like an output standard ("F" meaning fast slew rate).  Did you use a generic IBIS model or did you generate it for your design using IBISwriter?

-- Gabor
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Visitor
Visitor
6,276 Views
Registered: ‎12-16-2013

I used the generic ibis model provided by Xilinx

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Participant
Participant
6,189 Views
Registered: ‎01-21-2014

Source termination resistor value should be:

Rs = <trace impedance> - <driver output impedance>.

 

So putting 22..33 Ohm instead of 50 Ohms should do the trick.

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