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Contributor
Contributor
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Registered: ‎10-05-2015

drive strength in Kintex 7 IBIS model

Hi,

I am using Kintex 7 FPGA (XC7K160T-1FFG676C) along with its IBIS model downloaded from Xilinx website (kintex7_ibis_v2_0).

 

I'm using this model in Hyperlynx. Generally, with higher drive strength, we generally have a shorter rise time and a lesser output impedance. I can see this for LVCMOS18, slow HP.

 

For the particular case of (LVCMOS18, slow HR), I am getting identical information for drive strengths for 8 mA as well as 12 mA.

Rise Time (20-80) = 569 ps

Output Impedance = 30 ohms

 

Is this an error of the model?

 

Regards,

Binayak

 

 

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Xilinx Employee
Xilinx Employee
5,510 Views
Registered: ‎08-01-2008

Drive strength determines the output impedance of the driver. When you do a signal integrity analysis of your board, before you get it fabricated, using the layout and the IBIS models, you then get to see if it will work as designed.

Too weak, and you may not meet timing. Too strong, and you may have simulaneous swicthing noise, over shoot, under shoot, ringing, excessive jitter, and data errors (or worse).

For example 6 mA or 8 mA is a very good match for pcb 50 ohm impedance controlled traces.

http://www.xilinx.com/products/technology/signal-integrity/
Thanks and Regards
Balkrishan
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Scholar
Scholar
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Registered: ‎02-27-2008

b,

 

No.  All models are from hSpice(TM Synopsys) of the actual IO circuits converted to IBIS models, using TSMC models for process.  They are verified by the characterization team before production.

 

It is likely that once 'strong enough,' you see no more 'improvement' in the metrics mentioned.  Not at all unusual.

 

 

Austin Lesea
Principal Engineer
Xilinx San Jose
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Contributor
Contributor
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Registered: ‎10-05-2015

Hi Austin,

 

I think I have got my engineering intuition off by a little! Hope you will guide me on this!

 

I thought that having a bigger drive strength would decrease the rise time. I did some simulations. I just took some drivers and tested their rise times without connecting them to any circuits. I then connected them to a simple resistor circuit to get the output impedance. I did it for virtex 6 (XC6VLX75T-1, FFG484) as well as kintex 7(XC7K160T-1, FFG676). All the rise times (RT) are (10-90)%.

 

I am sharing this information here:

 

Virtex 6

1) LVCMOS18_SLOW - 8mA      =>   RT = 570 ps;     Zout = 42 ohms

2) LVCMOS18_SLOW - 12mA    =>  RT = 388  ps;     Zout = 29 ohms

 

3) LVCMOS25_SLOW - 8mA      =>  RT = 580 ps;     Zout  = 43 ohms

4) LVCMOS25_SLOW - 12mA    =>  RT = 396 ps;     Zout  = 30 ohms

 

Kintex 7

5) LVCMOS18_SLOW_HP - 8mA    =>   RT= 762 ps;    Zout = 46 ohms

6) LVCMOS18_SLOW_HP - 12mA  =>   RT= 533 ps;    Zout = 29 ohms

 

7) LVCMOS18_SLOW_HR - 8mA    =>   RT= 881 ps;    Zout = 30 ohms

8) LVCMOS18_SLOW_HR - 12mA  =>   RT= 881 ps;    Zout = 30 ohms

 

Except the case in #8, if we see the cases, drive strength of 8 mA is giving shorter rise time and smaller output impedance.

 

Hence I got confused for scenario #8 of LVCMOS18_SLOW_HR for the Kintex device IBIS model. Have I done something wrong here?

 

Warm Regards,

Binayak

 

 

 

 

 

    

  

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Contributor
Contributor
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Registered: ‎10-05-2015

Sorry for the error in the last post.

 

I wanted to say that the in all cases except in the scenarios of #7 and #8, drive strength of 12 mA is giving shorter rise time and smaller output impedance compared to those with the drive strength of 8 mA.

 

Warm regards,

Binayak

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Contributor
Contributor
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Registered: ‎10-05-2015

any updates?

 

regards,

binayak

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