On-chip DIFF_TERM of XILINX Spartan-6 and Virtex-5

This is an examination of FPGA internal differential termination resister of XILINX Spartan-6 and Virtex-5.
For reproducibility, high frequency signal is not used.

XILINX FPGA offers on-chip terminators by the name of "DIFF_TERM".

(Taken from XILINX user guide: UG381:Spartan-6 FPGA SelectIO resource user guide )

*Some pictures are copied from manufucturer's web-page.

 


Environment

Item Used Model number Picture Manuf. Note
Spartan-6 FPGA SP601
Evaluation Board
SP601 XILINX with FMC-XM105
Virtex-5 FPGA board XCM-203-LX30 HuMANDATA with ZKB-016
Clock signal generator CG635   Stanford Research Systems, Inc.  
Oscilloscope 204Xi  - Lecroy Bandwidth limited at 200MHz
FET Probe ZS1500 - Lecroy  


Left: SP601 and FMC-XM105(daughter board),  Right: XCM-203-LX30 and ZKB106 (HuMANDATA products)

 


Waveform

 

  • Waveform picture: Signal name from top: P-signal, N-signal, P minus N calclated.
  • The time axis is not unified. Please see amplitude.
No. Source Signal Receiver Waveform Note
 1 Spartan-6

(IOSTANDARD = LVDS_25)
 Spartan-6 (DIFF_TERM = OFF)
-
Spartan-6 (DIFF_TERM = ON)
Good
100 Ohm Load only
Refefence
No. Source Signal Receiver Waveform Note
2 Virtex-5

(IOSTANDARD = LVDS_25)
Spartan-6(DIFF_TERM = OFF)
-
Spartan-6(DIFF_TERM = ON)
Good
100 Ohm Load only
Reference
No. Source Signal Receiver Waveform Note
3

CG635


(Output: LVDS setting)

Spartan-6(DIFF_TERM = OFF)
-
Spartan-6(DIFF_TERMA = ON)
NG
100 Ohm Load only
Reference
No. Source Signal Receiver Waveform Note
4 CG635 + DC cut adaptor

(Output: LVDS setting)
 Spartan-6(DIFF_TERM = OFF)
-
 Spartan-6(DIFF_TERM = ON)
Good
No. Source Signal Receiver Waveform Note
5 Spartan-6

(IOSTANDARD = LVDS_25)
Virtex-5(DIFF_TERM OFF)
-
Virtex-5(DIFF_TERM = ON)
Good
100 Ohm Load only
Reference
No. Source Signal Receiver Waveform Note
6 Virtex-5

(IOSTANDARD = LVDS_25)
Virtex-5(DIFF_TERM = OFF)
 
Virtex-5(DIFF_TERM = ON)
Good
100 Ohm Load only
Reference
No. Source Signal Receiver Waveform Note
7 CG635

(Output: LVDS setting)
Virtex-5(DIFF_TERM = OFF)
 
Virtex-5(DIFF_TERM = ON)
Good
100 Ohm Load only
Reference

 


Result

    In case "Signal generator to Spartan-6", a DIFF_TERM did not work.

Source Signal FPGA DIFF_TERM effect
OK
OK
DIFF_TERM does not work?
OK
OK
OK
OK
OK

  Some HuMANDATA's XCM series FPGA boards did not use DIFF_TERM. They have footpatterns for on-board terminations (Not for all pins)


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