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

• CG635 output waveform with no load
• CG635 output waveform with 100-Ohm load

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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