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
|
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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