Programmable differential internal termination for a low voltage differential signal input or output buffer

• US 6,963,219 B1
• Filed: 04/08/2003
• Issued: 11/08/2005
• Est. Priority Date: 04/08/2003
• Status: Active Grant

• Current Assignee: Xilinx, Inc. (Advanced Micro Devices, Inc.)
• Original Assignee: Xilinx, Inc. (Advanced Micro Devices, Inc.)
• Inventors: Ghia, Atul V., Sodha, Ketan
• Primary Examiner(s): Chang, Daniel D.
• Application Number:
  US10/409,891
• Time in Patent Office: 945 Days
• Field of Search: 326/30, 326/86, 327/321, 327/328
• US Class Current: 326/30
• CPC Class Codes: H04L 25/0278 Arrangements for impedance matching

Abstract

A configurable low voltage differential signal (LVDS) system is located on a chip, such as a programmable logic device. The configurable LVDS system includes a pair of I/O pads, an LVDS transmitter for driving a differential output signal onto the I/O pads, an LVDS receiver for receiving a differential input signal from the I/O pads, and a termination resistor coupled across the pair of I/O pads, wherein the termination resistance can be enabled for use with either the LVDS transmitter or the LVDS receiver. Control circuitry is provided to control the selective enabling and disabling of the LVDS transmitter, the LVDS receiver and the termination resistance. This control circuitry can be configured in response to configuration data values stored on the chip.

Claims

1. A configurable low voltage differential signal (LVDS) system located on a chip, the configurable LVDS system comprising:

a pair of conductive terminals;

an LVDS transmitter for driving a differential output signal to the pair of terminals;

an LVDS receiver for receiving a differential input signal from the pair of terminals;

a termination resistor selectively coupled across the pair of terminals, wherein the termination resistor is selected via a control signal for use with either the LVDS transmitter or the LVDS receiver; and

a control circuit configured to selectively enable and disable the LVDS transmitter, the LVDS receiver and the termination resistor, wherein the control circuit is further configured to enable and disable one or more non-LVDS receivers, and in response to the control circuit enabling one of the one or more non-LVDS receivers, disabling the LVDS receiver and the LVDS transmitter.

2. The configurable LVDS system of claim 1, wherein the pair of conductive terminals comprise input/output pads located on the chip.

3. The configurable LVDS system of claim 1, wherein the termination resistor comprises one or more transistors coupled between the pair of terminals.

4. The configurable LVDS system of claim 1, wherein the control circuit is configured to select the termination resistor by activating the one or more transistors with the control signal, and to deselect the termination resistor by deactivating the one or more transistors with the control signal.

5. The configurable LVDS system of claim 1, wherein the control circuit is configured to enable the termination resistor and either the LVDS transmitter or the LVDS receiver.

6. The configurable LVDS system of claim 1, wherein the control circuit comprises one or more configuration memory cells, wherein the configuration memory cells store configuration data values that are used to enable and disable the LVDS transmitter, the LVDS receiver, the termination resistor, and the one or more non-LVDS receivers.

7. The configurable LVDS system of claim 1, wherein the LVDS transmitter comprises a first set of transistors configured to drive a first signal to a first one of the terminals, and a second set of transistors configured to drive a second signal, complementary to the first signal, to a second one of the terminals, wherein the control circuit disables the LVDS transmitter by turning off the first and second sets of transistors.

8. The configurable LVDS system of claim 1, further comprising:

an LVDS transmitter extension connected in parallel with the LVDS transmitter across the pair of terminals; and

a second termination resistance associated with the LVDS transmitter extension, and coupled across the pair of terminals.

9. A method of implementing a configurable low-voltage differential signal (LVDS) system on a chip, the method comprising:

selectively coupling an LVDS transmitter,

an LVDS receiver, and

a termination resistor to a pair of input/output (I/O) pads on the chip; and

selectively decoupling the LVDS transmitter, the LVDS receiver, and the termination resistor in response to coupling a non-LVDS receiver to the pair of I/O pads on the chip.

10. The method of claim 9, further comprising:

loading one or more configuration data values into one or more corresponding configuration memory cells on the chip; and

controlling the I/O pad coupling in response to the configuration data values.

11. The method of claim 9, wherein the step of selectively coupling the termination resistor comprises turning on one or more transistors coupled across the pair of I/O pads.

12. The method of claim 11, wherein the step of selectively decoupling the termination resistor further comprises turning off the one or more transistors coupled across the pair of I/O pads.

13. A configurable low voltage differential signal (LVDS) system located on a chip, the configurable LVDS system comprising:

a pair of electrically conductive input/output (I/O) pads;

an LVDS transmitter coupled to the pair of I/O pads;

a termination resistor coupled to the pair of I/O pads;

an LVDS receiver coupled to the pair of I/O pads;

means for selectively enabling and disabling the LVDS transmitter, the LVDS receiver and the termination resistor; and

at least one non-LVDS receiver coupled to the pair of I/O pads, wherein the at least one non-LVDS receiver is enabled in response to disabling the LVDS transmitter, the LVDS receiver and the termination resistor.

14. The configurable LVDS system of claim 13, wherein the means for selectively enabling and disabling comprise a plurality of configuration memory cells that store a corresponding plurality of configuration data values.

15. The configurable LVDS system of claim 13, wherein the termination resistor comprises one or more transistors coupled across the I/O pads.

16. A configurable low voltage differential signal (LVDS) system located on a chip, the configurable LVDS system comprising:

a pair of conductive terminals;

an LVDS transmitter for driving a differential output signal to the pair of terminals;

an LVDS receiver for receiving a differential input signal from the pair of terminals; and

a termination resistor selectively coupled across the pair of terminals, wherein the termination resistor includes,

a first transistor network selectively coupled via a first control signal; and

a second transistor network selectively coupled in parallel to the first transistor network via a second control signal.

17. The configurable LVDS system of claim 16, wherein the first transistor network comprises:

a first pair of series connected transistors coupled across the pair of terminals; and

a second pair of series connected transistors coupled in parallel to the first pair of series connected transistors.