Methods and Systems for Reducing Supply and Termination Noise

• US 20110019760A1
• Filed: 07/09/2010
• Published: 01/27/2011
• Est. Priority Date: 07/21/2009
• Status: Abandoned Application

• Current Assignee: Rambus Inc.
• Original Assignee: Rambus Inc.
• Inventors: Nguyen, Huy M.
• Application Number:
  US12/833,835
• Publication Number:
  US 20110019760A1
• Time in Patent Office: Days
• US Class Current: 375/267
• CPC Class Codes: H03K 19/0005 Modifications of input or output impedance
• CPC Class Codes: H03K 19/00346 Modifications for eliminating interference or parasitic voltages or currents
• CPC Class Codes: H04L 1/0028 Formatting
• CPC Class Codes: H04L 25/0276 Arrangements for coupling common mode signals
• CPC Class Codes: H04L 25/0282 Provision for current-mode coupling
• CPC Class Codes: H04L 25/0294 Provision for current-mode coupling

Abstract

A transmitter expresses continuous-time signals on alternate, parallel channels with reference to different supply voltages such that the signals on alternate channels have different common-mode voltages. At the transmitter, expressing the symbols using alternate supply voltages limits the maximum supply current used to express the signals and to transition between adjacent symbol sets. Limiting supply current ameliorates problems associated with simultaneous switching noise (SSN). At the receiver, the different common-mode voltages tend to balance the current to and from termination nodes, and consequently place reduced stress on a reference voltage. Providing different common-mode voltages on alternate channels may additionally reduce cross-talk between channels.

Claims

1. A transmitter comprising:

first and second voltage nodes to respectively provide relatively high and low voltages;

a signal source to provide a plurality of parallel signal streams, including a first signal stream and a second signal stream;

a pull-up amplifier having a first control terminal coupled to the signal source to receive the first signal stream, a first current-handling terminal coupled to the first voltage node to receive the relatively high voltage level, and a second current-handling terminal to provide an amplified version of the first signal stream as a first continuous-time output signal, the first continuous-time output signal having a first common-mode voltage; and

a pull-down amplifier having a second control terminal coupled to the signal source to receive the second signal stream, a third current-handling terminal coupled to the second voltage node to receive the relatively low voltage level, and a fourth current-handling terminal to provide an amplified version of the second signal stream as a second continuous-time output signal, the second continuous-time output signal having a second common-mode voltage between the first common-mode voltage and the low voltage.

2. The transmitter of claim 1, wherein the first continuous-time output signal has a first voltage swing, the second continuous-time output signal has a second voltage swing, and a difference between the first common-mode voltage and a mid-level between the relatively high and low voltages is greater than half the first voltage swing.

3. The transmitter of claim 2, wherein a second difference between the second common-mode voltage and the mid-level between the relatively high and low voltages is greater than half the second voltage swing.

4. The transmitter of claim 3, wherein the first and second voltage swings are about equal.

5. The transmitter of claim 1, further comprising a first signal pad coupled to the second current-handling terminal to convey the amplified version of the first signal stream and a second signal pad, adjacent the first signal pad, coupled to the fourth current-handling terminal to convey the amplified version of the second signal stream.

6. The transmitter of claim 1, wherein the first and second signal streams are discrete signals.

7. A receiver to receive a plurality of parallel continuous-time signals on a corresponding plurality of terminals, including a first terminal to receive a first continuous-time signal and a second terminal to receive a second continuous-time signal in parallel with the first continuous-time signal, the receiver comprising:

first and second reference-voltage nodes to respectively provide relatively high and low reference voltages;

a termination node to receive a termination voltage between the high and low reference voltages;

a first termination impedance disposed between the termination node and the first terminal;

a second termination impedance disposed between the termination node and the second terminal;

a first receive amplifier having a first reference node coupled to the first reference-voltage node, to receive the relatively high reference voltage, and a first input node coupled to the first terminal to receive the first continuous-time signal; and

a second receive amplifier having a second reference node coupled to the second reference-voltage node, to receive the relatively low reference voltage, and a second input node coupled to the second terminal to receive the second continuous-time signal.

8. The receiver of claim 7, wherein the first continuous-time signal has a first voltage swing, the second continuous-time signal has a second voltage swing, and a difference between the first common-mode voltage and the termination voltage is greater than half the first voltage swing.

9. The receiver of claim 8, wherein a second difference between the second common-mode voltage and the termination voltage is greater than half the second voltage swing.

10. The receiver of claim 9, wherein the first and second voltage swings are about equal.

11. The receiver of claim 7, wherein the first and second terminals are adjacent pads on a integrated circuit.

12. A parallel communication system comprising:

at least two sets of single-ended communication channels, including a first set of channels to convey a first set of continuous-time signals and a second set of channels to convey a second set of continuous-time signals;

wherein each of the first set of continuous-time signals has a first voltage swing and a first common-mode voltage and each of the second set of continuous-time signals has a second voltage swing and a second common-mode voltage; and

wherein the difference between the first and second common-mode voltages is greater than half the sum of the first and second voltage swings.

13. The communication system of claim 12, wherein each member of the first set of communication channels is physically separated from the other members of the first set of communication channels by one or more members of the second set of communication channels.

14. The communication system of claim 12, further comprising a pull-up amplifier coupled to each of the first set of communication channels to express the first set of continuous-time signals and a pull-down amplifier coupled to each of the second set of communication channels to express the second set of continuous-time signals.

15. The communication system of claim 12, further comprising a receiver having a first set of receive amplifiers referenced to a relatively high voltage coupled to the first set of channels to receive the first set of signals and a second set of receive amplifiers referenced to a relatively low voltage coupled to the second set of channels to receive the second set of signals.

16. The communication system of claim 15, the receiver further having a termination resistor connecting each channel to a termination voltage between the first and second common-mode voltages.

17. A method of conveying parallel bit streams, including first and second sets of bit streams, simultaneously over parallel communication channels, the method comprising:

terminating the communication channels to a termination node;

selectively applying current to the termination node via a first subset of the communication channels and responsive to the first set of bit streams to produce a first set of continuous-time signals having a first common-mode voltage; and

selectively drawing current from the termination node via a second subset of the communication channels and responsive to the second set of bit streams to produce a second set of continuous-time signals having a second common-mode voltage.

18. The method of claim 17, wherein a first one of the first set of continuous-time signals has a first voltage swing and a second one of the second set of continuous-time signals has a second voltage swing, the sum of the first and second voltage swings being less than a voltage difference between the first and second common-mode voltages.

19. The method of claim 17, wherein selectively applying current to the termination node via the first subset of the communication channels comprises pulling at least one of the first subset of the communication channels up toward a first supply voltage and selectively drawing current from the termination node via the second subset of the communication channels comprises pulling at least one of the second subset of the communication channels down toward a second supply voltage.

20. The method of claim 17, further comprising comparing the first set of continuous-time signals with a first reference voltage to recover the first set of bit streams and comparing the second set of continuous-time signals with a second reference voltage to recover the second set of bit streams.

21. The method of claim 20, wherein the first reference voltage approximates the first common-mode voltage and the second reference voltage approximates the second common-mode voltage.