Electronic digital logic circuitry – Signal sensitivity or transmission integrity – Output switching noise reduction
Reexamination Certificate
2000-03-14
2001-10-16
Tokar, Michael (Department: 2819)
Electronic digital logic circuitry
Signal sensitivity or transmission integrity
Output switching noise reduction
C326S030000, C326S083000
Reexamination Certificate
active
06304098
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates in general to data communication systems, and in particular to method and circuitry for providing differential immunity from noise sources in data communication channels that share a reference signal among multiple data signals.
Conventional differential signaling uses a pair of wires to transmit an electrical signal. Due to its differential nature, this type of signal transmission rejects common-mode noise sources and thus enjoys higher immunity to noise and crosstalk. Differential signaling, however, doubles the amount of I/O hardware and integrated circuit (IC) pin count per signal. This overhead becomes prohibitively costly in applications where the IC processes multiple parallel data channels. Differential signaling based on the same principles but using a single reference channel that is shared by multiple single-wire data channels has been developed to reduce the required hardware overhead while retaining the noise immunity advantages. This type of differential signaling, however, is not as effective in rejecting noise at higher frequencies. Due to the fact that the reference channel is routed and connected to multiple input circuits, while a given data channel is routed and connected only to its one dedicated input circuit, inherent physical differences exist between the data channels and the share reference channel. These differences contribute to the mismatch between the parasitic capacitive loading of the data channels as compared to that of the shared reference channel. As a result, high frequency noise sources couple differently in the data channels versus the reference channel, introducing differential noise which reduces the signal to noise ratio for the communication system.
Accordingly, there is a need for method and circuitry to further improve the noise immunity of differential transmission at higher frequencies.
SUMMARY OF THE INVENTION
The present invention improves noise immunity of differential data channels that use a shared reference channel, by substantially matching their respective noise transfer functions. The invention takes advantage of the fact that noise from noise sources (e.g., power buses) do not couple into a signal based on the value of a single circuit component but rather based on the ratios of circuit components as determined by the transfer function from the noise source to the signal under attack. By substantially matching the transfer function from noise source to shared reference signal to the transfer function from noise source to data signals, the present invention improves noise immunity for the shared reference signaling to levels comparable to the ideal case of a dedicated data/reference pair. For example, although the parasitic capacitance of the reference channel is higher due to the extra routing and loading, an exemplary embodiment of the present invention reduces the reference channel's termination resistance and transmission line impedance to maintain a constant RC time constant for the noise transfer function. The noise transfer function typically includes other parameters such as inductance due to the packaging of the integrated circuit (e.g., bond-wire and lead frame). This allows the application of the same principle to the remaining components of the reference channel noise transfer function until it matches the data channel noise transfer function for all frequencies.
Accordingly, in one embodiment, the present invention provides a method of reducing noise for a communication system wherein data transmission occurs over a plurality of data channels that share a reference channel, the method including a step of substantially matching a transfer function of noise from a noise source to the reference channel, to a transfer function of noise from a noise source to one of the plurality of data channels. The substantially matching step includes a step of reducing a termination resistance of the reference channel. The substantially matching step further includes a step of reducing an impedance of the reference channel transmission line.
In another embodiment, the present invention provides an integrated circuit including an output data driver coupled to drive a data signal onto an external data pin, and a reference signal driver coupled to drive a reference signal onto an external reference pin, wherein information is conveyed by the data signal in relation to the reference signal, and wherein the reference signal driver is configured to have a programmable output impedance. In a specific embodiment, the reference signal driver includes a plurality of parallel-coupled resistive elements that are programmably coupled to the external reference pin. The plurality of parallel-coupled resistive elements include field effect transistors coupled between the external reference pin and a power bus. The field effect transistors are biased to operate in linear region.
A better understanding of the nature and advantages of the noise transfer matching technique of the present invention may be gained with reference to the following detailed description and the accompanying drawings.
REFERENCES:
patent: 4217553 (1980-08-01), Winebarger
patent: 5977795 (1999-11-01), Lee
patent: 6087885 (2000-07-01), Tobita
Mooney et al., “A 900 Mb/s Bidirectional Signaling Scheme”, IEEE Journal of Solid-State Circuits, vol. 30, No. 12, pp. 1538-1543 (Dec. 1995).
Sidiropoulos et al., “A 700-Mb/s/pin CMOS Signaling Interface Using Current Integrating Receivers”, IEEE Journal of Solid-State Circuits, vol. 32, No. 5, pp. 681-690 (May 1997).
Drost Robert J.
Wilhelm Neil C.
Le Don Phu
Sun Microsystems Inc.
Tokar Michael
Townsend and Townsend / and Crew LLP
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