The U.S. Patent and Trademark Office on Tuesday tentatively invalidated several Rambus Inc patents key to the microchip designer's disputes against graphics chip maker Nvidia Corp.

The patent office has now initially rejected all of the patent claims asserted by Rambus against Nvidia in the ITC, or U.S. International Trade Commission, Nvidia general counsel David Shannon said in a statement.

Rambus replied that the game is hardly over.

The recent action by the (patent office) to re-examine Rambus patents is part of the process and Rambus will have an opportunity to respond, Rambus said.

Rambus, which has spent $300 million on legal disputes with chip makers since 2000, wants Nvidia to pay royalties on memory circuits used in Nvidia graphics chips.

Nvidia appears to have won one round in a multi-round contest, so we have a lot more decisions to make before they can declare final victory, said David Wu of Global Crown Capital.

The dispute began in June, 2008, when Rambus filed suit in federal court in San Francisco charging that Nvidia had infringed 17 patents. No dollar amount was attached to the suit, but Rambus has won millions in similar suits.

In November, Rambus also filed before the International Trade Commission in Washington, D.C., a U.S. agency that may bar disputed imports, concerning nine of those patents.

The ITC administrative process is quicker than the federal court and could influence the judge in San Francisco, Susan Illston. Illston suspended proceedings to await the ITC ruling.

In Washington, Nvidia defended itself in the ITC case by challenging Rambus's patents, telling the U.S. Patent and Trademark Office they were based on old ideas.

In June, a U.S. patent examiner agreed with Nvidia that the office had made a mistake in issuing the patents. This week, the patent examiner agreed again on even more claims.

In between those two decisions, Rambus asked the ITC to drop some of its claims. Now Rambus and Nvidia will each argue their side in writing to the patent office.

Analysts say their bigger concern is over another suit involving Nvidia, in a dispute with Intel over a licensing agreement. That is being played out in a Delaware state court.

(Reporting by David Lawsky; editing by Carol Bishopric)

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