Xilinx Inc. (XLNX), which makes programmable microchips, Monday cut its sales estimate for the September quarter, citing weakness in Asia-Pacific demand and other factors, sending shares down more than 15 percent.

The company said it now expects to report sales down 1 percent to 2 percent sequentially for the period, compared with previous forecasts for sales to be flat to up 4 percent.

Analysts had expected the company's revenue in the fiscal second quarter to reach $415.6 million, according to Reuters Estimates.

Wall Street analysts, some of whom had established buy ratings over the past month after Xilinx reaffirmed its forecast in early September, were blindsided by the news and a dearth of details.

"It caught everybody by surprise," said Satya Chillara, an analyst at American Technology Research, who maintained its buy rating on the stock. "The details are very sketchy ... We don't know which end-market is low or what is happening."

Caris & Co. analyst Rick Whittington cut his rating on Xilinx and rival Altera Corp.(ALTR) , both to "average" from "above average" on fears that both were locked in a price war, leading customers to hold off on buying components.

Sales from companies with manufacturing operations in the Asia-Pacific region fell by double-digit percentages sequentially, the company said. The company also cited less than expected sales in "mainstream products."

The San Jose, California, company said it still expects to report gross margins for the quarter of 61 percent to 62 percent.

Xilinx shares were down $4.22 to $22.90, after touching a new annual low of $22.79; shares of Altera fell 19 cents, or 1 percent, to $18.15 on Nasdaq after trading as low as $17.75.