Corporate raider Carl Icahn has the option to purchase 9.5 percent of the stock in Wilsonville-based Mentor Graphics.
The company is taking a defensive measure against the purchase, a "poison pill" that will allow existing shareholders to purchase discounted stock if they own more than a certain number of the company's shares.
Mentor's "shareholder rights" plan will distribute one incentive stock-purchase right for each share held as of the close of business July 6. The right is triggered if any person or entity acquires control of more than 15 percent of the company's stock.
Mentor makes software for designing computer chips and other electronic devices. It's the largest technology company headquartered in Oregon -- Intel is based in California -- and roughly a quarter of its 4,400 employees work at its Wilsonville headquarters.
Read more at OregonLive.com.