The 31-year-old said a set number of appearances would trigger an automatic option for an extra year, but he refused to reveal the figure and admitted he wants to convince the club to keep him, regardless of the clause.
"It is games-related and I'm hoping it will just roll on into next season, but I've still got plenty to play for. It is the club's option in the summer so we will wait and see what happens," he told the Express & Star.
"I want to carry on playing in the Premier League. A lot of the season has been hampered with a couple of injuries but I just want to keep playing from now until the end of the season. "