1. An electronic device comprising: a display screen configured for displaying images; a touch sensor configured to detect user input in a sensing region, wherein said sensing region overlaps with an active area of said display screen; a host processing system; and a touch screen control system comprising: a memory configured for holding rules; and control circuitry configured for: updating said display screen in response to image data from said host processing system; operating according to a first set of rules held in said memory; in response to receiving an indication from said host processing system, switching from operating according to said first set of rules to operating according to a second set of rules, wherein said second set of rules is held in said memory, and wherein at least one of said first set of rules and said second set of rules defines when said control circuitry updates said display screen in response to said user input; and operating according to said second set of rules.