An ignition apparatus including a primary (22) and a secondary (24) winding for producing a spark voltage, an ion current detection circuit (32) for producing an ion current signal (33), a slope detector (36) and a knock detector (34). The slope detector (36) determines when the ion current signal (33) exhibits a slope indicative of a time period (t2-t3) when knock is likely to occur, and de-asserts a knock reset signal (64). The knock reset signal (64) holds the knock detector (34) in reset, thus disallowing or masking the processing of signals by the knock detector (36) that may be mistaken for knock.