An integrated circuit (11) has a frequency detection circuit (22) which provides one or more digital signals (50) to a current source (26) based upon a detected frequency of operation of a generated reference clock (48). The signals (50) allows the current source (26) to change its operational state between two or more discrete current output levels in a digitally-controlled manner. Using signals (50), a high current output level can selected and provided by the current source (26) to the external oscillator circuit (16) during a start up mode to ensure that the integrated circuit (11) can start up in an optimally reduced time period. After a start up operation is complete, the signals (50) can then be used to switch the current source (26) into a lower current operational mode whereby electromagnetic interference (EMI) effects are reduced during the normal modes of operation occurring after start up.