Abstract: Circuits and methods for switching between an internal clock and an external clock without causing an interruption or an artifact in the switched clock signal are disclosed. To achieve this, the internal clock signal is synchronized with the external clock signal prior to switching. The synchronization may be accomplished using two possible clock-synchronization methods: a first method that passively waits for the clocks to synchronize over time and a second that adjusts a period of the internal clock signal to actively synchronize the clocks. The method selected for use requires the fewest clock cycles to reach synchronization, which is determined by a frequency difference between the two clock frequencies. After clock-synchronization, the output clock signal spectrum will be substantially the same before and after switching between the clock signals, and therefore is suitable for use with spread spectrum clocks.

Abstract: Circuits and methods for switching between an internal clock and an external clock without causing an interruption or an artifact in the switched clock signal are disclosed. To achieve this, the internal clock signal is synchronized with the external clock signal prior to switching. The synchronization may be accomplished using two possible clock-synchronization methods: a first method that passively waits for the clocks to synchronize over time and a second that adjusts a period of the internal clock signal to actively synchronize the clocks. The method selected for use requires the fewest clock cycles to reach synchronization, which is determined by a frequency difference between the two clock frequencies. After clock-synchronization, the output clock signal spectrum will be substantially the same before and after switching between the clock signals, and therefore is suitable for use with spread spectrum clocks.