A synchronization circuit receives an external clock input. The circuit includes an internal oscillator; a clock detection circuit, coupled to the external clock input, for determining whether a clock signal at the external clock input is valid; circuitry for keeping the frequency of the internal oscillator constant until the clock detection circuit determines that an external clock signal is valid; and circuitry for switching the output of the synchronization circuit from the internal oscillator to the external clock input when the clock detection circuit determines than an external clock signal is valid.