Apparatus for synchronously selecting different oscillators as system clock source

An apparatus for synchronously selecting different oscillators as a system clock within a system having a plurality of oscillators and a control means for generating selection signals indicating which oscillator is to be selected and sequence control signals for controlling the clock selection process of said apparatus, said control signals being asynchronous to said oscillator signals, said apparatus comprising:

• a first oscillator selection means, receiving the output of each of said plurality of oscillators and said selection signals, for selecting the output of one of said plurality of oscillator in response to said selection signals and for providing said selected output of said selected oscillator as the A CLOCK;
• a second oscillator selection means, receiving the output of each of said plurality of oscillators and said selection signals, for selecting the output of one of said plurality of oscillator in response to said selection signals and for providing said selected output of said selected oscillator as the B CLOCK;
• and
• a clock control means, receiving said A CLOCK, said B CLOCK and said control signals, for switching between said A CLOCK and said B CLOCK in response to said control signals to provide either said A CLOCK or said B CLOCK as the system clock to said system and for providing a STATUS signal to said control means for indicating whether said A CLOCK or said B CLOCK is presently selected as the system clock, said clock control means including a blanking means for preventing clock signals or noise from appearing on the system clock during the time that said first or second oscillator selection means is in the process of selecting a new oscillator, interlocking means for controlling the sequence of switching between said A CLOCK and said B CLOCK thereby preventing said A CLOCK and said B CLOCK from appearing as the system clock as the same time and a synchronizing means for synchronizing said control signals to said A CLOCK or said B CLOCK during a switching process.