US Patent 10326457 Reference-locked clock generator

Clock generation from an external reference by generating a reference clock gating signal using a reference clock gating circuit; enabling a ring-oscillator-injection mode using the reference clock gating signal to disable a first buffer of a ring oscillator and to enable a reference clock injection buffer, the first buffer and the injection buffer having parallel connected outputs that connect to a next buffer input; receiving a reference clock transition of a reference clock signal at the injection buffer and injecting it into the next buffer; and enabling a ring-oscillator-closed-loop mode by using the reference clock gating signal to enable the first buffer and to disable the reference clock injection buffer.