Abstract: A clock source selector for selecting either a first clock signal A or a second clock signal B in accordance with a switch request signal includes three retiming circuits each consisting of two clocked flip-flops. The switch request signal is first retimed relative to clock A to give a signal P, is then retimed relative to clock B to give a signal Q, and finally is retimed relative to clock A to give a signal R. Selector circuitry operates such that when signal Q is asserted, the second clock signal B is output, when neither signal P nor signal R, as combined by a NOR gate, are asserted, the first clock signal A is output, and at other times a zero level is output. The clock source selector can be used in an integrated circuit to form a glitch-free multiplexer.

Abstract: A digital phase comparator circuit that determines and adjusts the relative phase of two digital clock signals derived from the same digital clock. The circuit having two inputs, one connected to receive each of the clock signals to be compared and including a latch circuit to receive one clock signal at the clock input, and the other clock signal at a data input. The latch circuit is arranged so that the output is equal to the signal at the data input when measured at the clock edge. The output is therefore a logic “1” when the second clock leads the first clock, and a logic “0” when the second clock lags the first clock.

Abstract: A digital frequency divider includes phase control of the output signal in increments of whole or half cycles of the input frequency. Whole cycle phase control is achieved by varying (logically or physically) the tap off point of a shift register loaded with a bit pattern for appropriate division. Half cycle phase changes are achieved by a multiplexer selecting one of two signals every half cycle.