The ICS2495 Clock Generator is an integrated circuit dual phase-locked loop frequency synthesizer capable of generating 16 video frequencies and 4 memory clock frequencies for use with high performance video display systems. Utilizing CMOS technology to implement all linear, digital and memory functions, the ICS2495 provides a low-power, small-footprint, low-cost solution to the generation of video dot clocks. Outputs are compatible with XGA, VGA, EGA, MCGA, CGA, MDA, as well as the higher frequencies needed for advanced applications in desktop publishing and workstation graphics. Provision is made via a single level custom mask to implement customer specific frequency sets. Phase-locked loop circuitry permits rapid glitch-free transitions between clock frequencies. In addition to providing 16 clock rates, the ICS2495 has provisions to multiplex an externally-generated signal source into the VCLK signal path. Internal phase-locked frequencies continue to remain locked at their preset values when this mode is selected. This feature permits instantaneous transition from an external frequency to an internally-generated frequency. Printed circuit board testing is simplified by the use of these modes as an external clock generated by the ATE tester can be fed through, permitting synchronous testing of the entire system.