The VCT2001A is a dual reference input 3.3V clock generator module with an HCMOS output. Reference inputs are selectable by a one-bit select control input. An alarm output can be used to detect a problem on the selected reference, such as a loss of lock or a loss of reference. A logic level "1" is indicated from the alarm pin when there is a problem with the selected reference. The unit operates in the free run mode when there is a problem with the selected reference, or when free run is invoked by enabling the forced free run input. Free run accuracy will keep the output to within 20 ppm. The reference will not automatically switch upon loss of reference. Also, all outputs may be tri-stated for external testing purposes with a signal to the Tri-State/Reset input.