The runt pulse monitor uses a window detector to validate input clock pulses that arrive within the nominal clock period minus a programmable early window threshold (T_EARLY). When an input pulse arrives after T_EARLY, the pulse is considered valid and the runt pulse flag will be cleared. When an early or runt input pulse arrives before T_EARLY, the monitor will set the flag immediately to disqualify the input.

Typically, T_EARLY should be set lower than the input's shortest clock period (including cycle-to-cycle jitter). The early pulse monitor can act as a coarse frequency detector with faster detection than the ppm frequency detector. The early pulse monitor is supported for input frequencies between 2 kHz and f_VCO/12 and should be disabled when outside of this range.

Users must enable missing clock detect in order to use early clock detect. Early clock detect cannot be enabled alone.

Figure 8-16 Early and Late Window Detector Examples