An on-chip phase-locked loop (PLL) uses a fractional divider
[8], [9] to generate Ratioed Synchronous Clocks with support
for a wide range of integer and fractional divide ratios. The
distribution of these clocks uses a combination of H-trees
and grids. This ensures they meet tight clock skew budgets
while keeping power consumption under control. Clock Tree
Synthesis is used for routing the asynchronous clocks. Asynchronous
clock domain crossings are handled using FIFOs
and meta-stability hardened flip-flops. All clock headers are
designed to support clock gating to save clock power.
Fig. 11 shows the block diagram of the PLL. Its architecture
is similar to the one described in [8]. It uses a loop filter capacitor
referenced to a regulated 1.1 V supply (VREG). VREG is
generated by a voltage regulator from the 1.5 V supply coming