Clock tree for SiS 645/648 — what this part does
The Cypress CY28342OC is a PLL-based clock synthesizer for the SiS 645 and SiS 648 chipset platforms. It takes a single-ended clock input and fans it out to 20 clock outputs, two at 24 MHz and the rest at 48 MHz, with differential output capability. The part lives in a 48-pin SSOP package and operates from a 3.135 V to 3.465 V supply.
Output frequencies and fanout — what the 1:20 ratio means for the board
The 1:20 input-to-output ratio means one reference clock feeds 20 device clocks — enough to drive the CPU, memory controller, PCI bus, and peripheral clocks on a SiS 645/648 motherboard without an external fanout buffer. The 24 MHz output typically serves the PCI bus or USB controller; the 48 MHz lines handle the chipset core and memory interface. The differential outputs (No/Yes) give you better noise immunity on the longer traces between the clock generator and the chipset.
Temperature grade and environment
Rated for 0°C to 70°C — commercial grade only. This part is intended for indoor, office-temperature equipment. Not suitable for industrial enclosures, automotive cabins, or outdoor telecom cabinets where ambient temperatures push past 70°C.