The XRT91L30 is a fully integrated SONET/SDH transceiver for SONET/SDH 622.08 Mbps STS-12/ STM-4 or 155.52 Mbps STS-3/STM-1 applications. The transceiver includes an on-chip Clock Multiplier Unit (CMU), which uses a high frequency PhaseLocked Loop (PLL) to generate the high-speed transmit serial clock from a slower external clock reference. It also provides Clock and Data Recovery (CDR) function by synchronizing its on-chip Voltage Controlled Oscillator (VCO) to the incoming serial data stream. The internal CDR unit can be disabled and bypassed in lieu of an externally recovered received clock from the optical module. Either the internally recovered clock or the externally recovered clock can be used for loop timing applications. The chip provides serial-to-parallel and parallel-to-serial converters using an 8-bit wide LVTTL system interface in both receive and transmit directions. The transmit section includes an option to accept a