The XRT91L80 is a fully integrated SONET/SDH transceiver for SONET OC-48/STM-16 applications supporting the use of Forward Error Correction (FEC) capability. The transceiver includes an on-chip Clock Multiplier Unit (CMU), which uses a high frequency Phase-Locked Loop (PLL) to generate the highspeed transmit serial clock from a slower external clock reference. It also provides Clock and Data Recovery (CDR) functions by synchronizing its onchip Voltage Controlled Oscillator (VCO) to the incoming serial data stream. The chip provides serialto-parallel and parallel-to-serial converters and 4-bit LVDS system interfaces in both receive and transmit directions. The transmit section includes a 4x9 Elastic Buffer (FIFO) to absorb any phase differences between the transmitter clock input and the internally generated transmitter reference clock. In the event of an overflow, an internal FIFO control circuit outputs an OVERFLOW indication. The FIFO under the