The ACS8526 is a highly integrated single-chip solution for protection switching between two SECs (SDH/SONET Equipment Clocks) from Master and Slave SETS clock cards, for line cards in a PDH, SONET or SDH Network Element. The ACS8526 has fast activity monitors on the inputs and will raise a flag on a pin if there is a loss of activity on the currently selected input. The protection switching between the input reference clock sources is controlled by an external pin. The ACS8526 has two SEC reference clock input ports, configured for expected frequency by setting hardware pins or by writing to registers via the serial interface. The ACS8526 can perform frequency translation, converting, for example, an 8 kHz SEC input clock from a backplane into a 155.52 MHz clock for local line cards. The ACS8526 generates two independent SEC clock outputs, one on a PECL/LVDS port and one on a TTL/CMOS port, at spot frequencies configured by hardware pins, or by writing to registers via the serial interface. The hardware selectable spot frequencies range from 1.544 MHz up to 155.52 MHz, with further options for N x E1/DS1 and 311.04 MHz via register selection. The ACS8526 also provides an 8 kHz Frame Sync output and 2 kHz Multi-Frame Sync output, both with programmable pulse width and polarity. Advanced configuration possibilities are available via the serial port (which can be SPI compatible), however the basic configuration of I/O frequencies and SONET/SDH selection by hardware make the device suitable for standalone operation, i.e., no need for a microprocessor.