CDC9842

The CDC9842 is a high-performance clock synthesizer/driver that generates the system clocks necessary to support PentiumE/82430X/82430VX and PentiumPro 82440FX chipsets. Four host-clock outputs (HCLKn) are programmable to one of three frequencies (50 MHz, 60 MHz, or 66 MHz) via the SEL0 and SEL1 control inputs. Six PCI-clock outputs (PCLKn) are half the frequency of CPU clock outputs and are delayed 1 ns to 4 ns from the rising edge of the CPU clock. In addition, a universal serial bus (USB) clock output at 48 MHz (SBCLK) and three 14.318-MHz reference clock outputs (REF0, REF1, REF2) are provided.

All CDC9842 output frequencies are generated from a 14.318-MHZ crystal input. A reference clock can be provided at the X1 input instead of a crystal input.

Two phase-locked loops (PLLs) CDC9842 are used to generate the host clock frequency and the 48-MHz clock frequency. On-chip loop filters and internal feedback eliminate the need for external components. The PCI-clock frequency is derived directly from the host-clock frequency. The PLL circuit can be bypassed in the TEST mode (i.e., SEL0 = SEL1 = H) to distribute a test clock provided at the X1 input.

The host- and PCI-clock outputs CDC9842 provide low-skew/low-jitter clock signals for reliable clock operation. All outputs are 3 state and are enabled via OE.

Because the CDC9842 is based on PLL circuitry, it requires a stabilization time to achieve phase-lock of the PLL. This stabilization time is required following power up and application of a fixed-frequency, fixed-phase signal at the X1 input, as well as following any changes to the OE or SELn inputs.