Rambus™ XDR™Clock Generator

The 9214 clock generator provides the necessary clock signals to support the Rambus XDRTM memory subsystem and Redwood logic interface. The clock source is a reference clock that may or may not be modulated for spread spectrum. The 9214 provides 4 differential clock pairs in a space saving 28-pin TSSOP package and provides an off-the-shelf high-performance interface solution. Figure 1 shows the major components of the 9214 XDR Clock Generator. These include the a PLL, a Bypass Multiplexer and four differential output buffers. The outputs can be disabled by a logic low on the OE pin. An output is enabled by the combination of the OE pin being high, and 1 in its SMBus Output control register bit. The PLL receives a reference clock, CLKINT/C and outputs a clock signal at a frequency equal to the input frequency times a multiplier. Table 2 shows the multipliers selectable via the SMBus interface. This clock signal is then fed to the differential output buffers to drive the enabled clocks. Disabled outputs are set to Hi-Z. The Bypass mode routes the input clock, CLKINT/C, directly to the differential output buffers, bypassing the PLL. Up to four 9214 devices can be cascaded on the same SMBus. Table 3 shows the SMBus addressing and control for the four devices.