The Direct Rambus clock generator (DRCG) provides the necessary clock signals to support a Direct Rambus
memory subsystem. It includes signals to synchronize the Direct Rambus channel clock to an external system or
processor clock. It is designed to support Direct Rambus memory on a desktop, workstation, server, and mobile
PC motherboards. DRCG also provides an off-the-shelf solution for a broad range of Direct Rambus memory
The DRCG provides clock multiplication and phase alignment for a Direct Rambus memory subsystem to enable
synchronous communication between the Rambus channel and ASIC clock domains. In a Direct Rambus
memory subsystem, a system clock source provides the REFCLK and PCLK clock references to the DRCG and
memory controller, respectively. The DRCG multiplies REFCLK and drives a high-speed BUSCLK to RDRAMs
and the memory controller. Gear ratio logic in the memory controller divides the PCLK and BUSCLK frequencies
by ratios M and N such that PCLKM = SYNCLKN, where SYNCLK = BUSCLK/4. The DRCG detects the phase
difference between PCLKM and SYNCLKN and adjusts the phase of BUSCLK such that the skew between
PCLKM and SYNCLKN is minimized. This allows data to be transferred across the SYNCLK/PCLK boundary
without incurring additional latency.
User control is provided by multiply and mode selection terminals. The multiply terminals provide selection of one
of four clock frequency multiply ratios, generating BUSCLK frequencies ranging from 267 MHz to 533 MHz with
clock references ranging from 33 MHz to 100 MHz. The mode select terminals can be used to select a bypass
mode where the frequency multiplied reference clock is directly output to the Rambus channel for systems where
synchronization between the Rambus clock and a system clock is not required. Test modes are provided to
bypass the PLL and output REFCLK on the Rambus channel and to place the outputs in a high-impedance state
for board testing.
The CDCFR83A has a fail-safe power up initialization state-machine which supports proper operation under all
power up conditions.
The CDCFR83A is characterized for operation over free-air temperatures of 40°C to 85°C.
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