Clocking
8.2
PLL Clocks
The PLL receives a reference clock at 1/2X, where X is the DDR command frequency.
The PLL generates the internal clocks shown below in Table 8-1.
Table 8-1.
PLL Clocks
Clock
Notes
DDRCA1X
This clock is unused in the AMB. It is still used for setting the alignment of the DDRCA2X clock
within the PLL.
DDRCA2X
DDR1X
Rising edge aligned to DDRCA1X. Also allows command/address bus to run at DDR data rate
in LAI mode.
Along with DDR2X and DDR2X#, used by DDRI/O cluster to generate DDR command clock,
DDR DQS output, and DDR DQ output. This is also used as core clock.
DDR2X
Rising edge aligned to DDR1X
Inverted DDR2X
DDR2X#
DDRRD1X
Used by DDRI/O cluster to advance the DDR Read FIFO read-pointer. It is also by Northbound
logic. This is driven by an independent divider and can be moved w.r.t to the core clock in 2UI
increments to align it with the DDR data availability.
HVMCLK
This is a fixed clock at the same frequency as the core clock but is driven by a independent
divider.
FBDCK (4)
Four phases of 6X clock. Provided to FBD Northbound, Southbound high-speed I/O clusters.
These signals are connected by abutment.
There are additional PLL modes used for testing and bring up. Details are in Section
8.9, “Additional Clock Modes” on page 87.
8.3
Reference Clock
A low-jitter differential reference clock (REFCLK) is routed to the host and each DIMM
from a common clock source on the system board. This reference clock uses HCSL
(High-Speed Current Steering Logic) signaling and its detailed requirements are
documented in the FB-DIMM Draft Specification: High Speed Differential P2P Link at
1.5V. The AMB uses the reference clock to generate internal buffer clocks and to
generate the clocks to the DRAMs located on each DIMM. The frequency of the
reference clocks (133 to 200 Mhz) is one half the frequency of the DRAM base clock
(267 to 400 Mhz), that is, it is one half the command-rate of the DRAM devices located
behind the AMB. For example, for DDR2 667 DRAM devices the reference clock
frequency would be 167 MHz. The reference clock is the basis for the various Core, FBD
and DDR internal clocks.
It is a requirement for the FBD channel to operate in the presence of Spread Spectrum
Clocking (SSC), which is commonly used to reduce EMI. The reference clocks for FBD
have to meet a jitter specification.
The reference clocks to the host and each DIMM are mesochronous, that is, they have
an unknown but fixed phase relationship to each other or the memory channel. This
simplifies PCB routing since no precise length matching is required. However an upper
bound for the clock length mismatch is necessary since the maximum phase difference
between the data sent out with the transmitter clock and the receiver clock needs to be
limited in the presence of SSC. It is required that all the reference clocks for a given
FBD channel originate from a single clock source, for example, a common clock
synthesizer or clock oscillator, and travel through the same jitter spectrum modifying
components (for example, PLL clock buffer) thereby ensuring that there is no frequency
mismatch or frequency drift between FBD agents.
Intel® 6400/6402 Advanced Memory Buffer Datasheet
85
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