1240pin Fully Buffered DDR2 SDRAM DIMMs
4.3 SMBus Slave Interface
The Advanced Memory Buffer supports an SMBus interface to allow system access to configuration regis-
ters independent of the FB-DIMM link. The Advanced Memory Buffer will never be a master on the SMBus,
only a slave. Serial SMBus data transfer is supported at 100 kHz. SMBus access to the Advanced Memory
Buffer may be a requirement to boot and to set link strength, frequency and other parameters needed to
insure robust configurations. It is also required for diagnostic support when the link is down. The SMBus
address straps located on the DIMM connector are used by the unique ID.
4.4 FBD Channel Latency
FB-DIMM channel latency is measured from the time a read request is driven on the FB-DIMM channel pins
to the time when the first 16 bytes (2nd chunk) of read completion data is sampled by the memory con-
troller. When not using the Variable Read Latency capability, the latency for a specific DIMM on a channel
is always equal to the latency for any other DIMM on that channel. However, the latency for each DIMM in
a specific configuration with some number of DIMMs installed. As more DIMMs are added to the channel,
additional latency is required to read from each DIMM on the channel. Because the channel is based on
the point to point interconnection of buffer components between DIMMs, memory requests are required to
travel through N-1 buffers before reaching the Nth buffer. The result is that a 4 DIMM channel configura-
tion will have greater idle read latency compared to a 1DIMM channel configuration.The Variable Read
Latency capability can be used to reduce latency for DIMMs closer to the host. The idle latencies listed in
this section are representative of what might be achieved in typical AMB designs. Actual implementations
with latencies less than the values listed will have higher application performance and vice versa.
4.5 Peak Theoretical Throughput
An FB-DIMM channel transfers read completion data on the FBD Northbound data connection. 144 bits of data are
transferred for every FBD Northbound data frame. This matches the 18-byte data transfer of an ECC DDR DRAM in a
single DRAM command clock. A DRAM burst of 8 from a single channel or a DRAM burst of four from two lock stepped
channels provides a total of 72 bytes of data(64 bytes plus 8 bytes ECC)
The FBD frame rate matches the DRAM command clock because of the fixed 6:1 ratio of the FBD channel clock to the
DRAM command clock. Therefore, the Northbound data connection will exhibit the same peak theoretical throughput
as a single DRAM channel. For example, when using DDR2 533 DRAMs, the peak theoretical throughput as a single
DRAM channel.For example, when using DDR2 533 DRAMs, the peak theoretical bandwidth of the Northbound data
connection is 4.276 GB/sec.
Write data is transferred on the FBD Southbound command and data connection, via Command+Wdata frames. 72
bits of data are transferred for every FBD Command+Wdata frame. Two Command+Wdata frames match the 18-byte
data transfer of and ECC DDR DRAM in a single DRAM command clock. A DRAM burst of 8 transfers from a single
channel, or a burst of 4 from two lock-step channels provides a total of 72 bytes of data(64 bytes plus & bytes ECC)
When the FBD frame rate matches the DRAM command clock, the Southbound command and data connection will
exhibit one half the peak theoretical throughput of a single DRAM channel. For example, when using DDR2 533
DRAMs, the peak theoretical bandwidth of the Southbound command and data connection is 2.133 GB/sec.
The total peak theoretical throughput for a single FBD channel is defined as the sum of the peak theoretical through-
put of the Northbound data connection and the Southbound command and data connection. When the FBD frame rate
matches the DRAM command clock, this is equal to 1.5 times the peak theoretical throughput of a single DRAM chan-
nel. For example, when using DDR2 533 DRAMs, the peak theoretical throughput of a DDR2 533 channel would be
4.267 GB/sec, while the peak theoretical throughput of and FBD -/+533 channel would be 6.4 GB/sec.
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