IEEE Solid-State Circuits Magazine - Summer 2016 - 39

The early days at Rambus gave Mike and Mark
an opportunity to innovate on the structure of
the company culture as well.

delay. The original proposal (shown
in Figure 2) envisioned sending a
clock past all devices and then back
in a U-shaped trajectory. The average
of the two clocks' arrival time t1+t2
for each device is identical; circuitry
on board would produce a local clock
that was synchronous everywhere. As
Mark and the Rambus team worked
on this design, they quickly realized
that 1) it was difficult to implement
and 2) not really what is needed since
the topology really needed to cancel
out signal propagation delay. After
much design and experimentation,

systems that Rambus shipped used a
modified form of source synchronous
clocking, as shown in Figure 3, where
a single clock source was looped at
the master end of the bus. Thus each
DRAM would see the clock twice, as
two source-synchronous clocks, a
clock "to master" (CTM) and clock
"from master" (CFM). Each DRAM
sent or received data relative to the
clock propagating in the appropriate direction.
This new high-speed signaling
presented both opportunities and
challenges: while the system could

VTERM
RTERM

Data
Ctrl

CPU

t1

CLK

DRAM

DRAM

DRAM

t2
DRAM

Figure 2: The original topology with fully synchronous clocking architecture.

VTERM
RTERM

Data
Ctrl
CFM
CTM

CPU

DRAM

DRAM

DRAM

CLK

DRAM

to avoid any reflection from the
transmitter or receiver. Any reflections could corrupt the next highspeed bit and require the system
to slow down for the reflections to
settle. A compact layout and limited amount of connectivity on the
CPU side made it desirable to build a
"multidrop" configuration where the
same signals would pass each DRAM.
Controlled impedance, termination,
and a multidrop bus drove the decision to use high-impedance current
mode drivers, both at the CPU end
and at each memory. This solved one
problem neatly; as a signal made its
way from the CPU past each DRAM,
the DRAM's driver (high-Z when not
driving) would not present a significant impedance change, and thus
reflections were minimized with
most of the signaling energy continuing down toward the terminator. New
terminology was created; the CPU
side would become the "master" of
the bus.
The original system (shown in
FIgure 2) envisioned DRAMs with
current mode drivers and termination at the far end of the bus. When
a DRAM drove the bus, the current
would split in two, dividing equally
and propagating in both directions
at half the final swing. As the wavefront reached the unterminated CPU
master end of the bus, it would hit
the high impedance as an "open,"
redoubling the signal and allowing
it to be received normally. Signals
from the CPU to the DRAMs had no
such split and traveled at full swing
until being received at the terminator. As all of the drivers were in
current mode, swing was controlled
by binary weighted NMOS devices
biased so that they would operate in
saturation. Properly setting V TERM so
devices would remain in saturation
when active was an important concern to minimize reflections.
Clocking was another challenge.
Mark knew he wanted to transmit and
receive on both edges of the clock
(double-data rate), but he considered
a variety of schemes to counteract the
clock skew introduced by propagation

Figure 3: The final architecture with source-synchronous clocking.

IEEE SOLID-STATE CIRCUITS MAGAZINE

su m m E r 2 0 16

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