Digital Signal Processing Reference
In-Depth Information
high-impedance state). In some power-sensitive applications, all of the drivers are
turned off when the bus is not actively driving data. As shown in the Problems, this
greatly reduces the power dissipated by the termination compared to the situation
where the bus is statically held in a state (often low) for extended periods. How-
ever, with all the transmitters turned off, resistors
R
1 and
R
2 force the bus voltage
to become equal to
V
term
. If this is close to
V
tt
, the result may be receiver chattering
and the creation of power supply or RF noise, along with an increase in receiver
power dissipation. These effects are especially pronounced on bidirectional buses
where each node on the bus has a three-state driver and receiver. In this situation
the termination voltage should be selected that is guaranteed to exceed the receiver
switch point under all environmental and worst-case tolerance conditions.
12.3.2 Thevenin Termination
From circuit theory the two resistors and the
V
dd
power supply in Figure 12.3 can
be replaced with a Thevenin equivalent resistance in series with a Thevenin voltage
supply [shown in Figure 12.3(b) as
R
term
and
V
term
]. This Thevenin termination is
often used to terminate signals driven to memory devices, such as double data rate
(DDR) memories.
For instance, a 60
Ω
Thevenin termination centered at 1.25V is formed with a
single 60
resistor (
R
term
) connected to a 1.25-V power supply (
V
term
). This ap-
proach uses half the number of resistors and generally allows the single resistor
to be placed closer to the point of termination than is possible with two resistors.
When done properly, this improves signal quality and, when many lines are ter-
minated, can be lower in cost. However, the creation of
V
term
must be done with
care, and in small systems it may cost more to properly create
V
term
than is saved
by eliminating half of the termination resistors.
It is generally inappropriate to use a simple three-terminal voltage regulator for
the
V
term
supply. Those regulators usually only source current, but the termination
requires that it both source and sinks current. Instead, regulators especially de-
signed for termination applications should be used, and the manufacturer's guide-
lines concerning placement and value of decoupling capacitors must be closely
followed.
High amounts of rapidly changing current flow when many signals are termi-
nated, and to keep the termination voltage noise-free, the circuit board layout must
provide a low-inductance path from the terminator resistor to
V
term
.
Narrow traces are too inductive to connect the regulator to the terminators.
For this reason in large, high-performance systems, a voltage plane (or a portion
of a plane) is sometimes dedicated to supplying
V
term
, much as a plane is used to
supply
V
dd
or ground. Besides providing a low-inductance connection from
R
term
to
V
term
, a plane frees up routing channels that would otherwise be consumed by
wide traces feeding
V
term
to various locations.
As explored in the Problems, Thevenin termination can dissipate less system
power than parallel termination, especially when a three-state bus is intentionally
placed in the high-impedance state to save power.
Ω
