Intel Embedded Intel486 Hardware Reference Manual page 305

resistors, so the drivers can supply less current than needed in parallel termination. The resistor
value can be adjusted to bias the lines towards the V
the two are equal, providing balanced performance. The Thevenin's circuit provides good over-
shoot suppression and noise immunity.
Due to power dissipation, this technique is best suited for bipolar and mix MOS devices and is
not suitable for pure CMOS implementations. The reasons for not having Thevenin's equivalent
for the pure CMOS system design are as follows:
CMOS circuits have very high impedance to both ground and V
is 50% of the supply voltage. Besides dissipating more power, multiple input crossing may occur
creating output oscillations.
The main problem is high power dissipation through the termination resistors in relationship to
the total power consumption of all of the CMOS devices on the board. Most designers prefer se-
ries terminations for CMOS to CMOS connections, because as this does not introduce any addi-
tional impedance from the signal to the ground. The main advantage of the series termination
technique, apart from its reduced power consumption, is its flexibility. The received signal am-
plitude can be adjusted to match the switching threshold of the receiver simply by changing the
value of the terminating resistor. This is a very useful technique for interconnecting the logic de-
vices with long lines.
AC Termination
AC termination is another technique which can be used for designs which cannot tolerate high
power dissipation of parallel termination and delays created by series termination. It consists of
a resistor and a capacitor connected in series from the line to the ground. It is similar to the par-
allel termination technique in functionality except that the capacitor blocks the DC component of
the signal and thus reduces the power dissipation. This technique is shown in
Driver
The main disadvantage of this technique is that it requires two components. Further the optimum
value of the RC time constant of the termination network is not easy to calculate. It usually begins
as a resistive value which is slightly larger than the characteristic line impedance. It is critical to
PHYSICAL DESIGN AND SYSTEM DEBUGGING
CC
R
C
Figure 10-16. AC Termination
or V . Ordinarily it is adjusted such that
SS
and their switching threshold
CC
Receiver
Figure 10-16.
10-21