
EXTERNAL DESIGN INTERFACE CONSIDERATIONS
INTRODUCTION TO STD80/STDM80
STD80/STDM80
1-24
SEC ASIC
Usually there are three types of input receivers in
ASIC libraries; TTL input buffer, CMOS input buffer,
and various Schmitt trigger input buffers.
TTL input buffer has relatively poor noise
characteristics because of its shifted logic threshold
voltage. CMOS input buffer is better than TTL against
a noise because the logic threshold voltage is near
2.5volt. If an input signal has relatively large noise
spikes, it could cause an unwanted input signal.
When an input signal is very noisy, the noise can be
filtered by using a Schmitt trigger input buffer. As
shown in Figure 1-22. Effect of Schmitt Trigger Input
Buffer, Schmitt trigger input buffers have two different
input thresholds for positive- and negative-going
signals. This hysteresis between positive- and
negative-going voltage signals can filter a noisy signal
to a wanted one.
According to applications, the most suitable one can
be chosen among the various Schmitt trigger input
buffers having different levels of threshold voltage.
Output Pad Cell
As incoming signals to a chip have a noise, the noise
can also be induced by the operation of the chip itself.
There are several sources of a noise, but the greatest
singular source of a noise is the switching of an output
with high capacitive load.
Figure 1-23.
Simple Model of Output Pad Cell
Figure 1-23. Simple Model of Output Pad Cell shows
the simple model of an output driver considering the
external interface. L1 and L2 are parasitic inductances
of the package and C
L
is an output load. Vout will fall
as Vin rises and the current i flows through n-transistor
discharging the loaded charge (V
DD
×
C
L
).
The details of this operations are described in Figure
1-24. Ground Bounce Phenomenon.
The important phenomenon which can be observed in
this figure is that the voltage level Vn shifts relative to
the system ground. Vn is the ground of the chip.
This phenomenon is called as a “ground bounce” that
is the chip reference shift caused by the external
inductance and the transient current flow to the
ground.
The amount of voltage level shifted by the ground
bounce is
Vn = -L
×
(di / dt)
When the output driver makes a low-to-high transition,
the similar noise problem is generated on the power.
Figure 1-24.
Ground Bounce Phenomenon
i
C
L
:
Bonding Pad
L2
L1
Vin
System Ground
System Power Supply
Vout
R
L
(a) Vin
(b) Vout
(c) i
(d) Vn
t
t
t
t
i = – C
L
×
(dVout / dt)
Vn = L1
×
(di / dt)