
EXTERNAL DESIGN INTERFACE CONSIDERATIONS
INTRODUCTION TO STD80/STDM80
STD80/STDM80
1-28
SEC ASIC
For example, suppose C
L
= 100pF, V
DD
= 3.3Volt, t
F
=
5ns. From Figure 1-24. Ground Bounce Phenomenon,
the maximum current flow occurs at time 0.5
×
t
F
. Then
approximately,
i = C
L
×
(dv / dt)
C
L
×
(
V /
t),
and
i (max) = 100
×
10
-12
×
{5 / (2.5
×
10
-9
)} = 200
[mA].
If the number of SSOs is 5, and L is 4nH,
Vn = L
×
(di / dt)
×
N
L
×
(
i /
t)
×
N by
approximation,
Vn (max) = 4
×
10
-9
×
{0.200 / (2.5
×
10
-9
)}
×
5 =
1.60 [Volt].
From this calculation, 1.60V of noise spike is
expected. This is about logic threshold voltage of TTL.
This numerical estimate clearly shows that power bus
noise control is one of the fundamental problems in a
high-speed CMOS VLSI design. It is an important
design consideration to prevent the noise from
affecting the integrity of the logic operation of a chip.
Figure 1-28.
Noise Spike Induced by Ground
Bounce
How to Protect Ground Bounce
The fundamental solution to the ground bounce
problem is to reduce the inductance of the package.
However, in the boundary of a given packaging
technology, the following guidelines can be used for
reducing ground bounce:
(1) If possible, use separate power and ground buses
for input buffers and output drivers.
(2) The number of ground and power pads should not
be less than the required number of pads.
(3) If the design is not so much sensitive to speed,
use slew rate control, i.e., increase switching time,
to reduce the value of di / dt of an output driver.
SEC supports two levels of slew rate controlled
output buffers, SM and SH. You can see this effect
in the following figure.
Figure 1-29.
Effect on Reducing Peak Current
with Slew-Rate Control
(4) If you cannot use a slew rate cell because of the
speed requirement, you can stagger the output
driver as shown in Figure 1-30. Effect on Reducing
Peak Current with Staggering Output Drivers. This
is not a general-purpose solution. It makes sense
only when special relief in timing requirements
exists from a system architecture.
Noise Spike
L
i
t
t
t
t
3 SSOs
3 SSOs
V
i
V
i