
VDD/VSS RULES AND GUIDELINES
INTRODUCTION TO STD80/STDM80
STD80/STDM80
1-6
SEC ASIC
V
DD
/V
SS
RULES AND GUIDELINES
There are three types of V
DD
and V
SS
in
STD80/STDM80, each with its related bus and pad
cells. To support the use of mixed voltage, two different
V
DD
types are needed for 5V and 3.3V respectively.
(1) Core logic
– VSSI, VDD5I (for 5V)
(2) Input buffers (usable when requested)
– VSSP, VDD5P (for 5V), VDD3P (for 3.3V)
(3) Output buffers
– VSSO, VDD5O (for 5V), VDD3O (for 3.3V)
The number of V
DD
and V
SS
pads required for a
specific design depends on the following factors:
Number of input and output buffers
Number of simultaneous switching inputs
Number of simultaneous switching outputs
Number of used gates and simultaneous switching
gates
Operating frequency of the design.
Core Logic V
SS
Bus and VSSI Pad
Allocation Guidelines
The purpose of these guidelines is to ensure that
V
DD
/V
SS
bounce caused by a simultaneous gate
switching is kept to minimum. The voltage bounce on
the power bus can have a negative impact on a
gate-switching speed and even on the functionality of
macrocells like flip-flops and latches in an extreme
case.
Because of variations in package inductance, the
number of V
DD
/V
SS
pads required for a specific design
is the function of the operating frequency of a chip, i.e.,
designs operating at high frequency should use more
V
DD
/V
SS
pads.
V
DD
bus width and pad requirements are half of
V
SS
.
V
DD
/V
SS
buses and pads should be distributed
evenly in the core and on all sides of the chip.
Whenever possible, at least one VSSI pad should
be used on each side of the chip.
The total number of core logic V
DD
pads required
is half of VSSI.
The number of VSSI pads required for a design can be
calculated from the following expression:
G x S x F x 2.00e–5
,where
G = Total number of used gates,
S = % of simultaneous switching gates,
F = Switching frequency in MHz.
Input Buffer V
DD
/V
SS
Pad Allocation
Guidelines
These guidelines ensure that an adequate input
threshold voltage margin is maintained during a
switching.
One VSSP is required to support 32 input buffers,
and one input buffer V
DD
can support up to 64
inputs.
For simultaneous switching inputs, one VSSP pad
is required for every 20 inputs, and one input
buffer V
DD
pad for every 40 inputs.
Input buffer V
SS
/V
DD
pads should be placed in
such a way that they equally divide the input
buffers on either side.
Output Buffer V
DD
/V
SS
Pad Allocation
Guidelines
The number of VSSO pads required for a device can
be calculated from the following expressions.
In 5V
∑
(I
OL Simultaneous switching outputs
) / 40 +
∑
(I
OL Normal outputs
) / 64
In 3.3V
∑
(I
OL Simultaneous switching outputs
) / 50 +
∑
(I
OL Normal outputs
) / 80
The total number of output buffer V
DD
pads
required is half of VSSO.
Output buffer V
SS
/V
DD
pads should be placed in
such a way that output buffers are equally divided
on either side.