Sizing the Design - Selecting the Array
Last Edit July 22, 2001
Refining Interface Requirements
When the interface types and their power supply requirements are documented
and one or more arrays chosen as candidates for the final selection, the
interface require ments must be refined. There are several conditions
under which additional power or ground pads will need to be added to an
array beyond the fixed power and ground pads provided. These include:
- simultaneously switching outputs,
- package restrictions,
- high-speed signal isolation and
- ECL - TTL isolation.
Simultaneous switching TTL or ECL outputs is a potential source of system
noise, which can be reduced by the addition of TTL VCC - TTL Ground pairs
and/or ECL VCC.
Some arrays require that drivers be placed next to ground. Others require
that a ground exist between simultaneously switching TTL outputs and ECL
inputs, or between any TTL output and an ECL input. Isolation of CMOS
inputs from the faster switching TTL and ECL signals may also be required.
When a fixed ground is not available, then one must be added. The design
rules for any array series are called out in the Design Manual
for the array.
Variable Requirements for Power and Ground
Bipolar arrays require that all fixed power and ground be used or bonded
out to the package. Additional power and grounds are based on simultaneously
switching out puts or isolation requirements.
CMOS arrays have some or all of their fixed power and ground pads under
user -placement control. The vendor provided a list of how many would
need to be used depending on the signals used by the design. This type
of flexibility is detrimental to standard packaging; it is time consuming
In spite of the drawbacks, recent BiCMOS designs have returned to this
approach, providing the minimal number of power and grounds and allowing
other fixed-position power and grounds to go unbonded (unconnected). The
criteria for requiring that these fixed positions be used or that additional
power and grounds be added is based on the number and types of interface
When the power busses supporting the internal core are isolated from
the busses supporting the peripheral I, O or I/O cells, noise feedback
due to output switching is minimized. The threshold and reference voltage
generators for the logic array inter nal cells and I, O and I/O cells
should also be independent to insure steady operation.
Adding Extra Power and Ground pads
Adding a power pad or a ground pad to an array can be accomplished by
placing a power or ground macro on the desired pad (array-specific procedure.
AMCC arrays use the ITPWR (+5V), ITGND (0V) and IEVCC (ECL VCC) macros
to add power or ground. (See Figure 3-6.) For standard refer-ence ECL,
IEVCC represents a ground pad. For +5V REF ECL, IEVCC represents a power
Figure 3-6 Added power and Ground macros (AMCC)