Sizing the Design - Selecting the Array
Last Edit July 22, 2001
Dual-Function I/O Macros
Each added power and ground macro uses a pad and disables the cell that
is associ ated with that pad, reducing the number of these cells and pads
available for I/O operations. To offset this waste, many macro libraries
include dual-function macros that use the I/O cell for one function and
the pad for added ground.
Silicon efficiency can be achieved with the dual function macros. The
macros avail able are array series-specific and vary widely. If any of
these functions applies to the design, they can reduce silicon requirements
while maintaining functionality. (See Figure 3-7.)
Example macros include:
- input function with 3-state enable driver
- 3-state enable driver with added ground bidirectional input with
added ground
Figure 3-7 Example Dual-Function I/O Macro
Example - Simultaneously Switching Outputs
All AMCC arrays, with the exception of the Q20000 Bipolar Series and
the BiCMOS Q24008 array, use the following rules for adding power and
ground due to simul taneously switching outputs (SSO), called an output
group.
Allow 8 TTL SSO outputs per quadrant, then add one TTLPWR and one TTLGND
macro for each group of 1-8 after the first eight. This requires two cells,
two pads and, depending on the package, two package pins. Add another
pair for the next group of 1-8 and another for the next group of 1-8 and
so on. All TTL output counts are converted to "equivalent" 8
mA outputs. (See Table 3-16.)
For packages with internal power and ground planes, place the TTLPWR
and TTLGND macros so that they are interspersed with the simultaneously
switching outputs and can be bonded to the power or ground package plane.
Table 3-16 Sample Rules for Adding TTL Power and Ground
PER TTL SSO |
ADD TTLPWR, TTLGND PAIRS: |
0-8 |
do nothing |
9-16 |
add 1 pair |
7-24 |
add 2 pairs |
Etc. |
|
Allow 8 ECL SSO outputs per quadrant, then add one ECLVCC macro for each
group of 1-8 after the first eight. This requires one cell, one pad and,
depending on the package, one package pin. Add another pair for the next
group of 1-8 and another pair for the next group and so on.
For packages with internal power and ground planes, place the ECLVCC
macro so that it is interspersed with the simultaneously switching outputs
and can be bonded to the power or ground package plane as required. Note
that ECLVCC is a power pad in a +5V reference ECL circuit (5V REF ECL)
and a ground pad in a standard reference ECL circuit. (See Table 3-17.)
Table 3-17 Sample Rules for Adding ECL Power OR Ground
PER ECL SSO |
ADD ECLVCC |
Q20000 Rules |
0-4 |
do nothing |
do nothing |
4-8 |
do nothing |
add 1 |
9-12 |
add 1 |
add 2 |
13-16 |
add 1 |
add 3 |
17-21 |
add 2 |
add 4 |
21-24 |
add 2 |
add 5 |
Etc. |
Etc. |
|
The Q20000 Series requires one ECLVCC per additional 1-4 ECL SSO after
the first group of four. All output counts are converted to "equivalent"
50 ohm outputs. The extremely high speeds of these arrays require design
procedures to ensure minimal noise.
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