Logic Design for Array-Based Circuits
Copyright © 1996, 2001, 2002 Donnamaie E. White
Last Edit July 22, 2001
Design To Fit The Package
Also at issue at this point in the design stage is the desired package. When the package is selected, the package methodology for handling added power and grounds can be determined. Packages can be one-on-one, each added power and ground pad reaches an external package pin, or they may have internal power and ground planes.
Not all array pads will reach an internal power or ground plane therefore there are placement restrictions on the locations of the added power and ground macros. If these restrictions cannot be met due to other placement requirements or if the package does not have enough pads that can bond to internal planes, then the added power or ground macros will require external package pins. (Example, some packages offer ground planes and no power planes.)
Case 1 - Count of all array pads less than or equal to the number of total package pins
There are two approaches to checking the package against the design. The first is when there are no internal power or ground planes. In this case, count the number of inputs, outputs, bidirectionals, added power, added ground, fixed power, fixed ground, and any fixed I/O signals (such as on-chip thermal diodes and AC speed monitors). This number should be less than or equal to the total number of package pins.
Case 2 - Count of all signals less than or equal to the number of package signal pins
The second case involves making an estimate which can be refined after placement is completed and approved. In this case, count the number of array pads used by inputs, outputs, bidirectionals, added power, added ground and any fixed I/O signals such as on-chip thermal diodes or AC speed monitors. This number should be less than or equal to the total number of package signal pins. The package power and ground pins connect to the internal power and ground planes.
After placement, the number of signals will be reduced by the number of added power and ground macros that were placed to connect to internal package power and ground planes. Those macros will not use the external package signal pins.
A designer submitted a design with a desired package (a 149 PGA with internal power and ground planes). The package has 120 signal pins. He used 132 array pads for I/O signals and added power and grounds. There were no fixed thermal diode or AC speed monitor signals on the array. There were eight added power and grounds. The design, after careful placement of the added power and grounds, had four signals more than there were package signal pins.
This problem was not discovered until placement, i.e., until after all simulations and design validations were performed.
One solution is to look for expendable I/O cell usage. If there are extra grounds beyond the minimum, or more VBB or other voltage sources than is really required, they can be reduced.
Another solution is to add an 8:1 MUX, place eight non-critical outputs as inputs to the MUX, add three input signals to control the select lines and one output for the MUX. This reduced the total number of signal pins required to 120, which would fit the package.
What if neither of these solutions is acceptable? Then some other design change is in order if the package cannot be changed. A design change requires that all simulations and all checking be repeated.
Could this situation have been prevented? By checking the package limits during the optimization phase and using the package limits as a guide, the design changes or package changes could have been identified earlier, saving the iteration of the simulation loop. Remember that simulation is estimated to use approximately 50% of the CPU time used in a design process.
Figure 4-4 Optimization - Packaging Issues