PIN Assignment

• The purpose is to define the signal that each pin will receive
• It can be done during floorplanning,during placement,after placement is fixed
• For un-designed blocks, a good assignment of pins improves placement
• If the blocks are already designed, still some pins can be exchanged

Depending on the location of the block, we have to judiciously select where to put the pins, now this decision we have to take for the so called flexible blocks i.e Flexible blocks are the blocks which are not yet been designed their exact shapes and sizes are not fixed and also the exact location of the pins are not fixed.

INPUT:

• A placement of blocks
• Number of pins on each block, possibly an ordering
• A netlist

REQUIREMENT:

• To determine the pin locations on the block

OBJECTIVE:

• To minimize net-length

Functionally equivalent pins:Two pins which carry the same signal, exchanging the signals does not affect the circuit

Equipotential pins:Both signals are internally connected and represent the same net

The purpose of problem formulation is to optmimize the assignment of nets within a functionally equivalent(or equipotential) pin groups.Objective: To reduce congestion or reduce number of crossovers.

Classification of Pin Assignment Algorithms

Concentric Circle Mapping-Given two sets of pins

• Determine the circles:the blue pins we can rotate but red ones are fixed, so this mapping will draw two imaginary circles,one inside this group of six pins and other outside this group but inside external pins.

• Determine the points:From center we draw lines to these 6 pins(these lines will intersect at some point), similarly draw lines to external pins(they will also intersect at some point)

• Determine initial mapping:Try to rotate the pin a way such that interconnection become simpler, find optimum angle of rotation for which interconnection lengths is minimized

• Best mapping(complete rotation): Once we get the best rotation, connect them in straight lines.AFter that come back to original one, now we know from where pin locations are coming, so from there connect to external points which will be final pin positions

Topological Pin Assignment-Similar to concentric circle mapping.If a net has been assigned to more than two pins, then the pin closest to the center of primary component is chosen.

Nine Zone method-Based on zones in a cartesian co-ordinate system.The center of the co-ordinate system is located inside a group of interchangeable pins on a component called pin class.A net rectangle is defined by each of the nets connected to the pin class.There are 9 zones in which this rectangle can be positioned.

Channel Pin Assignment-It's a special method applicable to standard cell based design.A significant portion of the chip is used for channel routing.After placement phase, the position of terminals on the boundaries of a block are not fixed.They may be moved before routing begins. As per below example, we can mimimise the area that is required for interconnection