Wednesday, 30 April 2014

Tie Cells Insertion

Tie Cells Insertion

Here I am going to discuss about Tie Cells Insertion. Before going to know about Tie Cells Insertion, We have to know what Tie Cells are. 

Tie Cells:
Tie cells are special purpose standard cells whose output is Constant High or Constant Low. These cells are used to hold (tie) the input of other cells which are required to be connected Constant High (Vdd) or Constant Low (Vss) values.

Tie High Cell:
Tie High Cell is special purpose standard cell whose output is Constant High (Vdd).

Tie Low Cell:
Tie Low Cell is special purpose standard cell whose output is Constant Low (Vss).

There are some unused inputs in the design netlist. These unused inputs should not be floated. They should be tied to either Power (Vdd) or Ground (Vss). The inputs which are required to connect Vdd, connect to Tie High cells. The inputs which are required to connect Vss, connect to Tie Low cells. This is the purpose of Tie cells in the design

Why Tie cells are inserted?
In lower technology nodes the gate oxide of the transistor is so thin and sensitive to voltage fluctuations in the power supply. If the gate of the transistor is directly connected to the Power/Ground network (Power Grid Network), the gate oxide of the transistor might be damaged due to voltage fluctuations in the power supply. To overcome this disadvantage, Tie cells are inserted.

To perform Placement Optimization or Physical optimization, Automatic Insertion and Optimization of Tie offs are required in the design. The following commands are used to execute the same.

set_auto_disable_drc_nets -constant false
set physopt_new_fix_constants true
set_attribute [...] max_fanout 12
set_attribute [...] max_capacitance 0.2 -type float

Tie-off Optimization:
Tie cells optimization means using a tie cell to hold (tie) as many inputs as possible at given logic level, while meeting specified maximum Fanout and maximum capacitance constraints (Logical DRCs). 

The set_auto_disable_drc_nets command enables DRC on constant nets.

The set physopt_new_fix_constants variable to true causes placement tool to observe the maximum capacitance constraint during tie-off optimization.

The maximum capacitance constraint is determined by the max_capacitance attribute, which can be set with the set_max_capacitance or set_attribute command. The set_attribute command can be used to specify explicitly both the maximum fanout and maximum capacitance constraints for objects in the design.

Note: We can also insert tie cells manually with the command connect_tie_cells. The command inserts tie cells and connects them to specified cell ports, while meeting maximum fanout and maximum wire length specified in the command.


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