Understanding Timing Models in Cell Design Environments
Delve into the intricacies of timing models for cells in design environments, exploring factors affecting delay, output transitions, linear and non-linear models, and the use of table models in cell libraries to specify delays and timing checks. Discover how input transitions and output capacitance impact delay in inverter cells and how rising and falling transitions affect output delays.
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Submitted by HARSHITHA G H
Provide accurate timing for various instances of the cell in design environment. The timing models are specified for each timing arc of the cell. Define the delay arcs between nodes
Since it is an inverter, a rising (falling) transition at the input causes a falling (rising) transition at the output. The two kinds of delay characterized for the cell are: Tr : Output rise delay Tf : Output fall delay
Delay for timing arc through inverter cell is dependent on two factors: the output load, that is, the capacitance load at the output pin of the inverter the transition time of the signal at the input. Delay increases with load capacitance, input transition time. 1. 2.
Two types of Timing models Linear Timing Models Non-Linear Delay model
The delay and output transition time of cell are the linear functions of the two parameters. Input transition time and Output load capacitance. Example D=D0+D1*S+D2*C Where D is delay of Linear model D0,D1,D2 Constants S Input transition time C Output load capacitance
cell libraries include table models to specify the delays and timing checks for various timing arcs of the cell. The table models are referred to as NLDM (Non-Linear Delay Model) and are used for delay, output slew, or other timing checks.
In this example Delay of the output pin OUT are described. Rising and falling delay models for the timing arc from pin INP1 to pin OUT and max_transition at pin OUT. Separate models for the rise and labeled as cell_rise and cell_fall respectively. Can also be used for transition time output of a cell. fall delays, at the
In general, the timing arcs can be from each input to each output of the block logic path from input to output is non-inverting(positive unate), then the output has the same polarity as the input If it is an inverting logic path or negative unate, the output has an opposite polarity to input General combinational block
For synchronous inputs timing arcs: Setup check arc (rising and falling) Hold check arc (rising and falling) For asynchronous inputs timing arcs Recovery check arc Removal check arc
