nanoTAS

A tiny (hence nano) framework for characterizing stdcells with pyspice

Description

This experimental framework was created to support logic cell characterisation with PySpice. It is able to derive timing information for logic gate cells. It can compute delay and transition times for rising and falling transitions with different load capacitances and input slews.

The requirements are:

• PySpice (https://pyspice.fabrice-salvaire.fr/index.html)
• Transistor models in a format compatible with ngspice
• Spice netlists for the gate cells
• "driver" files which specify the gate ports, the test scenarios and other per-cell parameters
• A configuration file called "config.py"

A sample for the configuration file can be found in config.py.sample. Copy the file to "config.py" and edit it. For details see the comments inside this file.

For the driver files, samples are provided in "drivers-examples".

Basic usage

Check out this script framework somewhere and set the path to the directory containing "nanotas.py".

Make sure the "config.py" file is in the current directory.

Run "nanoTAS":

nanotas.py

Details about the implementation

The implementation is based on PySpice which is a Python library wrapping libngspice ("shared ngspice") and combining it with NumPy. In effect this is a very convenient framework for running mass simulations on synthetic spice netlists and combining them with automated analysis. For details about the PySpice API see https://pyspice.fabrice-salvaire.fr/overview.html.

nanoTAS implements a context class collecting the setup from the driver files. Technically the driver files are Python scripts executed in the context of this object. Upon "collect", PySpice is used to prepare a netlist for each inslew/capa load pair with the stimulus pulse, the DC power source, dummy loads for ngspice convergence and the capacitance loads.

After this, the simulation is started using the time interval and time resolution provided by the config file.

The collected waveforms are analysed to get the measurements:

• Input pin capacitances are computed by integrating over the input pin current (charge Q) and using C = Q/U.
• The energy per transition is computed by integrating the power supply current (Q) and using the energy equation: E = Q*U.
• The rise and fall times are computed as the time difference between the 10% and 90% levels.
• The delay times are measured between the 50% transitions of input and output.

TODO

• Simplify the production of driver files
• Support for hold/setup time extraction for latches and FF's
• Generation of .lib files
• Assertions for checking the correctness of the driver's logic
• Distributed simulations