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# Characterization
This page of the documentation explains the characterization of OpenRAM.
## Table of Contents
1. [Characterization Overview](#characterization-overview)
2. [Characterizer Organization](#characterizer-organization)
3. [Characterization Options](#characterization-options)
4. [Characterization Measurements](#characterization-measurements)
5. [Analytical Characterization](#analytical-characterization)
6. [Multiport Characterization](#multiport-characterization)
7. [Characterizer Unit Test Use](#characterizer-unit-test-use)
8. [Functional Simulation](#functional-simulation)
9. [Power/Delay Characterization](#powerdelay-characterization)
10. [Timing Graph](#timing-graph)
11. [Graph Creation Example: Buffer](#graph-creation-example-buffer)
12. [Graph Module Exclusion](#graph-module-exclusion)
13. [Timing Measurement Checks](#timing-measurement-checks)
## Characterization Overview
Measures the timing/power through SPICE simulation:
* Generates the SPICE stimulus: The stimulus is written in standard SPICE format and can be used with any simulator that supports this.
* Runs the circuit simulations: To produce the average power, setup/hold times, and timing delay of the memory design.
* Parses the simulator's output: The setup time, hold time, and delay are found using a bidirectional search technique.
* Produces the output in a Liberty (.lib) file.
## Characterizer Organization
* Core Modules
* `delay.py` - Generates the delays and power of input SRAM and corner
* `setup_hold.py` - Generates setup and hold timing of DFFs by corner
* `lib.py` - Characterizes SRAM and builds Liberty file
* `stimuli.py` - Generates SPICE stimulus file for characterization
* Auxiliary Modules
* `simulation.py` - Base class for SRAM characterization modules
* `trim_spice.py` - Removes portions of SRAM SPICE to speedup simulation
* `measurements.py` - Contains classes to aid SPICE measurements
* `char_utils.py` - Contains common functions used during characterization
* `logical_effort.py` - Helper class for analytical delay model
* Testing Support Modules
* Other modules are derivatives of the simulation module used in the unit tests
## Characterization Options
* Characterization by Configuration File
* The process, voltage, and temperature (PVT) for characterization are defined in the config file.
* Running OpenRAM generates a Liberty (.lib) file for every corner characterized.
* Delays, slews, power determined for each input slew and outputs load combination in config file.
* OpenRAM Characterization Mode
* Supports analytical and SPICE based characterization
* Analytical characterization is default
* SPICE characterization enabled with -c flag (requires SPICE simulator)
## Characterization Measurements
* Characterization is performed primarily to generate tables in .lib file
* cell_rise/cell_fall - Delay of from negative clock edge to DOUT when reading a 1 or 0 respectively
* rise_transition/fall_transition - Slew of DOUT when read 1 or 0 respectively
* Setup and hold time for inputs (setup_rising, hold_rising)
* Total power and leakage power
* Delays and slews and intended to be independent of clock period.
* Fall delays are copied to rise delays after characterization*
* Characterizer can help with debugging or optimization
* Delay characterizer also produces Sense Amp Enable Timing to help debug read failures.
* Delay class can edited or can be base class if other measurements are wanted.
> **Note**: Rise delays are dependent on the clock period if measured from negative clock edge due to precharge.
## Analytical Characterization
* Delay
* Assumes a worst case delay through decoder, word line, and bit lines
* Calculates RC delay at each stage using parameters for handmade cells, unit capacitances, and unit resistances which are defined in the technology setup
* Output slews are estimated based on delays
* Wire delays only estimated for bitline delay
* Power
* Dynamic power at each stage calculated using $C * V^2 * frequency$
* Leakage power estimated with parameters in technology setup
* Corners add linear variations in delay and power
## Multiport Characterization
* Supports any port configuration specified by the configuration file
* Any number of Read, Write, and Read/Write ports
* Any custom bitcells may require modifications for characterization
* Characterization Algorithm
* Feasible period found for each port
* Common minimum period is found for all ports
* Power and delay is characterized on each port
* Characterization Runtime
* Ports are characterized separately for accuracy
* Runtime increases linearly with ports and can be slow.
## Characterizer Unit Test Use
* OpenRAM will use the characterizer to generate data for .lib by default
* Characterizer modules can be instantiated separately from lib.py to generate and test data
* Example: `21_ngspice_delay_test.py`
* Delay module generates data to compare against previously generated data to error check any recent OpenRAM changes having a large effect on the delay
* Delay/Simulation module can be used as base class and altered to include custom measurements
## Functional Simulation
* Assumes uninitialized memory
* Random operations
* Uses multiple ports when available
* Read, write, or noop
* Random address
* Random data
* Memory checks
* Uses standard CMOS noise margins
* Compares read with last write (or feed-through value if being written)
## Power/Delay Characterization
* Prunes unused portions of circuit for run-time
* Setup time, hold time, and delay are found using a bidirectional search.
* Finds a feasible period
* Iterates until convergence
* Dynamic and leakage measured
* Output saved in Liberty (.lib) file.
* Uses NLDM
* Wish list: CCS
## Timing Graph
* OpenRAM has SPICE like modules and instances
* A timing graph is created using the SPICE netlist, names and paths through lower level modules.
* Graph used in characterizer for debugging timing issues associated with the Sense Amp Enable
## Graph Creation Example: Buffer
* SPICE instantiated module (vdd, gnd excluded from graph)
* Xbuf1 A Z vdd gnd Buffer
* Base module has its own naming for ports and internal signals
* Node names reduced to top-most SPICE level with internal signals maintained.
* Internal modules determine edges between nodes
* Most lower level modules (gates, PTX, FF, etc) determine edges by connecting every input to every output by default.
* Custom timing paths can be overridden in any module
## Graph Module Exclusion
* Modules can be excluded from the graph before it's built
* This can help trim timing paths that are known to not affect the overall timing
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| Graph without exclusion | Graph with exclusion |
## Timing Measurement Checks
In addition to measurements done for characterization. Several measurements are done to help debug memory failures.
* Bitline measurements - Voltage of bitlines measured the checked to have at least a 10% difference
* Bitcell Measurements - Voltage measured on internal storage of cells and check that they match the operation.
* Output voltage measurements - Output voltage (`DOUT`) checked at end of cycle so it matches operation.
* Sense Amp Enable Timing - Delay of `S_EN` should not exceed a half-period