librelane

# Timing Corners

To ensure a chip can continue to function under various conditions, the chip
must undergo static timing analysis under different **timing corners**.

The timing corners are a combination of characteristics, specifically four, two
being as a result of the process:

* **Parasitic/Interconnect Corners**: Metal layers may have slightly different
  geometry based on manufacturing, which will affect the wires' parasitics
  (capacitance and resistance.)
* **Transistor Corners**: Also called "process corners", more commonly, variance
  in transistor carrier mobility, denoted as follows:
  * `s`, `t`, `f` for whether NMOS transistors are slow, typical, or fast respectively.
  * Another `s`, `t`, `f` but for PMOS transistors.
    For example, a corner with fast NMOS and PMOS would be denoted `ff`.
    See [Process Corners](https://en.wikipedia.org/wiki/Process_corners#FEOL_corners) for more info.

And two that are dependent on the operation environment:

* **Temperature**: A higher temperature causes transistors to switch slower.
* **Voltage**: A higher voltage causes transistors to switch faster.

Common EDA files incorporate these corners as follows:

* `.spef` files are usually a function of interconnect corners, but temperature,
  voltage and process corners may also affect the parasitic values at smaller nodes.

* `.spice` files usually incorporate a specific combination of interconnect and transistor corners, but temperature and voltage are continuously variable.

* `.lib` files characterize a circuit at a full corner.

  * For standard cells, the parasitic effect is minimal, leading to the common
    acronym "PVT": **P**rocess/Transistor Corner, **V**oltage and **T**emperature
    for their lib files.
  * For macros, the parasitic effect is significant, and a lib file for one
    parasitic corner is not necessary applicable for others.

```{note}
The default extraction utility for OpenLane, OpenROAD OpenRCX, only accounts for
the interconnect corner.
```

## Default Flow Configuration

In its current state, the default OpenLane flow allows SCLs to configure the following:

* A list of PVT-corners with names that correspond to `.lib` files
* A list of interconnect corners with arbitrary names that _may_ correspond to any of:
  * `TECH_LEF`

The sky130A/sky130_fd_sc_hd SCL, for example, comes with configurations for these corners:

* PVT: Corner data stored in {var}`::LIB`

| Name           | Process {NMOS, PMOS} | Voltage (V) | Temperature (C) | Corresponding File                  |
| -------------- | -------------------- | ----------- | --------------- | ----------------------------------- |
| "tt_025C_1v80" | {T, T}               | 1.8         | 25              | `sky130_fd_sc_hd__tt_025C_1v80.lib` |
| "ss_100C_1v60" | {S, S}               | 1.6         | 100             | `sky130_fd_sc_hd__ss_100C_1v60.lib` |
| "ff_n40C_1v95" | {F, F}               | 1.95        | -40             | `sky130_fd_sc_hd__ff_n40C_1v95.lib` |

* Interconnect: Corner data stored in {var}`::TECH_LEFS` and {var}`OpenROAD.RCX::RCX_RULESETS`

| Name  | Description                     | Corresponding Technology LEF | Corresponding Ruleset               |
| ----- | ------------------------------- | ---------------------------- | ----------------------------------- |
| "nom" | The nominal interconnect corner | `sky130_fd_sc_hd__nom.tlef`  | `rules.openrcx.sky130A.nom.calibre` |
| "min" | The minimal interconnect corner | `sky130_fd_sc_hd__min.tlef`  | `rules.openrcx.sky130A.min.calibre` |
| "max" | The maximum interconnect corner | `sky130_fd_sc_hd__max.tlef`  | `rules.openrcx.sky130A.max.calibre` |

As a user, you are free to override these values as you would any other PDK/SCL
variables, however, it is your responsibility to keep the consistent.