Special Cell and Standard Cell Modeling

The IEEE 1801 standard is a must for the design and verification of low power integrated circuits, the correct identification and mapping of all power attributes, topology-related attributes, and the functions of low power special cells or standard cells. This is achieved by the proper registration of the special cells and the standard cell models. If cells are registered, they are identified in the schematic view accordingly.

Low power special cells and standard cells include:

Isolation cell
Level shifter cell
Power switch cell
Always on cell
State retention cell
Combinational/Sequential elements

Power Manager extracts level shifter strategy, isolation strategy, and power switch strategy in the power intent based on the definition provided in a 1801 special cell definition file or Liberty models. The in-design checking uses the same information to verify the power domain crossings with or without these special cell instances.

You can register these cells in any of the following two ways:

By using the Liberty models of the cells
Special cells and standard cells, which have their Liberty model available, are recognized for their topology and various supply-related attributes by including the corresponding Liberty files. The Liberty files enable the following:
- The pg_pin attributes specified in the Liberty files help in identifying the nets used inside the cells as power or ground.
```
pg_pin(VDD)  {
```
```
voltage_name         : VDD;
```
```
pg_type              : primary_power;
```
```
direction            : inout;
```
```
} 
```
- The voltage map specified in the Liberty files is used to extract the voltage levels of the corresponding supply and ground nets used.
```
voltage_map( VDD , 1.100000);
```
- The cell type specified in the Liberty files is used to identify the type of low power special cell or the standard cell, and extract it accordingly during 1801 power intent extraction or running In-Design Checks.
```
cell(Isolation_Cell) {
```
```
is_isolation_cell : true;
```
- The function attributes specified in the Liberty files are used as follows:
  - The switch/enable function of the low power special cells is used for finding the switch functions, enable conditions, and for backtracing the control/enable signals of the special cells, such as power switch cell, isolation cell, and enable level shifter cells, for any of the boundary ports.
```
pg_pin(VDDSW){
```
```
switch_function : "psw_en";
```
```
pin(out_iso) {
```
```
isolation_enable_condition : "en_iso";
```
  - The output function of the standard cells is used during backtracing of the control/enable signals of the low power special cells that are generated by some combinational logic, for example, output of a NAND gate. Here for a NAND gate Liberty function attribute of its output can be used.
```
pin (Y) {
```
```
direction : "output";
```
```
function : "(!((!AN) B))";
```
By using the 1801 special cell definition file
When you provide a special cell definition file at the location specified in the setup file, before extracting the power intent, Power Manager reads the given file and recognizes the cells specified in that file as special cells. The 1801 special cell definition file aids in modeling of these cells by specifying the related attributes in their definition.
- Cell type
- Supply ports/Switchable supply ports
- Data ports
- Enable/control ports
- Voltage range
- Cell-specific attributes
Example:
```
define_isolation_cell -cells ISOHLDX1_OFF -power ExtVDD -ground VSS -clamp_cell low -enable !ISO
```
```
define_level_shifter_cell -cells LSLHX1_TO -direction both -input_power_pin ExtVDD -output_power_pin VDD -ground VSS -input_voltage_range {{0.8 1.4}} -output_voltage_range {{0.8 1.4}}
```
```
define_power_switch_cell -cells HSWX1 -type header -power ExtVDD -power_switchable VDD -ground VSS -stage_1_enable PSO -stage_1_output PSO_out -exclude { HeadSwitch_WrapperA HeadSwitch_WrapperB }
```
Use this method to register special cells for the standard and verified standard cell libraries, which do not have the Liberty models available. This method can also be used to register custom standard cells.
Setup registration for Liberty models and the special cell definition file is as follows:
```
libFiles (
```
```
"libs/fast_vdd1v0_basicCells.lib"
```
```
"libs/fast_vdd1v2_extvdd1v0.lib"
```
```
"libs/macro_lib.lib"
```
```
"libs/Headswitch.upf"
```
```
)
```

Virtuoso Power Manager User Guide
Product Version IC23.1, June 2023