What are Always-On Logic Cells?
Some logic needs to remain active during shutdown, such as Power Switch Cells, Retention Register Cells, and Isolation Cells. These are referred to as Always-On Logic Cells. They can maintain a constant ‘on’ state regardless of whether the voltage area is powered down.
Where are they used?
The difference between these cells and regular cells is that they have an additional secondary power pin, which must be connected to the always-on stripe. Typically, due to their large area and the additional secondary power pin consuming routing resources, they are used infrequently. They are often added to feedthrough nets and can also be connected to the control side of SRPG cells or the enable side of isolation cells.
The UPF file describes the always-on buffer as follows:
define_always_on_cell -cells { VIAONPTBUFF} \ buffer cell name
-ground {GND} \ primary ground pin
-ground_switchable {GNDINT} \ secondary ground pin
-power {VDD} primary power pin
Liberty Always-On Buffer, for Reference
In the Liberty format description of these cells, there will be an attribute “always-on” set to true. Additionally, for Always-On Logic Cells, the pg_pin description generally includes two sets: primary and backup. When the tool identifies the cell as Always On, it will connect both sets of power and ground to the always-on power/ground.
cell(buffer_type_AO) {
always_on : true;
pg_pin(VDD) {
voltage_name : VDD;
pg_type : primary_power;
}
pg_pin(VDDB) {
voltage_name : VDDB;
pg_type : backup_power;
}
pg_pin(VSS) {
voltage_name : VSS;
pg_type : primary_ground;
}
...
pin (A) {
related_power_pin : VDDB;
related_ground_pin : VSS;
}
pin (Y) {
function : "A";
related_power_pin : VDDB;
related_ground_pin : VSS;
power_down_function : "!VDDB + VSS";
}
...
References
- Synopsys® Low-Power Flow
- https://blog.csdn.net/Tao_ZT/article/details/102456728