power switch in VLSI Chip

A power switch is used to implement the power gating for a power domain. Power switch that we create in UPF is an abstract model of the actual power switch network which will be implemented in the silicon. For example in coarse grain power gating many parallel power switches are used to implement the power gating. Creating the power switch in UPF serves two purposes. One it captures the power intent of the designer and two it can be used for low power verification.

power_switch

In UPF, a power switch can be created by using the create_power_switch command. The following command creates a power switch for the example in figure.

create_power_switch PDA_SWITCH

-domain PDA

-input_power_port {Vin VDD}

-control_port {Cin SLEEP}

-output_power_port {Vout VDD_GATED}

-ack_port {Aout ACK !SLEEP}

-ack_delay {Aout 100ns}

-on_state {PDA_ON VDD !SLEEP}

-off_state {PDA_OFF SLEEP}

Options input_power_port, control_port and output_power_port does the port mapping for the power switch. Where as option domain specifies the power domain in which the power switch is created. This domain option is synopsys specific but not actually specified in IEEE 1801 standard. While other options like on_state, off_state, ack_port and ack_delay are used to model the power switch. The syntax of which are shown below,

-ack_port {port_name net_name {boolean_function}}

-ack_delay {port_name delay}

-on_state {state_name input_supply_port {boolean_function}}

-off_state {state_name {boolean_function}}

These are required for low power verification. Another model of power switch internally have two separate switches one small and other large (also called mother daughter). Smaller switch is used during the power up. Once the circuit gets to certain voltage level the larger switch is turned ON for normal operation of the power domain logic cells. So two control signals are used for creating these type of power switches.

create_power_switch PDA_SWITCH

-domain PDA

-input_power_port {Vin VDD}

-control_port {CSin SLEEP1}

-control_port {CLin SLEEP2}

-output_power_port {Vout VDD_GATED}

-ack_port {Aout ACK {!SLEEP1 & SLEEP2}}

-ack_delay {Aout 100ns}

-on_state {PDA_ON VDD !SLEEP2}

-off_state {PDA_OFF SLEEP2}

Similarly many different models of power switch can be specified. Apart from the two types of power switches some examples of different types of switches are

Muxed switch: This type of power switch can function as a mux between two input power supplies which can be from different source or of different voltage. One control switch is used to control which of the two supplies are to be selected. Another control signal is used to gate the output supply.

Overlapping Muxed switch: This type of power switch can do a smooth switch over from one supply input to another. This is done by selecting both the supplies at the same time and switching off one after some time during the transition. The two input supplies can be from different source and may have different supply voltages also but they should have the same voltage during the transition. So two control signals for mux select and one control signal to gate the output supply are required.

Mapping power switch:

The supply nets specified in the create_power_switch should already exist in the design. Since the tool has to use a library cell to implement the power switch the designer can specify the library cell that should be used for the purpose by using map_power_switch command. An example is shown bellow,

map_power_switch PDA_SWITCH

-domain PDA

-lib_cells mvlib/headswitch

We must note that the domain specified in both create_power_switch and map_power_switch is the domain in which the power switch is to be placed.