Low Power Special Cell

• List of Low Power Cell:
• Isolation Cells
• Level Sifters
• Retention Registers
• Power Switches
• Always-ON Buffer/Inverter

1.  Isolation Cell:

• Powering down the region may result in crowbar current at the inputs of powered up blocks if they are talking to each other.
• Isolation cell is used to prevent the crow-bar current. It will clamp the output of the powered-down block.
• There are three basic types of isolation cells as below:
1. Clamp 0 : AND gate is used for clamping 0.
2. Clamp 1: OR gate is used for clamping 1.
3. State retention isolation: It retains last value of element.

• Example for isolation with UPF isolation strategy:

ISOLATION with UPF Strategy

        2. Level Sifter:  

• Level Sifter: Level Sifter is a buffer that translates the signal from one voltage level to another voltage level. It helps to reduce crow-bar current mostly in Low-to-High design path.
• Two types of level sifters:
• Unidirectional level sifter                      
• Bidirectional level sifter
• Further classified into:
• High-to-Low
• Low-to-High 
• Both

 

      3. Retention Register: 

• Registers used to preserve the state of the block before power-down. It consists of two registers: Main register and Shadow register.
• “Shadow register” used to preserve the state during power down and restore it at power up. It is working on high threshold voltage (Vth). Hence leakage will be less.
• There are three types of retention register as below:
1. Single Pin Retention: A single save/restore control pin used
2. Dual Pin Retention: Two separate save and restore signals are used
3. Zero Pin Retention: Clock and reset will control save/restore operation  

Retention Register

       4. Power Switches:

• Power switch is used to implement the power gating technique
• Fine grain power switch
• In fine grain power gating the switch is placed locally inside each standard cell in the library. Area will be more and structure is difficult
• Coarse grain power switch
  • In coarse grain power gating, a block of gates has its power switched by a collection of switch cells. Area is small compared to Fine grain. Structure is less difficult. 
  
  
  
  

  

  Types of Power switch w.r.t Arch

• Types of power switches based on controlling supply net:
  1. Header Power Switch to control power rail.
  2. Footer Power Switch to control ground rail.

     

     Header_Footer_Power_Switch

     
     

         5. Always-On Buffer/Inverter Cell:

• Always ON buffer/Inverter
• Always-on buffers/inverters = regular buffers/inverters + backup PG pins
• AON cell ensures that all critical signals (clk, rst, enable) path will remain alive during shutdown.  

 

Always On Buffer/Inverter