Routing

Overview of Routing

• We need the Clock Tree Synthesis database before going to the Route stage in physical design, where we have placed all the cells present in the design.
• Route creates physical connections to all the logical connections present in the design with the help of metal layers. These connections can be clock or data.

Types of Routes

• There are basically three routes:
• • 
    

    PG Route

    
    

    Power/Ground physical routing is completed during floorplan. Secondary PG routing happens just after STD cell placement.

  • 
    

    Clock Route

    
    

Clock physical routing is completed during CTS after clock buffer insertion.

• • 
    

    Signal Route

    
    

    All the signal routing are completed after all the cells are placed which is called as routing stage.

• Routing and its optimization are done after taking care of QORs and physical DRCs. Once these counts are reasonable, we go for the ECO stage.

Pre-Route Checks

The pre-route checks that need to be done are as follows:

• All the Physical cells and Standard cells should be placed properly inside the core area.
• Clock cells should be placed and clock routing should be completed. Clock NDR should be applied for the required or all clock nets.
• Acceptable congestion, Timing (Setup/Hold), Power and Logical DRCs.
• We should fix all the assign statements where 1’b0 should be connected via TIE low and similarly 1’b1 should be connected to TIE high cell.
• ATPG and scan coverage should be proper. The entire design should have testability.
• PDN report (Low Power report) should be clean.
• Logical Equivalence should be clean.
• Max Tran, Max Cap, Max Load etc. violations should not be too high. Marginal can be handled during routing or ECO stage.

Goals of Routing

• Route all the signal nets with minimal physical DRCs.
• Optimize Data path logic for timing, DRCs and Power.
• Quality of route in a way, post route CTO is optionally performed.