What is Magnet Placement, and how is it used during physical implementation?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Magnet placement is exactly what it sounds like: you designate a fixed object — an I/O port, a macro, or even one macro pin — as a "magnet," and the placer applies an attractive pull to every standard cell directly wired to it. Without this, interface registers connected to a fixed off-block port can end up scattered randomly across the core by unconstrained global placement, which then makes closing the interface's input-delay timing painful.
Technical Explanation
- Magnet placement is exactly what it sounds like: you designate a fixed object — an I/O port, a macro, or even one macro pin — as a "magnet," and the placer applies an attractive pull to every standard cell directly wired to it.
- Without this, interface registers connected to a fixed off-block port can end up scattered randomly across the core by unconstrained global placement, which then makes closing the interface's input-delay timing painful.
- The pull typically has a distance cap — e.g., "keep all direct receiver flops within 50 µm of this I/O pad" — so cells land close without collapsing into an unrealistic pile.
- The same trick pulls SRAM read-data registers close to the SRAM's Q-pins for the mirror-image reason: it eliminates long routing delay on the memory-access critical path, and it works best when applied before general standard-cell placement, not after.
Common Mistake
The Trap: Applying magnet placement with an overly tight distance constraint (e.g. 5 um), which creates an extreme placement density hotspot right in front of the I/O pin that fails legalization.
Follow-up Question & Model Response
"How does magnet placement differ from a fixed placement bound?"
Candidate Model Response: A placement bound creates a rigid geometric box that strictly confines specified instances. Magnet placement applies an attractive directional force without rigid geometric boundaries.
Practical Example
Executing Magnet Placement:
# Synopsys ICC2: Magnet placement pulling interface registers toward DDR ports
create_placement -magnet [get_ports ddr_data[*]] -cells [get_cells u_ddr_phy/rx_reg*]
# Cadence Innovus magnet placement
magnetMove [get_ports ddr_data[*]] -cells [get_cells u_ddr_phy/rx_reg*]PnR Flow Mentor Guide
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