hold-fix

Hold Fix — Post-CTS / Post-Route Hold Violation Repair

After CTS, clock insertion delay makes fast data paths arrive too early at capture flops, causing hold violations. This is expected and must be fixed by inserting buffers or delay cells on short paths. Hold fixing is iterative: fix -> check -> re-fix until all hold slack >= 0 at the fast (FF) corner.

When to use

• After CTS (Step 19): CTS introduces clock insertion delay that creates hold violations on short paths. This is the primary trigger.
• After routing: parasitics change path delays, potentially creating new hold violations.
• After ECO: any timing ECO can shift hold margins.
• Before signoff: hold must be clean at FF corner for tapeout.

Inputs

1. Post-CTS or post-route database (DEF + netlist)
2. SDC constraints with clock definitions
3. Liberty files for the fast (FF) corner — hold is worst at FF
4. STA hold report (report_checks -path_delay min)
5. Area budget — maximum area overhead for buffer insertion (typically 2-5%)
6. Hold slack margin target (e.g., 0 ps for nominal, +50 ps for guardband)

Workflow

Step 1: Analyze Hold Violations

Run STA at the FF corner (fast process, high voltage, low temperature):

# OpenSTA
read_liberty gf180mcu_fd_sc_mcu7t5v0__ff_1p10v_m40c.lib
read_verilog <design>.v
read_sdc constraints/<design>.sdc
report_checks -path_delay min -slack_max 0 -format full_clock_expanded

Collect:

• Total number of hold-violating endpoints
• Worst Hold Slack (WHS)
• Total Hold Slack (THS) — sum of all negative hold slacks
• Distribution: how many endpoints per slack bucket (0 to -100 ps, -100 to -200 ps, etc.)

Step 2: Hold Fix Strategy

The fix is always the same: slow down the data path by inserting buffers or delay cells.

Strategy selection (slack-bucket → strategy) is a pure numeric-threshold lookup — enforced by programs/hold_fix_planner.py (pick_strategy: > -50 ps → single buffer; -200..-50 ps → 2-3 buffer chain; < -200 ps → delay cell / restructure). Feed it the per-endpoint hold-slack list; do not re-read a prose table.