identify-diff-pairs

name	identify-diff-pairs
description	Identifies differential pairs on a KiCad PCB by pin function via datasheet lookup, catching pairs whose net names don't follow P/N conventions. Recommends per-interface routing parameters (differential impedance, gap, intra-pair matching) and outputs ready-to-use route_diff.py commands.

Identify Differential Pairs

When this skill is invoked with a KiCad PCB file, find all differential pairs — including ones name-based detection misses — and recommend routing parameters per interface.

Step 1: Baseline Name-Based Detection

python3 list_nets.py path/to/file.kicad_pcb --diff-pairs

This catches pairs following P/N naming conventions. Record the result as the baseline.

Step 2: Datasheet-Based Detection by Pin Function

For each IC likely to have differential interfaces (footprint/value keywords: FPGA, PHY, SERDES, USB, ETH, HDMI, LVDS, MIPI, SATA, PCIE, DDR, transceiver, redriver):

1. Check pad metadata first — KiCad symbols often carry it:

from kicad_parser import parse_kicad_pcb
pcb = parse_kicad_pcb('path/to/file.kicad_pcb')
fp = pcb.footprints['U3']
for pad in fp.pads:
    print(pad.pad_number, pad.pinfunction, pad.net_name)

Differential pin functions usually pair as ..._P/..._N, TX+/TX-, D+/D-.

2. Where pinfunction is missing or generic, WebSearch the part's datasheet pinout and map differential pin pairs to pad numbers, then to nets via the pads.

3. Trace through series passives: if a differential pin connects to a 2-pad R/C/L, follow to the net on the component's other pad — the pair often gets its "real" net name on the far side of AC coupling caps or series resistors.

A pair is confirmed when both nets trace back to a documented differential pin pair. Nets that look paired only by name or topology are suspected — list them separately for the user to confirm.

Step 3: Recommend Parameters per Interface

Interface	Differential impedance	Notes
USB 2.0 / 3.x	90 Ω	Intra-pair matching matters from USB 2.0 HS up
LVDS	100 Ω
Ethernet (MDI)	100 Ω
HDMI / TMDS	100 Ω
PCIe	85 Ω	AC coupling caps in-line on TX
SATA	90 Ω
MIPI D-PHY	100 Ω
DDR (DQS, CK)	100 Ω typical	Check the memory datasheet
CAN	120 Ω	Low speed; tolerances relaxed

From the impedance, derive the routing flags:

• --impedance <Z> computes per-layer widths and gap from the stackup (preferred when the stackup is realistic — see /recommend-stackup), or set --track-width/--diff-pair-gap explicitly.
• --diff-pair-intra-match for interfaces fast enough that P/N skew matters (USB HS+, PCIe, SATA, HDMI, LVDS at high rates).
• Classify every pair's polarity-swappability from the datasheet pin functions and report a machine-readable verdict per pair: polarity_swappable: yes|no|unknown. yes = an endpoint can compensate a P/N swap (FPGA/CPLD generic I/O pin pairs, PCIe lanes, SerDes with RX/TX polarity-invert, 1000BASE-T MDI); no = polarity-critical (USB D+/D-, MIPI, TMDS/HDMI/DP, CAN, RS-485/422, DDR CK/DQS, fixed-function receivers, connectors to unknown hardware, or an asymmetric attachment like a single-sided pull-up on one net); unknown = treat as no. Feed the yes pairs into route_diff.py --polarity-swap-nets <patterns> (swaps are denied by default, #279).

Step 4: Output

1. Confirmed pairs, grouped by interface, each group with a ready-to-run command:

# USB is polarity-critical: no --polarity-swap-nets (swaps denied by default)
python3 -X utf8 route_diff.py board.kicad_pcb board_usb.kicad_pcb \
    --nets "USB_DP" "USB_DM" \
    --impedance 90 --diff-pair-intra-match \
    2>&1 | tee /tmp/route_usb.txt

# FPGA generic-I/O pairs CAN swap (pin functions reassigned in gateware)
python3 -X utf8 route_diff.py board.kicad_pcb board_lvds.kicad_pcb \
    --nets "lvds_*" --impedance 100 \
    --polarity-swap-nets "lvds_*" \
    2>&1 | tee /tmp/route_lvds.txt

2. Suspected pairs with the evidence (matching pin functions but unverified part, name-only pairing), asking the user to confirm before routing them as pairs.

3. A note of any pairs found by Step 1 but not corroborated by pin function — possible false positives of name matching (e.g. unrelated nets that happen to end in P/N).