identify-diff-pairs

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 py_router/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-.

1. 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.

2. 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