allure-boards

Two small boards that take the place of the Harman Kardon Allure's LinkPlay A88 module.

The Allure's brain is a LinkPlay A88 module that plugs into the mainboard with an M.2 key A card edge. Both boards here have the same card edge and outline as that module, so they plug into the same slot.

Board	What it does
A88 Pi Zero carrier	Holds a Raspberry Pi Zero 2 W on a 2x20 socket and connects it to the speaker: power, I2C, the front panel's UART, I2S to the DAC and from the microphones, the CX20924's reset and wake-word pins. This is the board allure-os runs on.
A88 breakout	Brings every A88 signal to a 2x20 header with its name on the silkscreen: for measuring what the stock module does, or wiring something else to the speaker.

docs/pinout.md lists every contact of the card edge, what each board does with it, and what allure-os uses it for.

Status

Rev B, the boards in this repository, is untested until built. Rev A, drawn by hand without a schematic, runs in two speakers today. Rev B keeps its routing and adds:

• schematics, so every connection has a name and KiCad checks it (ERC, DRC and schematic parity are clean)
• all ground and supply contacts of the card edge, and on the carrier all of the Pi's ground pins and both 5 V pins
• a ground pour on both sides with stitching vias, kept clear of the contacts
• a 0.8 mm board, the thickness the M.2 socket expects
• wider spacing where Rev A's I2C vias sat 0.1 mm apart, and tracks that no longer touch the neighbouring contacts

Ordering

Each board folder has a fab folder with Gerbers and drill files (*-gerbers.zip), the BOM, the placement file and the schematic as PDF.

Setting	Value
Layers	2
Thickness	0.8 mm (M.2 socket)
Size	42.5 x 63.3 mm
Surface	ENIG, or hard gold on the card edge
Card edge	bevelled (chamfered) at 20°, the M.2 contacts' lead-in
Minimum track / clearance	0.2 mm / 0.15 mm (the card edge contacts are 0.35 mm at 0.5 mm pitch)
Minimum via	0.45 mm, 0.2 mm drill

Parts:

Board	Part	Footprint
Pi Zero carrier	2x20 socket, 2.54 mm, SMD, vertical	PinSocket_2x20_P2.54mm_Vertical_SMD
Pi Zero carrier	Raspberry Pi Zero 2 W with a 2x20 pin header	—
Breakout	2x20 pin header, 2.54 mm, SMD, vertical	PinHeader_2x20_P2.54mm_Vertical_SMD

Files

a88-pizero/          the Pi Zero carrier: KiCad project, schematic, PCB, rules
  fab/               Gerbers and drill files, BOM, placement, schematic PDF
  images/            renders and 2D plots
a88-breakout/        the breakout, with the same layout
docs/pinout.md       the A88 card edge, contact by contact

The projects open in KiCad 10. Each carries its own library for the card edge footprint (LinkPlay_A88.pretty) and symbols (allure-boards.kicad_sym); the headers come from KiCad's standard libraries.

Sources

The A88's pin names, directions, ground and supply pins and the card thickness come from its specification: Linkplay WiFiAudio-A88 Specification, WMB20170414 Rev 1.5, 2018-04-02, published in the FCC equipment authorization database. The connections themselves were traced from Rev A.

Related

• allure-os: the firmware these boards run. Its docs/hardware.md describes how the Pi talks to the chips on the mainboard.
• harman-kardon-allure-kernel, branch allure/rpi-6.18.y: the kernel drivers and the rpi-cxsmartspk-i2s overlay.

License

CERN-OHL-P-2.0, the permissive CERN Open Hardware Licence.