I took a deep look into the WiFi hardware on the Raspberry Pi Zero W.

The chip used is the BCM43438 (which actually seems to be designed by
Cypress Semiconductor), and also the same one used on the Pi3 Model B https://www.alldatasheet.com/datasheet-pdf/pdf/1018493/CYPRESS/BCM43438.html

First, it's not USB connected as I'm sure I've heard suggested
before. The chip actually supports either SPI or SDIO communications
and the description of the sdio overlay in /boot/overlays/README
suggests that the Pi defaults to communicating with it using SDIO
in the 50MHz "high-speed 4-bit mode" (200Mbps maximum speed quoted
in the datasheet). SDIO is the protocol used by SD cards for
high-speed data transfer.

It also has a built-in broadcast FM radio receiver, controlled via
the same UART interface as the Bluetooth controller. This is able to
output analogue stereo audio, or provide a digital stream. Most
likely the pin that the FM antenna needs to be connected to isn't
connected on the Pi circuit board, though you never know maybe it
goes to one of the undocumented test pads?

Not mentioned on this list for the Pi3B (though some entries are
blank): https://raspberrypi.stackexchange.com/questions/91901/what-are-the-functions-of -the-test-pads-on-the-pi-3b-3b

I don't know of a comprehensive list for the Pi Zero W's test pads.
The schematics are useless, they don't even show the WiFi chip!

The commands for controlling it over the UART connection are probably documented in Cypress' WICED SDK package. https://community.cypress.com/message/179340

It would certinly be fun to combine this with the well known GPIO FM transmitter hack, to make a Raspberry Pi walkie talkie. But I'm
guessing that the required pin/s are probably stuck under the
chip (curse these modern chip packages! :) ).

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Computer Nerd Kev <not@telling.you.invalid> wrote:

It also has a built-in broadcast FM radio receiver, controlled via
the same UART interface as the Bluetooth controller. This is able to
output analogue stereo audio, or provide a digital stream. Most
likely the pin that the FM antenna needs to be connected to isn't
connected on the Pi circuit board, though you never know maybe it
goes to one of the undocumented test pads?

Nice idea. The pin is pad E1 which is actually on one edge of the chip.
Here's a picture:

https://i.stack.imgur.com/EuIHC.jpg

The orientation is unclear from that photo but it looks like pin A1 is the bottom left, and the small capacitor to the right of the writing 'C252' is connected to F1 and G1. I can't see what E1 does from behind it, and
whether something could get at the ball from the side, or whether it's grounded.

You can always try bonding an antenna to the PCB anyway, and relying on capacitive coupling ;-)

The commands for controlling it over the UART connection are probably documented in Cypress' WICED SDK package. https://community.cypress.com/message/179340

It's likely Android sources include it too, since FM using the wifi chip is common on phones.

Theo

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