Pico W First Blink: Why the LED Lives on the CYW43 (Not GPIO25)

Pico W First Blink: Why the LED Lives on the CYW43 (Not GPIO25)

The quickest honest test of a new Raspberry Pi Pico W is the blink example that ships inside Raspberry Pi's own SDK. It is also where a lot of "my board is dead" tickets start, because the on-board LED is not an RP2040 GPIO. On the classic Pico W it lives on the CYW43439 wireless chip, and you only reach it after the CYW43 driver is initialized.

Everything below comes from open-source, Raspberry Pi–maintained sources:

• raspberrypi/pico-sdk, the official Pico SDK
• raspberrypi/pico-examples, including examples/blink and pico_w/wifi/blink
• Board header src/boards/include/boards/pico_w.h

I verified the board header and blink sources against upstream copies fetched for this write-up (pico_w.h, blink/blink.c, blink/CMakeLists.txt, pico_w/wifi/blink/picow_blink.c). I did not flash hardware for this post, and I did not run a full pico-sdk compile on the machine I wrote on, so treat the steps as source-verified, not as a flash log.

This post is about the classic Pico W (RP2040 + CYW43439). It is not the Pico 2 W (PICO_BOARD=pico2_w). If your silkscreen says Pico 2 W, use that board definition instead.

Disclosure: further down I link one eBay listing for the board. That link is an eBay affiliate link (customid=blog01); if you buy through it I may earn a commission at no extra cost to you. Check the listing yourself before you pay.

The two facts that matter before you flash anything

1. There is no PICO_DEFAULT_LED_PIN on Pico W. The board header says so in plain text:

// --- LED ---
// no PICO_DEFAULT_LED_PIN - LED is on Wireless chip
// no PICO_DEFAULT_WS2812_PIN

Instead it defines:

#ifndef CYW43_WL_GPIO_LED_PIN
#define CYW43_WL_GPIO_LED_PIN 0
#endif

and sets PICO_CYW43_SUPPORTED. The LED is WL_GPIO0 on the wireless chip — not RP2040 GPIO25.

2. Building as plain pico leaves the LED dark. The non-wireless Pico does put the LED on GPIO25. On Pico W, GPIO25 is reused as the CYW43 SPI chip-select (CYW43_DEFAULT_PIN_WL_CS is 25u in the same header). If you configure with the wrong board, the classic gpio_put(25, …) path either never compiles into the CYW43 branch, or you are poking the Wi-Fi CS line — neither lights the user LED.

Official examples/blink/blink.c branches on the macros:

#if defined(PICO_DEFAULT_LED_PIN)
    gpio_init(PICO_DEFAULT_LED_PIN);
    gpio_set_dir(PICO_DEFAULT_LED_PIN, GPIO_OUT);
#elif defined(CYW43_WL_GPIO_LED_PIN)
    return cyw43_arch_init();
#endif

and later:

cyw43_arch_gpio_put(CYW43_WL_GPIO_LED_PIN, led_on);

The blink CMakeLists.txt only links the driver when the board supports it:

if (PICO_CYW43_SUPPORTED)
    target_link_libraries(blink pico_cyw43_arch_none)
endif()

So the fix is not "try a different GPIO number." It is -DPICO_BOARD=pico_w so the CYW43 path is compiled in.

Classic Pico W vs Pico / Pico 2 W

If the listing photos show a green RP2040 board with a separate wireless module can, you almost certainly have classic Pico W. Confirm the silkscreen before you pick a PICO_BOARD value.

The board I'd point you to

If you still need a board, this is the listing I checked while writing: NEW Raspberry Pi Pico W (Wireless) - RP2040 on eBay (affiliate link).

When I opened it on Nov 7, 2026, the title was NEW Raspberry Pi Pico W (Wireless) - RP2040, condition was New, Buy It Now at US $23.01 with Free USPS Ground Advantage shipping from Chattanooga, Tennessee, United States. The seller (FindersKeepersTN) showed about 99.7% positive feedback from roughly 6,164 ratings, and returns were accepted for 30 days with the seller paying return shipping. Photos show two boards in clear factory packaging (green PCB + wireless module) — classic Pico W packaging, not Pico 2 W.

Whichever seller you buy from, check two things on arrival: the silkscreen says Pico W (not Pico 2 W), and you are willing to build with PICO_BOARD=pico_w so the CYW43 LED path is active.

1) Install the Pico SDK toolchain

Follow Raspberry Pi's Getting started with the Pico SDK for your OS (ARM GCC + CMake + the SDK). A typical layout:

## clone SDK (once)
git clone https://github.com/raspberrypi/pico-sdk.git
cd pico-sdk && git submodule update --init && cd ..
export PICO_SDK_PATH="$PWD/pico-sdk"

You also need a Pico-capable ARM toolchain (arm-none-eabi-gcc) and CMake. When setup is done, arm-none-eabi-gcc --version and cmake --version should both print something useful. Use a pico-sdk release that includes the pico_w board header.

2) Clone pico-examples and configure for Pico W

git clone https://github.com/raspberrypi/pico-examples.git
cd pico-examples
mkdir -p build && cd build
cmake -DPICO_BOARD=pico_w ..

-DPICO_BOARD=pico_w is the critical line. It pulls in boards/pico_w.h, defines PICO_CYW43_SUPPORTED, and leaves PICO_DEFAULT_LED_PIN undefined so blink takes the CYW43 branch.

3) Build the blink example (and know about picow_blink)

cmake --build . --target blink

That builds examples/blink, which auto-selects CYW43 when the board header defines CYW43_WL_GPIO_LED_PIN.

There is also an explicit Wi-Fi-folder example that does the same thing without the #ifdef dance:

cmake --build . --target picow_blink

Upstream pico_w/wifi/blink/picow_blink.c is short on purpose: cyw43_arch_init(), then cyw43_arch_gpio_put(CYW43_WL_GPIO_LED_PIN, 1/0) in a loop, linking pico_cyw43_arch_none (Wi-Fi stack not required just to toggle the LED GPIO on the wireless chip).

Either target is fine for a first LED check. Prefer blink if you want one tree that also works for plain Pico; prefer picow_blink if you want the Pico W path spelled out with no macros.

4) Flash via BOOTSEL UF2

Hold BOOTSEL, plug the Pico W into USB, release BOOTSEL. A mass-storage volume named something like RPI-RP2 should appear. Copy the UF2 onto it:

## Linux example — adjust the mount path / volume name for your OS
cp blink/blink.uf2 /media/$USER/RPI-RP2/
## or, for the dedicated example:
## cp pico_w/wifi/blink/picow_blink.uf2 /media/$USER/RPI-RP2/

The board reboots on its own. If the CYW43 path is active, the on-board LED should blink at 250 ms on / 250 ms off (the delay used in both official examples).

Use a data USB cable. Charge-only cables are still the most common fake "dead board," and they never present the BOOTSEL drive.

Troubleshooting in one table

Takeaway

When a new Pico W seems dead after a "GPIO25 blink," do not start by swapping pin numbers the way you would on a plain Pico. On the classic Pico W the official answer is already in pico-sdk / pico-examples: there is no PICO_DEFAULT_LED_PIN, the LED is CYW43_WL_GPIO_LED_PIN (0) on the wireless chip, and you must build with -DPICO_BOARD=pico_w so cyw43_arch_init() / cyw43_arch_gpio_put actually run.

Affiliate disclosure: the eBay link above is an affiliate link (customid=blog01). Prices, stock and seller details change; I describe the listing as I saw it on Nov 7, 2026.