The ESP32 on the bench

One band on most chips

The ESP32, S2, S3, C3 and C6 have a 2.4 GHz radio only. The ESP32-C5 also works at 5 GHz. A 2.4 GHz board never sees a network that is on 5 GHz alone, so it reports reason 201, no access point found, not a wrong password. The ESP32-P4 has no radio at all: it gets Wi-Fi from a second Espressif chip wired to it.

The S3 can use 40 MHz channels, but in a crowded 2.4 GHz band that is rarely wise. Setting the country with esp_wifi_set_country() picks the channels and power limits for where you are; it is not a licence to exceed the local rules.

The antenna end

A PCB antenna needs clear space. Espressif's layout guide puts the module's antenna outside the base board or over a cut-out, keeps copper out of the module datasheet's keep-out zone, and asks for at least 15 mm of clearance inside the housing. A hand, a metal case or a ground pour close to the antenna takes signal away, by an amount nobody can quote in advance, so test the range in the final enclosure. Where the board has to sit in metal, use a module with a connector for an external antenna, such as the ESP32-WROOM-32UE.

A burst of current

The radio draws most when it transmits: 340 mA peak by the S3's datasheet, 335 mA for the C3 and 354 mA for the C6 by theirs. Espressif asks for a 3.3 V supply of at least 500 mA and at least 10 µF at the power entrance, and warns that the sudden jump can collapse the rail. Through a thin USB cable or a weak regulator the dip can trip the brownout detector, and the board resets the moment Wi-Fi starts, which looks like a software fault. The rail itself is in power and the 3V3 rail.

ADC2 and the radio

On the ESP32, S2 and S3, ADC2 is shared with Wi-Fi. While Wi-Fi uses it, adc_oneshot_read() can fail with ESP_ERR_TIMEOUT and no valid result. On the S3, ADC1 is GPIO 1 to 10: put analog sensors there (reading the ADC).

Bluetooth shares the radio as well. Wi-Fi and Bluetooth take turns on one 2.4 GHz front end, and ESP-IDF divides the time between them, so heavy use of one slows the other.

Common mistakes

• Pointing a board at a 5 GHz-only network. Reason 201 means not found. Turn on the router's 2.4 GHz network, or use a C5.
• Burying the antenna. A metal case or copper under the antenna end costs range. Keep the antenna end clear, or use an external antenna.
• Debugging code for a reset that comes from the cable. A board that restarts as Wi-Fi starts needs a short, good cable and a supply of at least 500 mA before it needs a code change.