Design question for NRF54L15

[Wu Yewan]

Hi Nordic Team,

Does the reset of 54L15 need to be handled? Still pull up a 1k resistor. Also, do I need two wires for debugging?

The customer has another question. They want to use the nfc function. The usage scenario is that we are a portable tag with wireless charging function. The coil of the nfc and the wireless charging coil are on the same FFC and concentric, that is, in the same magnetic field.

If IC is entering deep sleep. When wireless charging comes in, will the ic mistake it for nfc and then start up? I asked the AI of your system. It seems that if it is awakened by nfc, it will reset, and there is also a protection device in nfc that will be disabled. In that case, would nfc be damaged in this scenario?

I want to reduce the impact of wireless charging on the nfc circuit. I want to connect two 50pf capacitors in series to the pins of the NFC. Is this feasible? A 50pf capacitor is placed between the ic and the resonant circuit.

Customer: SHENZHEN FEASYCOM TECH CO LTD.

Distributor：Future electronics.

[Kenneth]

Hello,

Reset pin: The nRESET pin on the nRF54L15 has an internal pull-up resistor, so you don't need an external pull-up. You can leave the pin unconnected or route it only to a test pad. If you want extra immunity to noise, which is a reasonable precaution next to the wireless charging coil, keep the nRESET trace short and away from the coil, and optionally add a small capacitor to GND. Note that the capacitor makes the reset pulse a bit longer.

Two-wire debugging: Yes. Programming and debugging use Serial Wire Debug (SWD), which only needs SWDIO and SWDCLK, plus GND and a VDD reference for the debugger. The debugger can reset the chip, and also erase and recover it, through the debug port, so nRESET doesn't have to be connected to the debug connector.

NFC and Charging coil: When the NFC pins are in NFC mode, the nRF54L15 has on-chip NFC pad protection. Do not configure the NFC pins as GPIOs, because that disables the protection.

With a concentric Qi coil, the main risk is current. At Qi frequencies the NFC coil is close to a short, so the induced current would flow through the on-chip clamp with little to limit it. Adding series capacitors should help: 50 pF should block most of the Qi energy. However, 50 pF will impact NFC matching network, so make sure to retune the antenna to 13.56 MHz, then verify read range. 

A strong Qi field may also trigger NFC field detection and wake the device from System OFF (the reset reason will show NFC when waking up from System OFF). We recommend checking the charger status in firmware and either turning off NFC sensing while charging or don't enter system OFF with NFC wake up during charging.

Hope that helps,
Kenneth