5340 + 21540 timing

RE: 5340 + 21540 timing

skajam66 — Tue, 06 Jan 2026 15:08:49 GMT

> So something is off the way the FEM is being controlled.

Agreed. I can look into this. Just wanted a second opinion.

Many thanks,

RE: 5340 + 21540 timing

Bendik Heiskel — Tue, 06 Jan 2026 14:10:41 GMT

Ok, this looks more normal with regards to the default state of the PDN pin, but it is being de-asserted briefly during the transmission in the first plot. This will cause the nRF21540 to exit TX the state and return to PDN:

So something is off the way the FEM is being controlled.

RE: 5340 + 21540 timing

skajam66 — Tue, 06 Jan 2026 13:52:33 GMT

Hi Bendik,

Many thanks for your response.

On your last comment about the 21540 PDN pin, the screenshot that I sent was zoomed in so that you could see the timing to the 5340 and 21540 signals. Below, I have zoomed back out and you can see that the PDN pin is being deasserted between transmissions.

Kind regards,

RE: 5340 + 21540 timing

Bendik Heiskel — Tue, 06 Jan 2026 13:25:52 GMT

Hi,

This timing is not normal at all, when the start task is triggered the radio is already ramped up and the carrier is already started:

https://docs.nordicsemi.com/bundle/ps_nrf5340/page/radio.html#ariaid-title7

The FEM_TXEN should have been asserted "long" before the start task.

The nRF21540 needs a minimum of 27.5us from power down to TX, so the order the should be PDN pin asserted followed by TX at least 17.5us later. This should happen early enough so that the FEM is in TX mode before the radio is finished ramping up. The rampup time of the radio is 40us, so the startup of the FEM can be started at the same time as the TXEN task is started.

https://docs.nordicsemi.com/bundle/ps_nrf21540/page/chapters/device_control/doc/state_transition_timing.html

Here is a example plot of the correct timing of the PDN and TXEN pin in relation to RF out of the radio:

Channel 1 is showing a power detection connected to the output of the ANT pin of the nRF5340, the transition from high to low is due to the radio transmitting.

Ignore the first pulse on the TX_EN pin, this was a artifact of the measurement setup.

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The behavior of the PDN pin is also not correct, In normal operation the PDN pin will be low by default, keeping the nRF21540 in power down by default, only exiting power down for the radio events, TX/RX. Here the nRF21540 is being kept on by default, and momentarily shut off during the transmission.

Best regards,

Bendik