nRF54 or nRF52 with nRF91

[JackT]

Hello everyone,

We're currently working on a project where we're looking to add LTE-M functionality to our existing hardware — or rather, to a slightly upgraded version of it.

We have a custom PCBA based on the nRF52840, and we’re planning to update the design to include new features.
This includes upgrading to the nRF54L15 and adding the nRF9151 for LTE-M.

Now, looking at Nordic's Thingy:91(x) reference designs, I noticed that the nRF91 is typically the main application processor, with the nRF52 acting as a secondary chip handling Bluetooth.
In our case, it's the opposite — our current setup uses the nRF52 as the main application processor, and we’d prefer to keep it that way for simplicity. The majority of our applications will continue to focus on Bluetooth for now.

So I have a few questions for anyone with experience in this kind of setup:

1. Can the nRF5x control the nRF91xx, including managing its IO?

2. Can the nRF91xx be used to perform OTA updates on the nRF5x (e.g., fetching and flashing new firmware)?

3. Is a UART connection between the two sufficient for sending commands and data?

Has anyone tried this kind of configuration before?

Any help or insight would be greatly appreciated!

Thanks in advance.

Kind regards,
Jack

[khelmutlord]

Hello,

1. Because the nRF91 has an open application core, it may be worth investigating custom extensions on top of our "serial LTE modem" sample: https://docs.nordicsemi.com/bundle/ncs-2.9.1/page/nrf/applications/serial_lte_modem/README.html Note that this already includes AT commands for setting GPIOs among other things.

2. I would investigate MCUmgr. I could see a system where the nRF54L acts as an "SMP server" https://docs.zephyrproject.org/latest/samples/subsys/mgmt/mcumgr/smp_svr/README.html and the nRF9151 acts as an "SMP client" https://github.com/zephyrproject-rtos/zephyr/tree/d0fdd384db0be275566330ed785e5ce7504985a4/tests/subsys/mgmt/mcumgr/smp_client w/ some custom AT commands to signify when a new DFU image is available.

3. A UART configuration should be just fine. I would look at our "low power UART" https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/samples/peripheral/lpuart/README.html which uses two extra pins alongside TX/RX (and instead of RTS/CTS) to achieve lower power consumption than a typical UART.

Let me know if this is helpful

Thanks,

Helmut Lord