nRF9151 Low Power Mode.

[Medical Device]

I am using the nRF9151 DK, using the Serial LTE Modem firmware from the nRF Connect quick start.

I am planning to use nRF9151 as a serial modem, and connect it to my Host MCU using UART.

I am using the Power Profile II to measure the current of VDD_nRF.

Questions:

1. Is this the product code that I should be using for my final product?
2. Power save mode:
   1. I set the modem LPM. The average current is about 500uA in LPM. Why is it still so high? What is nRF9151 drawing current for? Is it because nRF9151 UART is still enabled?
   2. I press button 1 on the DK, the average current went down to 3uA.
   3. I press button 1 again, it seems to wake the nRF9151 up, with average current about 500uA again, but I lost all network configurations.
   4. Questions:
      1. What does button 1 do exactly? Any documents that describe exactly what it does?
      2. What is the proper way to go into deep power save mode, assuming I am using this Serial LTE Modem firmware in my final product?
      3. Is there a even lower power mode that the button 1 method? i.e. average current < 1uA maybe?
3. There are some instances, when I leave the DK alone for maybe 10 mins, then I tried to send a AT command, the DK is not responsive in the sense it did not send back "OK" or network messages. However, my AT command was executed (based on the current profile of the Power Profiler II). Is this a know issue? Below image shows this error. In the image, bottom right, a few AT commands are sent but there were not response received.

[Simon D-M]

Hi,

Is this the product code that I should be using for my final product?

I'm not sure to understand what you are asking here... If you are asking if the serial modem add-on (previously called "Serial LTE Modem (SLM)") is the right firmware to flash on the nRF9151 if you want to use as a modem-only solution; yes it is. However, if you want to use it with a host MCU, you probably want to use the devicetree overlay "overlay-external-mcu.overlay" file.

What does button 1 do exactly? Any documents that describe exactly what it does?

The button is connected to the UART DTR pin. All the documentation about the UART is here.

What is the proper way to go into deep power save mode, assuming I am using this Serial LTE Modem firmware in my final product?

All our normal power saving techniques (link) still applies for the Serial Modem add-on.

I don't know how relevant it is but the nRF9151 is not only a modem, it has an application MCU. If the embedded MCU of the nRF9151 is enough for your needs you don't need a host MCU which can help you save power.

Is there a even lower power mode that the button 1 method? i.e. average current < 1uA maybe?

There are the "System OFF" and "System Disabled" mode, but it greatly depends on your application, as with "System OFF", the core system functionality is powered down and ongoing tasks terminated, and only the reset and the wake-up functions are available and responsive. And with "System Disabled" the chip is basically fully shut down. You can find the documentation about these modes here.

There are some instances, when I leave the DK alone for maybe 10 mins, then I tried to send a AT command, the DK is not responsive in the sense it did not send back "OK" or network messages. However, my AT command was executed (based on the current profile of the Power Profiler II). Is this a know issue? Below image shows this error. In the image, bottom right, a few AT commands are sent but there were not response received.

This is not an issue that I know of. I wasn't able to replicate it. Can you please document what you did a bit better so that I can try to replicate it?

Best regards,

Simon D-M

[Medical Device]

Hi Simon,

Thanks for your reply.

I have a external host MCU that will communicate with nRF9151 using AT commands. I want to use nRF9151 as a Serial LTE Modem. Meaning I do not want to modify any of the firmware (application and modem). I will just program whatever production-certified modem firmware and application firmware without any recompilation, which means I won't need to use overlay-external-mcu.overlay. Is this possible?

I am able to get the modem to PSM but the current is not low enough as the application MCU is still operating. Based on my use case, I won't be able to modify any nRF9151 internal registers directly, unless it is accessible through AT commands. Are there AT commands that set nRF9151 to deep sleep? I understand I can wake it up from deep sleep using some GPIOs. Which GPIOs have the wake up function besides the DTR pin and Enable pin?

Also I need your advise on the power saving. If my device only transmit once a day, will setting the nRF9151 to deep sleep be more power efficiency, or disabling it using the ENABLE pin be better? I am not sure what is the overhead and the extra current required to initialize nRF9151 when I toggle the ENABLE pin to wake it up.

For the error, let me try to record the steps to reproduce it and post it here.

Thanks,

Alex.

[Simon D-M]

Hi Alex,

Medical Device said:

I have a external host MCU that will communicate with nRF9151 using AT commands. I want to use nRF9151 as a Serial LTE Modem. Meaning I do not want to modify any of the firmware (application and modem). I will just program whatever production-certified modem firmware and application firmware without any recompilation, which means I won't need to use overlay-external-mcu.overlay. Is this possible?

Yes it is possible, we have a version already compiled with the "overlay-external-mcu.overlay" on the Github page of the application.

Medical Device said:

I am able to get the modem to PSM but the current is not low enough as the application MCU is still operating. Based on my use case, I won't be able to modify any nRF9151 internal registers directly, unless it is accessible through AT commands. Are there AT commands that set nRF9151 to deep sleep? I understand I can wake it up from deep sleep using some GPIOs. Which GPIOs have the wake up function besides the DTR pin and Enable pin?

All the AT commands usable with the Serial modem add-on are split between the "normal AT commands" and the "custom AT commands".

The command to get into different power modes are %XSLEEP and %XSHUTDOWN. 

I believe that every GPIO pins can trigger the wake-up action. There is more information about this in the hardware design guidelines for the nRF9151.

Medical Device said:

Also I need your advise on the power saving. If my device only transmit once a day, will setting the nRF9151 to deep sleep be more power efficiency, or disabling it using the ENABLE pin be better? I am not sure what is the overhead and the extra current required to initialize nRF9151 when I toggle the ENABLE pin to wake it up.

I believe that if your data sending frequency is only once a day, it might be better to disabling the chip using the ENABLE pin. However, it is very much a case to case scenario. You probably want to try and measure multiple solutions and compare them to see what is actually better.

Best regards,

Simon D-M

[Medical Device]

Hi Simon,

Thanks for your reply.

'Yes it is possible, we have a version already compiled with the "overlay-external-mcu.overlay" on the Github page of the application.'

Great that there is an 'off the shelf' hex file that we can use immediately without any coding and compilation required.

• Is this hex file the same one used in nRF Connect Desktop, Serial Modem? i.e. Is the one used in the quick start in nRF Connect Desktop, serial modem, compiled with "overlay-external-mcu.overlay"?
• Is this hex file for the application MCU only? Or does it also includes the modem firmware/stack? Do I need a separate hex file, i.e. 1 for application MCU and another for modex?
• Can I use this hex file in the DK? Will it have any hardware conflict?

'I believe that every GPIO pins can trigger the wake-up action. There is more information about this in the hardware design guidelines for the nRF9151.'

Thanks for the reference to the guidelines. I believe this requires a bit of customization to set a particular GPIO pin as the wakeup pin. With the context of just using the standard serial modem hex file, is there currently a pin that is already configured for this purposes? Is it DTR, P0.31 (active low, pull-up)?

Additional questions:

1. I am a bit confused on all the different AT commands. I believe each of the type of commands are for different use scenarios or hosts. Considering the use case of external MCU sending commands to the nRF9151 as Serial Modem, what is the exact commands that the external MCU needs to send? Will it be something like AT+CFUN? Or  AT%.... Or AT#.... Or just the command without AT, like ATE1?
2. https://docs.nordicsemi.com/bundle/addon-serial_modem-latest/page/app/at_generic.html
   1. For the commands listed in the above link, is it application to my use case? (i.e. standard serial modem using AT commands from external MCU).
   2. I tried sending commands like ATE1, ATE0 to the DK from the PC serial terminal but it didn't wake. Does the debugger (U3 - nRF5340) interpret these commands?
   3. If I send these commands directly to nRF9151 instead of through the debugger, will it work? If not, how do I use these commands.
   4. Similar questions for AT commands starting with '#'. Does the debugger interpret these commands? Will it work if I send these command directly to nRF9151 instead of going through the debugger, will it work? And what are these '#' commands for and how are these command being used?
3. https://docs.nordicsemi.com/bundle/addon-serial_modem-latest/page/uart_configuration.html. In this link, under 'Serial Modem application (nRF9151)', there are 'USB UART with PC host' and 'external MCU as host'.
   1. Are these applicable for the Serial Modem application of the DK? From the DK schematic 'PCA10171_Schematic_And_PCB', the pin assignment does not match those defined in the above link.
   2. If I want to use the pins allocated for USB UART with PC Host in my final design, can it be done?
4. https://docs.nordicsemi.com/bundle/ug_nrf9151_dk/page/UG/nrf91_DK/hw_description/sip_enable.html.
   1. This link says: The nRF9151 System in Package (SiP) can be enabled by pulling pin 101 high or disabled by pulling pin 101 low. By default, the enable signal is pulled high by resistor R1. Where is pin 101? The schematic of the DK shows it is connected to pin 10 of nRF9151.
   2. Also, the DK seems to be going into sleep/active mode by pressing Switch 1, and based on the schematic, switch 1 is connected to the DK connector P10 pin 1, so I guess 101 = P10.1? Assuming switch 1 is connected to the ENABLE pin of nRF9151, the behavior is not as per the datasheet. The nRF9151 datasheet mentioned this pin is level triggered, but on the DK, it seems to be edge triggered. I need to press and release switch 1 in order to toggle between active and sleep mode. So should it be a edge or level trigger pin to switch between Enable and Disable mode?

Thanks,

Alex.

[Simon D-M]

Hi Alex,

Medical Device said:

Is this hex file the same one used in nRF Connect Desktop, Serial Modem? i.e. Is the one used in the quick start in nRF Connect Desktop, serial modem, compiled with "overlay-external-mcu.overlay"?

No, the one flashed by the "Quick Start" application from nRF Connect for Desktop is not compiled with the overlay. It may also be a few versions older than the one on the GitHub.

Medical Device said:

Is this hex file for the application MCU only? Or does it also includes the modem firmware/stack? Do I need a separate hex file, i.e. 1 for application MCU and another for modex?

In our nRF91 chips, we have 2 cores: The Application core, which contains the Serial Modem add-on in your case. And a modem core which contains the modem firmware. They are both programmable independently.

The .hex you download are only for the application core. If you want to update the modem core, you can follow this guide. You can also download the modem firmware on the nRF9151 product page, in the "Download" section. (if you download it from the product page, do not unzip the file before programming. You need to give the whole zip to the "programmer" app).

Medical Device said:

Can I use this hex file in the DK? Will it have any hardware conflict?

This .hex file was compiled for the nRF9151 DK. Here is some more information about the UART configuration for the Serial modem add-on.

Medical Device said:

Thanks for the reference to the guidelines. I believe this requires a bit of customization to set a particular GPIO pin as the wakeup pin. With the context of just using the standard serial modem hex file, is there currently a pin that is already configured for this purposes? Is it DTR, P0.31 (active low, pull-up)?

I don't think that by default, there are any wake-up pins configured. The DTR pin can wake the chip up when it is in low power mode, but cannot wake it up when it is in "System OFF" mode. For the DTR part, it is documented in here.

Medical Device said:

I am a bit confused on all the different AT commands. I believe each of the type of commands are for different use scenarios or hosts. Considering the use case of external MCU sending commands to the nRF9151 as Serial Modem, what is the exact commands that the external MCU needs to send? Will it be something like AT+CFUN? Or  AT%.... Or AT#.... Or just the command without AT, like ATE1?

I would recommend you taking our "Cellular IoT Fundamentals" course on DevAcademy. It explains most of what you are asking here in the Lesson 2.

Medical Device said:

https://docs.nordicsemi.com/bundle/addon-serial_modem-latest/page/app/at_generic.html

1. For the commands listed in the above link, is it application to my use case? (i.e. standard serial modem using AT commands from external MCU).
2. I tried sending commands like ATE1, ATE0 to the DK from the PC serial terminal but it didn't wake. Does the debugger (U3 - nRF5340) interpret these commands?
3. If I send these commands directly to nRF9151 instead of through the debugger, will it work? If not, how do I use these commands.
4. Similar questions for AT commands starting with '#'. Does the debugger interpret these commands? Will it work if I send these command directly to nRF9151 instead of going through the debugger, will it work? And what are these '#' commands for and how are these command being used?

When you connect to the Serial terminal, you can consider the debugger as a USB-UART converter. It does not look at what is being sent, it acts as a pass-through. So, when you'll connect directly to the nRF9151 UART it will be the exact same.

Most of your question about AT commands are explained in the DevAcademy course I recommended you earlier. Try to take the course, you should get a better understanding of how to use AT commands. If you still have question after that, feel free to ask them. Also, you can ask our AI assistant (blue circle bottom left of your screen), It works very well for questions that we documented.

Medical Device said:

https://docs.nordicsemi.com/bundle/addon-serial_modem-latest/page/uart_configuration.html. In this link, under 'Serial Modem application (nRF9151)', there are 'USB UART with PC host' and 'external MCU as host'.

1. Are these applicable for the Serial Modem application of the DK? From the DK schematic 'PCA10171_Schematic_And_PCB', the pin assignment does not match those defined in the above link.

I'm not sure to follow you here, I believe they are matching...

Medical Device said:

If I want to use the pins allocated for USB UART with PC Host in my final design, can it be done?

If you do your own board, yes, you can use these pins. But even if, from what I understood, you don't really want to do any programming, I would still recommend you to make some modification to the application at least to map the pins like you want. And to build your own version of the application.

Medical Device said:

This link says: The nRF9151 System in Package (SiP) can be enabled by pulling pin 101 high or disabled by pulling pin 101 low. By default, the enable signal is pulled high by resistor R1. Where is pin 101? The schematic of the DK shows it is connected to pin 10 of nRF9151.

Oh, good catch! The documentation is wrong here, it has most likely been copied from the nRF9161 or the nRF9160, which both have the "ENABLE" pin on pin 101. For the nRF9151, It is indeed pin 10 on the chip that has to be used.

Medical Device said:

Also, the DK seems to be going into sleep/active mode by pressing Switch 1, and based on the schematic, switch 1 is connected to the DK connector P10 pin 1, so I guess 101 = P10.1? Assuming switch 1 is connected to the ENABLE pin of nRF9151, the behavior is not as per the datasheet. The nRF9151 datasheet mentioned this pin is level triggered, but on the DK, it seems to be edge triggered. I need to press and release switch 1 in order to toggle between active and sleep mode. So should it be a edge or level trigger pin to switch between Enable and Disable mode?

No, this is not the ENABLE pin. The button is connected to a normal GPIO. All the power management done with this pin is done and configured in the application firmware.

Best regards,

Simon D-M

[Medical Device]

Hi Simon,

"This .hex file was compiled for the nRF9151 DK. Here is some more information about the UART configuration for the Serial modem add-on.".

The UART pin configuration shown in the link is different from the schematic of the DK. Can it still work?

I did went through lesson 2. It did not mention anything related to AT# commands. I know where to find these commands on the Nordic website but some AT# commands is able to run (eg AT#XCLAC), some can't (eg AT#XSLEEP), and can only be recognized before enabling the radio (AT#XCLAC). Once radio is enabled with AT+FUNC=1, this AT#XCLAC command cannot be recognized anymore. I can't find any documentation on this.

'Oh, good catch! The documentation is wrong here, it has most likely been copied from the nRF9161 or the nRF9160, which both have the "ENABLE" pin on pin 101. For the nRF9151, It is indeed pin 10 on the chip that has to be used.'

Is there any way to access the enable pin? Can find any connection from the enable pin to any connectors. What does the red X in the DK schematic on the enable pin represent?

Thanks,

Alex.

[Simon D-M]

Hi Alex,

Medical Device said:

The UART pin configuration shown in the link is different from the schematic of the DK. Can it still work?

You are correct again... The pins in the documentation are wrong! The right ones are the ones you see on the datasheet (UART1). However, only the documentation is wrong. The firmware is configured to use the right pins.

Honestly, it is a bit strange that we didn't catch that earlier. Sorry for the wrong documentation.

Medical Device said:

I did went through lesson 2. It did not mention anything related to AT# commands. I know where to find these commands on the Nordic website but some AT# commands is able to run (eg AT#XCLAC), some can't (eg AT#XSLEEP), and can only be recognized before enabling the radio (AT#XCLAC). Once radio is enabled with AT+FUNC=1, this AT#XCLAC command cannot be recognized anymore. I can't find any documentation on this.

The AT commands with # are custom AT command that are interpreted by the application. They are implemented by the Serial Modem Add-on. You cannot use them in any other firmware.

For the AT#XCALC, I was able to use it fine... Can you please share some logs? Here are my logs:

Ready
> AT+CFUN?

+CFUN: 0

OK
> AT#XCLAC

ATE0
ATE1
AT#XAPOLL
AT#XBIND
AT#XCLAC
AT#XCLOSE
AT#XCONNECT
AT#XDATACTRL
AT#XDFUAPPLY
AT#XDFUINIT
AT#XDFUWRITE
AT#XFOTA
AT#XGETADDRINFO
AT#XGNSS
AT#XGNSSDEL
AT#XMODEMRESET
AT#XMQTTCFG
AT#XMQTTCON
AT#XMQTTPUB
AT#XMQTTSUB
AT#XMQTTUNSUB
AT#XNRFCLOUD
AT#XNRFCLOUDPOS
AT#XPING
AT#XRECV
AT#XRECVCFG
AT#XRECVFROM
AT#XRESET
AT#XSEND
AT#XSENDTO
AT#XSHUTDOWN
AT#XSLEEP
AT#XSMS
AT#XSMVER
AT#XSOCKET
AT#XSOCKETOPT
AT#XSSOCKET
AT#XSSOCKETOPT
AT#XUUID

OK
> AT+CFUN=1

OK
> AT#XCLAC

ATE0
ATE1
AT#XAPOLL
AT#XBIND
AT#XCLAC
AT#XCLOSE
AT#XCONNECT
AT#XDATACTRL
AT#XDFUAPPLY
AT#XDFUINIT
AT#XDFUWRITE
AT#XFOTA
AT#XGETADDRINFO
AT#XGNSS
AT#XGNSSDEL
AT#XMODEMRESET
AT#XMQTTCFG
AT#XMQTTCON
AT#XMQTTPUB
AT#XMQTTSUB
AT#XMQTTUNSUB
AT#XNRFCLOUD
AT#XNRFCLOUDPOS
AT#XPING
AT#XRECV
AT#XRECVCFG
AT#XRECVFROM
AT#XRESET
AT#XSEND
AT#XSENDTO
AT#XSHUTDOWN
AT#XSLEEP
AT#XSMS
AT#XSMVER
AT#XSOCKET
AT#XSOCKETOPT
AT#XSSOCKET
AT#XSSOCKETOPT
AT#XUUID

OK
> AT+CFUN=0

OK
> AT#XCLAC

ATE0
ATE1
AT#XAPOLL
AT#XBIND
AT#XCLAC
AT#XCLOSE
AT#XCONNECT
AT#XDATACTRL
AT#XDFUAPPLY
AT#XDFUINIT
AT#XDFUWRITE
AT#XFOTA
AT#XGETADDRINFO
AT#XGNSS
AT#XGNSSDEL
AT#XMODEMRESET
AT#XMQTTCFG
AT#XMQTTCON
AT#XMQTTPUB
AT#XMQTTSUB
AT#XMQTTUNSUB
AT#XNRFCLOUD
AT#XNRFCLOUDPOS
AT#XPING
AT#XRECV
AT#XRECVCFG
AT#XRECVFROM
AT#XRESET
AT#XSEND
AT#XSENDTO
AT#XSHUTDOWN
AT#XSLEEP
AT#XSMS
AT#XSMVER
AT#XSOCKET
AT#XSOCKETOPT
AT#XSSOCKET
AT#XSSOCKETOPT
AT#XUUID

OK

Medical Device said:

Is there any way to access the enable pin? Can find any connection from the enable pin to any connectors. What does the red X in the DK schematic on the enable pin represent?

There is no good way to have access to the ENABLE pin in the nRF9151DK, unfortunately.

The red X in the schematic is just a warning from Altium. (As the ENABLE pin is set as an input pin, it detects that no output pin have been connected to it and make a warning.) It can be completely ignored in that scenario.

Best regards,

Simon D-M

[Medical Device]

Hi SImon,

Thanks again for your replies. How can I generate the box of the log like you did?

Some follow up and additional clarifications.

Some how after I tried to program the serial_modem_v0.3.0_nrf9151dk_ext_mcu.hex" firmware and then back to the nRF Connect AT command firmware, it works. Anyway I see some difference in the log.

> AT#XCLAC

AT#XACCEPT
AT#XBIND
AT#XCLAC
AT#XCONNECT
AT#XDATACTRL
AT#XFOTA
AT#XFTP
AT#XGETADDRINFO
AT#XGPIO
AT#XGPIOCFG
AT#XGPS
AT#XGPSDEL
AT#XHTTPCCON
AT#XHTTPCREQ
AT#XLISTEN
AT#XMODEMRESET
AT#XMQTTCFG
AT#XMQTTCON
AT#XMQTTPUB
AT#XMQTTSUB
AT#XMQTTUNSUB
AT#XNRFCLOUD
AT#XNRFCLOUDPOS
AT#XPING
AT#XPOLL
AT#XRECV
AT#XRECVFROM
AT#XRESET
AT#XSEND
AT#XSENDTO
AT#XSHUTDOWN
AT#XSLEEP
AT#XSLMVER
AT#XSMS
AT#XSOCKET
AT#XSOCKETOPT
AT#XSOCKETSELECT
AT#XSSOCKET
AT#XSSOCKETOPT
AT#XTCPCLI
AT#XTCPHANGUP
AT#XTCPSEND
AT#XTCPSVR
AT#XTFTP
AT#XUDPCLI
AT#XUDPSEND
AT#XUDPSVR
AT#XUUID

OK

Did you generate this list from the DK? What is the application hex you used?

'You are correct again... The pins in the documentation are wrong! The right ones are the ones you see on the datasheet (UART1). However, only the documentation is wrong. The firmware is configured to use the right pins.'

Alex: By datasheet do you mean the DK schematic? There is no datasheet for the DK and I don't think the hardware guide show which pins are used.

'You can also download the modem firmware on the nRF9151 product page, in the "Download" section.'

Alex: This link points to "mfw_nrf91x1_2.0.4" which is modem firmware ver 2.0.4.

'If you want to update the modem core, you can follow this guide.'

Alex: This link points to the update guide as well as a link to download firmware for the DK (nRF9151 DK Downloads) with an older ver 2.0.2. I assume I should ignore the firmware in this link correct?

And the PTCRB cert indicates "mfw_nrf91x1_2.0.2". ver 2.04 is not certified by PTCRB yet. Shoudn't I be using ver 2.0.2? Is ver 2.0.4 currently under approval process?

Also, the AT Command file "nrf91x1_cellular_at_commands_v1.4" indicates only update to modem firmware ver 2.0.3. Are the AT commands still applicable for ver 2.0.4?

Also, I want to revisit the Application MCU firmware of the nRF9151DF again. In nRF Connect for Desktop, I ran quick start, under "select an application to run", I choose "AT commands". What exactly is the MCU firmware used? I read somewhere there AT command mode is not Serial Modem Mode? AT command mode is sending AT command directly to the Modem Firmware (AT command pass thru mode), and AT# commands are not supported? While Serial Modem mode, the AT commands are sent to the MCU firmware, which can interpret AT# commands. Is this correct? This is quick confusing. Is there anywhere that describe the different behavior and maybe dataflow or block diagram between PC/external MCU, nRF9151 MCU firmware, and nRF9151 modem?

So again, I just to talk to nRF9151 as a whole, using AT commands. Which mode should I use for my final product without the need to modify the nRF9151 firmware, be it the MCU app firmware or the modem firmware. Should it be the AT command "pass thru" firmware or the Serial Modem firmware?

By the way, I programmed "serial_modem_v0.3.0_nrf9151dk_ext_mcu.hex" into the DK using 'erase and write', I encountered error. I retried again and it was successful. See below log:

2026-01-31T03:44:48.427Z INFO Recovering Application core
2026-01-31T03:44:48.428Z INFO Recovering Application core 0%
2026-01-31T03:44:48.499Z INFO Recovering Application core 50%
2026-01-31T03:44:48.499Z INFO Recovering Application core 50%
2026-01-31T03:44:49.736Z INFO Recovering Application core 100%
2026-01-31T03:44:49.736Z INFO Recovering Application core completed
2026-01-31T03:44:49.736Z INFO Loading core information for Application core
2026-01-31T03:44:49.738Z INFO Update files regions according to Application core
2026-01-31T03:44:49.738Z INFO Parse memory regions for file
2026-01-31T03:44:49.740Z INFO Loading core information for Application core 0%
2026-01-31T03:44:49.740Z INFO Loading core information for Application core 100%
2026-01-31T03:44:49.741Z INFO Loading core information for Application core completed
2026-01-31T03:44:49.742Z INFO Update files regions according to Application core
2026-01-31T03:44:49.742Z INFO Parse memory regions for file
2026-01-31T03:44:50.263Z INFO Writing HEX to Application core
2026-01-31T03:44:50.264Z INFO Writing HEX to Application core 0%
2026-01-31T03:44:50.364Z INFO Writing HEX to Application core 33%
2026-01-31T03:44:50.485Z INFO Writing HEX to Application core 33%
2026-01-31T03:44:50.797Z INFO Writing HEX to Application core 67%
2026-01-31T03:44:50.797Z INFO Writing HEX to Application core 67%
2026-01-31T03:44:52.886Z ERROR Failed "writing hex to application core". Error: code: 1, description: Generic, message: Batch task program failed, Device error: NVMC Failed to write to NVMC.CONFIGNS: failed: Failed to read from memory address 0x50003100: Failed to write DebugPort register 0x08: Unknown error in the J-Link DLL (error code =-1797082992)
2026-01-31T03:45:36.199Z INFO Recovering Application core
2026-01-31T03:45:36.200Z INFO Recovering Application core 0%
2026-01-31T03:45:36.264Z INFO Recovering Application core 50%
2026-01-31T03:45:36.264Z INFO Recovering Application core 50%
2026-01-31T03:45:37.487Z INFO Recovering Application core 100%
2026-01-31T03:45:37.488Z INFO Recovering Application core completed
2026-01-31T03:45:37.488Z INFO Loading core information for Application core
2026-01-31T03:45:37.488Z INFO Loading core information for Application core 0%
2026-01-31T03:45:37.489Z INFO Update files regions according to Application core
2026-01-31T03:45:37.489Z INFO Parse memory regions for file
2026-01-31T03:45:37.491Z INFO Loading core information for Application core 100%
2026-01-31T03:45:37.491Z INFO Loading core information for Application core completed
2026-01-31T03:45:37.492Z INFO Update files regions according to Application core
2026-01-31T03:45:37.492Z INFO Parse memory regions for file
2026-01-31T03:45:37.900Z INFO Writing HEX to Application core
2026-01-31T03:45:37.900Z INFO Writing HEX to Application core 0%
2026-01-31T03:45:37.965Z INFO Writing HEX to Application core 33%
2026-01-31T03:45:38.033Z INFO Writing HEX to Application core 33%
2026-01-31T03:45:38.322Z INFO Writing HEX to Application core 67%
2026-01-31T03:45:38.322Z INFO Writing HEX to Application core 67%
2026-01-31T03:45:40.333Z INFO Writing HEX to Application core 83%
2026-01-31T03:45:40.333Z INFO Writing HEX to Application core 100%
2026-01-31T03:45:40.338Z INFO Writing HEX to Application core completed
2026-01-31T03:45:41.006Z INFO Reading readback protection status for Application core
2026-01-31T03:45:41.007Z INFO Reading readback protection status for Application core 0%
2026-01-31T03:45:41.072Z INFO Reading readback protection status for Application core 50%
2026-01-31T03:45:41.073Z INFO Reading readback protection status for Application core 50%
2026-01-31T03:45:41.074Z INFO Reading readback protection status for Application core 100%
2026-01-31T03:45:41.074Z INFO Application core protection status 'NRFDL_PROTECTION_STATUS_NONE'
2026-01-31T03:45:41.075Z INFO Reading readback protection status for Application core completed

I reset the DK, there were no initial messages which I were there when I use the AT command settings from the nRF Connect, and sending "AT", there is no response.

I use the nRF Connect to download the AT commands firmware and it works again. So it seems like the UART pin definition on the schematic is not the same as the "serial_modem_v0.3.0_nrf9151dk_ext_mcu.hex" firmware?

Also, I assume that firmware for EXT MCU can also be used on the DK using the Debug IC (i.e connecting the DK to PC using the USB cable and sending commands through the PC serial terminal)?