topic Re: Custom MCXA156VLH board always power-up in ISP mode in MCX Microcontrollers

Custom MCXA156VLH board always power-up in ISP mode

rj_engineer — Wed, 03 Sep 2025 14:12:03 GMT

Following the MCXA14x/15x hardware design guide and FRDM-MCXA156 schematics, we designed a board with the MCXA156VLH. However, the MCU always enter ISP after power-up ignoring the ISP_MODE pin logic level.

Here are the relevant schematics :

We checked multiple times that the ISP_MODE pin is P3_29 on MCXA156VLH (the P0_6 pin is only routed to a pull-up).

Here are our observations :

The ISP_MODE pin is at 3V3 after Power-up
The MCU enters Master boot after a warm reset (button, debug, blhost)
The ISP_MODE stays at 3V3 after soft reset
We see the normal rise and fall (capacitor charging) on the RESET and ISP_MODE pins during power-up and button press
interestingly, connecting the PE-Micro Multilink universal debug probe (even not powered) launch the user code (we suspect a reset)

We would greatly appreciate if you could give us any advice on the source of this error.

Sincerely,

Re: Custom MCXA156VLH board always power-up in ISP mode

Alice_Yang — Thu, 04 Sep 2025 10:01:15 GMT

Hello @rj_engineer

If there is no valid image in the internal flash, the MCU will enter ISP mode after a reset.

Please first download a valid image to the flash for testing.

If it still fails to operate normally, please send your schematic in PDF format. Additionally, please tell me how do you determine whether it has entered ISP mode.

Thank you.

Alice

Re: Custom MCXA156VLH board always power-up in ISP mode

rj_engineer — Thu, 04 Sep 2025 11:42:46 GMT

Hello Alice,

Using MCUXpresso IDE (and a debug probe), i flashed the SDK "dev_cdc_vcom_bm" and can interact with the board through serial terminal. That's why, i consider having a valid image. However, after power-cycling, the board enters in ISP mode.
I determine being in ISP because i can use blhost to interact with the board without a debugger.
Please find attached the relevant schematic in PDF format as well as the layout just in case

Re: Custom MCXA156VLH board always power-up in ISP mode

Alice_Yang — Fri, 05 Sep 2025 08:20:37 GMT

Hi @rj_engineer

Please remove the TVS connected to ISP pin to have a try.

Alice

Re: Custom MCXA156VLH board always power-up in ISP mode

rj_engineer — Fri, 05 Sep 2025 08:25:45 GMT

Hi Alice,

I removed the TVS and the behaviour remains the same, the board power-up in ISP mode.

Re: Custom MCXA156VLH board always power-up in ISP mode

Alice_Yang — Mon, 08 Sep 2025 04:13:38 GMT

Hi @rj_engineer

Using MCUXpresso IDE (and a debug probe), i flashed the SDK "dev_cdc_vcom_bm" and can interact with the board through serial terminal. That's why, i consider having a valid image. However, after power-cycling, the board enters in ISP mode.
I determine being in ISP because i can use blhost to interact with the board without a debugger.

->> Please create a video that demonstrates all the steps. Use the led_blinky demo (do not use USB cdc demo), and verify whether the LED continues blinking after power-cycling the board.

Thank you.

Alice

Re: Custom MCXA156VLH board always power-up in ISP mode

rj_engineer — Mon, 08 Sep 2025 07:23:14 GMT

Hi Alice,

Please find attached the requested video.

Re: Custom MCXA156VLH board always power-up in ISP mode

rj_engineer — Mon, 08 Sep 2025 12:27:35 GMT

Re: Custom MCXA156VLH board always power-up in ISP mode

Alice_Yang — Tue, 09 Sep 2025 04:06:29 GMT

Hi @rj_engineer

Thanks for your sharing.

It is strange,. Please measure and monitor the ISP pin, and also record a video of the process. Thank you.

Alice

Re: Custom MCXA156VLH board always power-up in ISP mode

rj_engineer — Tue, 09 Sep 2025 07:23:29 GMT

Hi Alice,

Please find attached the video with the following steps :

Activate the trigger of the oscilloscope on the ISP pin
Power-up the board
Observe the ISP rise curve
Activate the trigger of the oscilloscope on the ISP pin
Reset the board by shorting the Reset pin with Ground
Observe the ISP rise curve (a mistake might have shorted the ISP pin too)
Observe the LED blinking as the board left the ISP mode due to reset
Activate the trigger of the oscilloscope on the ISP pin
Press the ISP button (ISP short to ground) and release it (ISP back to 3V3)
Observe the ISP rise curve

Note : the oscilloscope is at 1V per square and 200us per square

Re: Custom MCXA156VLH board always power-up in ISP mode

Alice_Yang — Fri, 12 Sep 2025 03:19:16 GMT

Hi @rj_engineer

Thanks for your reply.

From your description, if the LED is blinking, I believe the board is working properly and has not entered ISP mode.

However, if I misunderstood and your board does enter ISP mode upon reset, I suspect that when you connected the RESET pin to GND, you may have also accidentally connected the ISP pin to GND.

You can try measuring both the ISP pin and RESET pin at the same time to check the state of the ISP pin during reset.

Thank you.

Alice

Re: Custom MCXA156VLH board always power-up in ISP mode

rj_engineer — Fri, 12 Sep 2025 06:40:09 GMT

Hi Alice,

It is the opposite, the board enters ISP mode at power-up and enter normal mode (master boot) after a reset.

Please find attached the ISP (yellow) and RESET (green) curve on power-up and after a software reset (blhost reset)

Re: Custom MCXA156VLH board always power-up in ISP mode

Alice_Yang — Mon, 15 Sep 2025 10:48:53 GMT

Hello @rj_engineer

From the image in your attachment, there are no issues with the RESET and ISP pins.
If there is no valid image in the flash, the device will enter ISP mode when powered on.

Alice

Re: Custom MCXA156VLH board always power-up in ISP mode

rj_engineer — Mon, 15 Sep 2025 11:12:34 GMT

Hi alice,

The image in the MCU is directly flashed by MCUXpresso Led_Blinky_example. I don't understand how it couldn't be valid.

An additional detail is that i use the OP29K revision of the MCXA156VLH.

Re: Custom MCXA156VLH board always power-up in ISP mode

Alice_Yang — Tue, 16 Sep 2025 10:48:56 GMT

Hello @rj_engineer

Thanks for your reply.

Yes, my FRDM-MCXA156 also uses the OP29K revision.

Do you have an FRDM-MCXA156 board? Does it work properly?

I've had you try all the possibilities I can think of, and I really don't know why this is happening. It might still be that the ISP pin is being pulled low during power supply. This is the only way to enter ISP mode when there's a valid image in the flash.

Alice

Re: Custom MCXA156VLH board always power-up in ISP mode

rj_engineer — Fri, 26 Sep 2025 07:35:31 GMT

Hi Alice,

We have found the root of our problem.

The P0_6 pin was pulled-down at boot by a 10k resistor. Removing the resistor make the board boot normally (led blinking), soldering it back make the board boot in ISP (led not blinking).

What puzzles us is that the "MCX A156, A155, A154, A146, A145, A144
Reference Manual" and the "UG10151 MCXA14x/15x Hardware Design Guide" clearly states that the ISP pin is P3_29 for LQFP64 and lower pin count (obviously including our MCXA156VLH OP29K), The P0_6 pin act as isp only for higher pin counts.

Page 123 of the reference manual :

Page 9 of the hardware guidelines :

Can you confirm that this behaviour is abnormal?

Re: Custom MCXA156VLH board always power-up in ISP mode

rj_engineer — Mon, 29 Sep 2025 07:00:23 GMT

To summarize what we observed so far :

On power-up, if P0_6 is low, the board will boot in ISP whether P3_29 is high or low, otherwise, it will always boot in normal mode (even if P3_29 is low)
On warm reset, if P3_29 is low BEFORE reset (not on reset release), the board will boot in ISP whether P0_6 is high or low, otherwise, it will always boot in normal mode

We also soldered the MCXA156VLH on the FRDM-MCXA153 dev board and observed the exact same behaviour.

Re: Custom MCXA156VLH board always power-up in ISP mode

rj_engineer — Mon, 13 Oct 2025 06:02:14 GMT

Hi Alice,

I think there is a misunderstanding.

While we Found the root of the problem, the observed behaviour is still abnormal.

As per the previously mentioned documentation, P0_6 shouldn't have any impact on the boot mode of the MCXA156VLH.

I look forward to continue investigating with NXP.

Sincerely

Re: Custom MCXA156VLH board always power-up in ISP mode

Alice_Yang — Tue, 14 Oct 2025 04:04:33 GMT

Hello @rj_engineer

Thank you for your reply.

I have tested on the FRDM-MCXA156 board, and found that only the P0_6 pin controls the ISP mode. As for the MCXA156VLH, I currently don't have a socket board for this chip, so I’ve created a private ticket to escalate the issue for further support.

Thanks for your understanding.

Alice

Re: Custom MCXA156VLH board always power-up in ISP mode

rj_engineer — Mon, 22 Dec 2025 10:37:03 GMT

Hi Alice,

Just to get an update on this case.

Is there a roadmap to solve the ISP issue on the hardware?

Sincerely