How to disable JTAG on i.MX 8M?

[christian_mauderer]

The i.MX 8M reference manual mentions a JTAG_SMODE[1:0] and a SJC DISABLE e-fuse that are necessary to disable JTAG access. But I can't find the address or location of the fuses. Where can I find them? Is there some document describing it?

[Nocker]

Hi,

I have the exact same question, can someone let me know the answer please?

And I agree with what someone else has posted, it is not very useful when the answer results in a private email.

Regards,

Chris

[bernhardfink · Accepted solution]

Here is the description of these fuse bits:

Fuse Address	Fuse Name	Function	Setting
0x470[21]	SJC_DISABLE	Disable/Enable the Secure JTAG Controller module. This fuse is used to create highest JTAG security level, where JTAG is totally blocked.	0 - Secure JTAG Controller is enabled 1 - Secure JTAG Controller is disabled
0x470[23:22]	JTAG_SMODE[1:0]	JTAG Security Mode. Controls the security mode of the JTAG debug interface	00 - JTAG enable mode 01 - Secure JTAG mode 1x - No debug mode

[Bhardwaj]

Hi,

When we are read the bank 1 word 3 in I.MX8MPLUS after the U-BOOT ,we observe the below value.

Does it means ,Is JTAG is Disable?

You said that  SJC_DISABLE  and JTAG_SMODE are set high ,that means JTAG is Disable.

Does the I.MX8M  fuse address are applicable for I.MX8MPLUS ?.

Fuse Address	Fuse Name	Function	Setting
0x470[21]	SJC_DISABLE	Disable/Enable the Secure JTAG Controller module. This fuse is used to create highest JTAG security level, where JTAG is totally blocked.	0 - Secure JTAG Controller is enabled 1 - Secure JTAG Controller is disabled
0x470[23:22]	JTAG_SMODE[1:0]	JTAG Security Mode. Controls the security mode of the JTAG debug interface	00 - JTAG enable mode 01 - Secure JTAG mode 1x - No debug mode

[bernhardfink]

You seem to have a closed part. All 3 relevant bits are set. Bit definitions and the location are valid for i.MX 8M Plus.

Make a crosscheck with a debugger, if you don't see any core appearing, then the JTAG is definitely closed.

Regards,
Bernhard.

[Bhardwaj]

1.How to enable JTAG Mode in I.MX8MPLUS ? 

2.Is there any possibility to override that register in I.MX8MPLUS ? for I.MX6 a bit is mentioned to override JTAG_SMODE by setting the BOOT_CFG_LOCK bits as shown below?.

[bernhardfink]

Closed is closed. If there would a way to re-open it, the closed configuration wouldn't make sense. The OTP bits are really just one-time-programmable.

The alternative way to access the chip is the Serial Download Protocol (SDP) on the USB port.

According to your readout, bit[28] = BT_FUSE_SEL is not set, so the device can still boot taking a configuration from external BOOT_MODE pins.

Regards,
Bernhard.

[M_J]

Hi @igorpadykov 

Could you forward to me details as well?

Kind regards,
Marcin

[bernhardfink]

Meanwhile these fuse bits have been documented in the Security Reference Manuals of 8M Nano and 8M Plus. It's the same structure as in 8M and 8M Mini.

I will check

• Why we haven't updated it for 8M Mini and 8M yet
• Why it is not part of the OTP fuse description in the normal Reference Manual

Regards,
Bernhard.

[igorpadykov]

Hi Christian

one can look at

Understanding how to burn eFuses on i.MX8 and i.MX8x families - i.MXDev Blog 

https://community.nxp.com/thread/505184 

Best regards
igor
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[christian_mauderer]

Hello Igor,

thanks for the reply. My problem is not how to set a fuse in general. It's more the problem where this specific fuses are located.

For example: The reference manual tells me that BT_FUSE_SEL is at 0x470[28] and it tells me that this is bank 1 word 3. But it doesn't tell me a location for JTAG_SMODE[1:0] and SJC DISABLE. For i.MX6 or i.MX7 there is another document that tells me the location (https://www.nxp.com/docs/en/application-note/AN4581.pdf ). But I didn't find one for i.MX8.

Best regards

Christian

[igorpadykov]

Hi Christian

I sent additional details by mail.

Best regards
igor

[xuxa]

Can you also send me the additional details?

Thank you.

[bernhardfink]

I marked my answer from last year as solution, so that it's easier to find.

[marcusfolkesson]

Please post the solution here instead of sending it to one single recipient. There are too many threads here that end with "I sent you a mail". The thread is useless for everyone but the thread creator.

I would like to have this information as well.

Thanks,
Marcus

[tgs]

Hi Igor

Can you please send me the same information as well.

Thanks in advance,

TG

[marcelziswiler]

Thanks, igorpadykov, very helpful to answer this by mail!

How about others like us who are also wondering about all this?

[christian_mauderer]

Hello Igor,

thank you. That solved my question.

Best regards

Christian