i.MX6DL - Boot behavior depends on state of USB_OTG_ID signal

[christian_neuwi]

Hi,

we're building our own hardware based on an i.MX6DL using an eMMC as primary boot device. The BOOT_MODE[1:0] pins are pulled low, i.e. 00b “Boot From Fuses”.

When such a board is in a 'virgin' state (i.e. no firmware installed and no fuses blown), the board behaves as expected as it will always enter "Serial Downloader" mode.

However, things change when a valid bootloader image is installed in eMMC. From that point onwards, "Serial Downloader" mode is entered only when the USB_OTG_ID signal is LOW. Otherwise the installed bootloader image is executed.

That happens even though we haven't programmed any fuses (meaning that also BT_FUSE_SEL = 0), but the iMX6DL reference manual clearly states that this shouldn't happen.

From chapter 8.2.3:

If set to Boot From Fuses, the boot flow is controlled by the BT_FUSE_SEL eFUSE
value. If BT_FUSE_SEL = 0, indicating that the boot device (for example, Flash, SD/
MMC) has not yet been programmed, the boot flow jumps directly to the Serial
Downloader.

What I'm looking for here is a confirmation from NXP that what we're seeing on our boards is actually the intended behavior of the i.MX6DL and the documentation is wrong or incomplete. Ideally, I'm looking for a reference to a different section in the reference manual (or some other document) that documents this behavior and - ideally - explains it.

We don't want to risk breaking our board production process at some point in the future should this behavior change w/o notice.

[igorpadykov · Accepted solution]

Chris, you are right this may be caused by GPIO_1 (ALT6) working as
SD1_CD (could be also USB_OTG_ID) as described in Table 4-2
Muxing Options, Table 8-18. SD/MMC IOMUX Pin Configuration
i.MX6SDL RM
http://cache.freescale.com/files/32bit/doc/ref_manual/IMX6SDLRM.pdf

Best regards
igor

[christian_neuwi]

Thanks, Igor.

[igorpadykov]

Hi Christian

behaviour may be caused by manufacture mode (enabled by default) ,

described in sect.8.11 SD/MMC Manufacture Mode i.MX6SDL Reference Manual:
boot will go to SD/MMC manufacture mode before serial download mode.
http://cache.freescale.com/files/32bit/doc/ref_manual/IMX6SDLRM.pdf

Best regards
igor
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[christian_neuwi]

Thanks for your response, Igor.

I also thought about SD/MMC manufacture mode, but why does the behavior depend on the state of the USB_OTG_ID signal?

Regards,

Chris.

[igorpadykov]

Hi Chris

probably your board bootloader uses this pin or it is

somewhere connected to other signals, which could affect behaviour.

ROM does not use it.

Best regards
igor

[dougplymale]

Hi igor,

From below, the pad of SD1_CD and SD2_CD are used to detect the card.  SD2_CD is also the USB_OTG_ID pin.  Could it be that we are really seeing the ROM check for the SD2_ID pin?

Thanks,

Doug

[christian_neuwi]

Igor,

Doug probably meant to say that SD1_CD_B and USB_OTG_ID share pin GPIO_01. Pin GPIO_01 also seems to be the only option for either of these functions. As such, it seems to be obvious that the Boot ROM's check for SD1_CD causes this behavior.

Can you confirm that?

Regards,

Chris.

[christian_neuwi]

Sorry, USB_OTG_ID can also use a different pad (i.e. ENET_RX_ER), but SD1_CD_B can't. USB_OTG_ID is connected to GPIO_01 on our hardware, though. Just to clarify that.