IMX6UL - Serial downloader works only after first power-up

[ehwsma]

Hi all,

We designed our custom board based on i.MX6UL processor, but I've got a strange behavior here :

• When I power-up the board for the first time (Blank processor and flash), the Serial Downloader feature is available from the USB port and I'm able to load a custom u-boot image to RAM.
  
  

  
  
  

• When I power the board OFF and ON (or use the "reset" command in u-boot), the HID USB interface for the Serial Downloader will not show again ! Instead, I'm having an "Unknown Device" appearing after a few seconds.
  
  
• If I power the board OFF again and wait a few minutes, the Serial Downloader feature is available back immediately when I power the board ON.

Would you have any explanation to this strange behavior? This is very annoying both for debugging and for production process, as it looks not being very stable...

Thanks in advance.

Best regards

serial downloader protocol‌ i.mx6ul‌

[igorpadykov]

Hi Sébastien

for reliable reboot without issues described above recommended to remove all power

from board as it is done on Sabre SD schematic spf-27392 using pmic pwron on p.19

Schematics (1)
"Design files, including hardware schematics, Gerbers, and OrCAD files"

SABRE Platform for Smart Devices|NXP 

Also it is necessary to verify that all power supplies capacitors were discharged

before powering-up board again, this is power-up sequence datasheet recommendation.

Best regards
igor
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[ehwsma]

Hi Igor, thanks for your reply.

We don't use a pmic but I don't think the power supply is the root cause of this. When I use the POR signal to restart the processor (all power supplies are correct), I keep having the issue ("Unknown Device" appears after ~15 secs).

I tried to investigate this a bit deeper, and found out that:

- some of our boards are NEVER impacted by this issue ("SE Blank 6UL" HID device always appearing correctly)

- some of our boards are impacted after first power-up (as described in first post)

- some of our boards are ALWAYS impacted by this issue (I never managed to get the "SE Blank 6UL" HID device appearing correctly).

Would you have any suggestion where to look after to find out what's happening?

Some precision:

• All the boards currently tested are "blank" processors
• Default Boot Mode "00" (eFuses) is configured by default (only internal Pull-Down on BOOT_MODE0 and BOOT_MODE1)
• Mounting 10k pull-up resistor to BOOT_MODE0 to enable "Serial Downloader" boot mode doesn't change anything
• eMMC is wired on USDHC1 and will be later used for boot
• Some pins of the LCD_DATA bus are connected to other slave devices, but no pull-up is used on those lines as eFuses are intended to be used
• I tried adding pull-down resistors on UART1_RX and UART2_RX lines with no change

Thanks in advance

BR

[igorpadykov]

Hi Sébastien

one can try to use latest script aids:

i.MX6ULL_LPDDR2_Script_Aid 

~igor

[ehwsma]

Unfortunately the issue occurs one step before... I can't load anything using the Mfgtool if the USB device is not available.

The script aids are not useful here.

Thanks

[igorpadykov]

actually script aids may be useful here, as many customers encountered similar issues when not resetting

whole power board. In particular were added add a Pre-Charge All and RD FIFO Pointer reset line commands for lpddr2/3 scripts.

However only whole power board reset may guarantee successful reset, as some modules can be initialized only once after power-up.

Best regards
igor