Entered into u-boot with high probability once powered up

[viper1]

IMX8MN entered into u-boot with high probability once powered up

1.Have you ever met this issue before?

2.Do i need to add some protective measures to UARTS? There were only two test points without anything connecting to UARTS when this issue occured.

3.Can you give some advices to avoid this issue?

[Yuri]

@viper1 
Hello,

   Please let me know more details about the issue.

What NXP Linux release is used in the case?

As for "with high probability once powered up" - does it mean
U-boot stops after boot (no kernel load) ?
Sometimes even U-boot is not loaded?

Have You tried the NXP memory test?

Regards,
Yuri.

[link]

Hi Yuri:

      The problem we encountered was that the system received serial port input during boot, which interrupted the normal startup process of the Uboot .

      The uboot we used is U-Boot 2020.04 and the Linux version is 5.4.70

U-Boot 2020.04-gbc8a51b (Feb 21 2022 - 17:11:20 +0800)

CPU:   i.MX8MNano Quad rev1.0 at 1200 MHz
Reset cause: POR
Model: NXP i.MX8MNano LPDDR4 EVK board
DRAM:  1 GiB
MMC:   FSL_SDHC: 1, FSL_SDHC: 2
Loading Environment from MMC... *** Warning - bad CRC, using default environment

In:    serial
Out:   serial
Err:   serial

 BuildInfo:
  - ATF f1d7187
  - U-Boot 2020.04-gbc8a51b

switch to partitions #0, OK
mmc2(part 0) is current device
Net:   No ethernet found.
Fastboot: Normal
Normal Boot
Hit any key to stop autoboot:  0 
u-boot=> gfhg
Unknown command 'gfhg' - try 'help'
u-boot=> 

[Yuri]

@link 
Hello,

   Do You use the NXP EVK or own board?
What terminal program is applied? 

Regards,
Yuri.

[link]

@Yuri 

    We found that when the board power is grounded, there is no problem.  When the board power is not grounded, there will be a probability of the above problems.

      During the Uboot phase, the RX of the UART is at low level and UART_TX is at high level .  We add a pull-up resistor to the UART_RX to change its initial state to high.  Problem disappear.

      We couldn't find a reasonable explanation for this problem.We wanted to know what might causie this problem ? Have you ever encountered similar problems ?

[Yuri]

@link 
Hello,

   You mentioned "when the board power is grounded, there is no problem".
This looks like as noisy environment influence because of non-proper 
board power supply unit. Please try to use other one. 

Regards,
Yuri

[viper1]

As i found in the datasheet, the default status of UART_RX is "input with PD".

Whether it is ok if i set UART_RX to "input with PU" with external resistance?

[Yuri]

@viper1 
Hello,

    the default status of UART_RX "input with PD" relates to state after power on / reset.
In Your case U-boot is already working, meaning that UART configuring is performed by U-boot. 

Regards,
Yuri.

[link]

@Yuri we use our own board and SecureCRT.