iMXRT1021 power on hang with MCUlink on UART1

[GREng]

I was experimenting with the suitability of the MCULink's VCOM to interact with the iMXRT1021 when it is in Serial Download mode. But experimenting on a MIMXRT1020-EVK, I found the iMXRT1021 will hang on powerup if the MCULink is attached and powered to UART1 as the MCU powers up. This is true regardless of the BOOT_MODE0/1 settings (Internal boot or serial download).

I must first power on the EVK without anything on UART1, to allow it to come up, then attach the MCULink.  This makes it inconvenient to leave the MCULink device attached while cycling power to the target.

My experimental set up is an MIMXRT1020-EVK (revA3):

-UART1 Rx, Tx, and ground from the Arduino interface are brought out and connected to the MCULink's 3-pin UART connector.

- I also pulled all the isolation jumpers around the onboard OpenSDA circuit, so the MCUlink should be the only thing on UART1.

-MCULink Firmware is version 3.108

My understanding of the power on sequences is that none of the UART1 pins are part of the BOOT_CFG parameters, and certainly not part of the BOOT_MODE0/1 setting.

Is this behavior peculiar to the MCULink or just the MIMXRT1020-EVK? How does the onboard OpenSDA's serial connection not hang the MCU on powerup?

My main concern is understanding the cause of this power-on hang in the context of our iMXRT1021-based custom hardware, which is entering layout soon. Is there something that needs to be added to UART1 circuit to prevent MCU hang if it finds UART1 is being driven during power up?

[diego_charles · Accepted solution]

Hi @GREng 

 I agree with your observations here:

• I must first power on the EVK without anything on UART1, to allow it to come up, then attach the MCULink.
• My understanding of the power on sequences is that none of the UART1 pins are part of the BOOT_CFG parameters, and certainly not part of the BOOT_MODE0/1 setting.

 Regarding  Is this behavior peculiar to the MCULink or just the MIMXRT1020-EVK? 

It is not an issue of the MCU-Link , just the MCU/EVK. This issue can be replicated with another UART line RX/TX that is pulled up to 3.3v as well.

Regarding How does the onboard OpenSDA's serial connection not hang the MCU on powerup?

This is the EVK's UART connection for OpenSDA IC and the RT1020.The UART RX/TX pins are being pulled up by default to 3.3v.

The pull comes  from VDD_SNVS domain, which is expected to be the first  rail to be powered on.

So, according to this, there is no issue on the EVK since UART RX/RX pins are being powered up along the chip.

Regarding this Is there something that needs to be added to UART1 circuit to prevent MCU hang if it finds UART1 is being driven during power up?

 From design perpective,  my first idea is  to  add an isolation  chip to avoid RX UART TX/RX being pulled-up before RT power sequence starts. Then when the power-up sequence of the RT starts or finishes let those pins to be pulled up. But please verify this on your design, if you are intending to follow this comment.

All the best, 

Diego