We have designed a PCB using IMX6UL and used UART1 as debug port. We are not fusing "BT_FUSE_SEL" to select the boot device(eMMC), and we use pull-up/pull-down resistors on "SD_CD", to detect the device as serial downloader(on OTG1 USB port) and boot from eMMC respectively. As suggested in the "Serial Downloader boot flow" on Pg305 of reference manual,the device checks for activity on UART1/2 before entering USB Serial downloader mode.Please find the image below for reference.
When debug UART cable not connected to PC and SD_CD = high(to flash the on-board eMMC),sometimes the device is not getting detected on USB.When the debug UART is connected to PC and SD_SD = high, the device was always detected on the PC.So we suspect noise on the UART lines that may assert false UART Data Ready and prevent the device from entering USB OTG1 download.
Can you please clarify the below doubts.
1.Is the noise on the UART lines asserts a false 'UART_Data Ready' on the UART controller, as suggested below( Pg307 of IMX6ULRM)
2.If that is the cause, can you please let us know the possible hardware design precautions to be followed, to avoid the noise on UART lines.
Thanks in anticipation,