Boot problems when UART_1 is used on i.MX7D

[wolframwadepohl]

We use UART_1 as console for U-Boot and Linux. There are no level-shifters and we use a FTDI USB-to-Serial 3V3 device to connect. When the Adapter is connected neither the reboot command from Linux (which uses the watchdog) nor the reset pushbutton, both rooted to PMIC PowerOn are working. When noting is connected to the UART Pins, pushbutton and reboot command (over SSH) works perfect. Somtimes it takes a few seconds, somtimes a couple of minutes and somtimes there is never a reboot when the UART is connected to the USB-to-serial device. PowerOn (disconnect and reconnect) always work, even in the USB-to-serial adapter is connected. How can we implement a working solution for the UART without level shifters/driver for RS-232 signals?

[JosephAtNXP]

Hi,

Thank you for your interest in NXP Semiconductor products,

When UART is connected, are the reset signals still?

Also, do you use the same DTS? It's probable that the pinmux for watchdog is changed/missed.

WDOGx_B output signals (for each one of the Watchdog modules) do not have dedicated pins, but are muxed out through the IOMUX. See the IOMUXC chapter of the i.MX 7Dual Application Processor Reference Manual (IMX7DRM) for detailed information

Regards

[wolframwadepohl]

That's not a matter of the IOMUX. Also the Reset Button which shares its signal with the watchdog output does not work under certain circumstances. Maybe it's a problem of the PMIC or the signal which triggers the PMIC PWRON input.

From the HW Designer: The PMIC provides an input signal to control the power state of the system by generating a turn-on event. This signal isprovided as PMIC PWRON, and can be used to bring the PMIC out of OFF and Sleep modes into the ON mode. By default, PMIC PWRON is pulled up with 100k to the VSNVS rail. The PMIC PWRON is pulled down by an open drain output rsp. a switch and a BAS 516 diode..