Mikkel,
You can give a 3.3v directly or use a small resistor to make the voltage on
the 4th pin of J1 higher than 3V. That should work. We rebooted a board
over hundreds of time by using an app on Linux. It did not fail a single
time.
We found this accidently by using serial cable and the board never fails to
boot when the cable is connected. The serial cable gives the 3.3v to the
4th pin which is the UART0_RX.
We tried to figure out what cause the problem and found a weird thing. We
measured the UART3_RX(D16), UART3_TX(D15), UART4_RX(pin11 on header P9) and
UART4_TX(pin13 on header P9). They are all 3.3V and they are in UART mode.
We removed the R165 which is the pull down resistor on the UART0_RX line.
The UART0_RX is around 1.4V and sometimes floating. We also checked the
device tree. The internal pull up of the UART0_RX is turned on.
uart0_pins: pinmux_uart0_pins {
pinctrl-single,pins = <
0x170 (PIN_INPUT_PULLUP | MUX_MODE0) /* uart0_rxd.uart0_rxd */
We do not know why the UART0_RX is not getting 3.3V, can you please check
the voltage on both UART0_TX and UART0_RX when U15 is removed?
Again, If somebody can tell me how to fix it in uboot or from software
perspective, I would really appreciate it.
Regards,
Shu
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