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Boot fails with the DPI interface plugged

Problem description

Sometimes with the DPI port interface plugged happen that a power cycle is not able to restart the board.

Solution

The problem is that the FTDI chip TX line is kept at 3.3V when the board is switched off.

This voltage enters the CPU through the Rx line and latches up the 3V3 power line in the CPU at over 2 Volts, preventing the CPU to correctly boot when powered up again.

To avoid this problem you can insert a series resistor on the TX line of the DPI FTDI chip. In this way the voltage loss on the series resistor is enough to prevent the latch up problem.

Cut the PCB track on the DPI board as shown here:

Then you have to put the series resistor of 47k between the two pads marked TX (shown a 56k resistor working equally well):

For a cleaner work you can use a SMD resistor like this:

The series resistor of course is slowing down the signal speed but at 115200 bit/s this is not much a problem. In the following photo you can see the signal in blue before the resistor and in green after the resistor. Even if the slew rate is lowered the signal integrity is maintained enough for correct working of the serial port.