Hello all,
At my target board I have several blocks. One of them is the micro (T89C51RD2) block and other is a USB (FT245AM) block. According to the USB device requirements he receive the power supply (5V) from the PC. So, the power of the USB block on my target has now connection with to the rest of the board power supply (I double checked it). Of course, there’s a common ground.
The problems starts when I connect the USB cable from the PC, my target start to work very strange. I also see that if I disconnect the main power supply and connect the USB cable, the USB block has is 5V but the rest of the card has about 3V where no voltage suppose to be.
I think that the problem came from the micro, because there is 4 control lines connected through serial resistors from the USB chip to the micro (port 1) and when I disconnect the resistor the voltage problem disappear.

Does anyone know why it happens and how can I fix it?