Synergy USB HS always powered

In these days we are dealing with Synergy USB HS interface used to read and write files on a USB Pen drive.

Our board uses the S7G2 controller in the LQFP176 package and unfortunately it has a shortage of pins.
For this reason, in the design phase we made some choices aimed at saving pins, resources and also costs.

To save a couple of pins compared to SK-S7G2, DK-S7G2 and PE-HMI cards, we have always kept the USB host connector powered.

Many months ago, before designing our board, we tried this solution by adding a USB host connector always powered to the SK-S7G2, as described in the following diagram.

This works regularly with a demo program derived from AN0239 which does not use the signals for power supply: the insertion and extraction of the Pen drive is recognized and the counter.txt file is read and written correctly.

Unfortunately our card does not work and even the insertion of the Pen drive is not recognized.

For a week we have been looking for the reason for these operating differences between the modified SK-S7G2 and our board but we have not found them yet.

Does someone have similar experience?
Any suggestions are welcome.

Thanks in advance and best regards.

  • Hi MPGA,
    the issue can be on D+ and D- differential lines. Have you a differential impedence of 90-100 ohm on PCB?
    Try to reduce the speed, from High to Full, disabling "Enable High Speed" on USBX.
  • In reply to RobertoG:

    Hi Roberto,

    thanks for the valuable information.

    I honestly don't know if we have a 90-100 Ohm differential impedance on our PCB.

    Your indication made us pay attention to the hardware and find a difference that we thought was insignificant: two pairs of clamping diodes on the USB lines.
    In the cards made up to now with other microcontrollers these diodes have always been used.
    With the Renesas S7G2 we had left them proposing to remove them during the test phase. Last week we had removed them but on another card "massacred" by the tests, which in light of today's results, has other problems / damages.

    Once these diodes were eliminated on a new card, it started working like the modified SK-S7G2.

    Thanks again and best regards