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USB UART in PicoZed FMC Carrier Card V2

oscargomezf

Hi,

 

I'm working on a new project based on the PicoZed and PicoZed FMC Carrier Card V2. I'm going to design a custom Carrier Card.

 

I'm trying to  understand the schematics and I have some doubts related to USB UART.

 

I don't understand, Why do they use a level shifter on the USB_UART_TXD and USB_UART_RXD with a p-mos BSS138?

 

I think it's a better option to eliminate the two bss-138 p-mos and connected them  (USB_UART_TXD  & USB_UART_RXD) directly, and feed the pin 5 (VIO) of the CP2104 IC to 1.8 Volts.

 

Does anyone know if I'm right?

 

Best regards.

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    The CP2104 has issues when you attempt any self-powering. The circuit must be 100% bus-powered or you must design in extra protection circuitry. This is our 3rd time using the CP2104, and we learned some of these lessons the hard way. Check out the Errata for MicroZed and the FMC-V1 for details.

     

    What you propose mixes a self-powered VIO while the rest of the circuit is bus-powered. If VIO were connected to the on-board 1.8V, here is the issue:

    1.   VIO is connected to net 1.8V
    2.   Q1 and Q2 are removed to allow a direct connection between CP2104 and Zynq
    3.   CP2104 RSTn (pin 9) is pulled-up to VIO
    4.   You plug in the USB cable prior to powering on the board
    5.   You then power on the board
    6.   The USB-UART does not enumerate in some cases

    For an explanation why, see http://microzed.org/content/usb-uart-pzcc-fmc-wont-enumerate

     

    According to the datasheet, the RSTn pin may also be an open-drain POR status. However, from what we heard from SiLabs, the RSTn circuit is powered internally by VIO, which is why the circuit examples show pulling it up to VIO. We worried that leaving RSTn floating with VIO unpowered would also cause issues in some cases.

     

    We considered adding a small 1.8V regulator sourced by CP2104 VDD. The level translator option that we used was cheaper.

     

    As the circuit is now, the CP2104 will consistently enumerate as soon as the cable is plugged in, regardless of whether board power is on or not.

     

    Bryan

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  • 0

    The CP2104 has issues when you attempt any self-powering. The circuit must be 100% bus-powered or you must design in extra protection circuitry. This is our 3rd time using the CP2104, and we learned some of these lessons the hard way. Check out the Errata for MicroZed and the FMC-V1 for details.

     

    What you propose mixes a self-powered VIO while the rest of the circuit is bus-powered. If VIO were connected to the on-board 1.8V, here is the issue:

    1.   VIO is connected to net 1.8V
    2.   Q1 and Q2 are removed to allow a direct connection between CP2104 and Zynq
    3.   CP2104 RSTn (pin 9) is pulled-up to VIO
    4.   You plug in the USB cable prior to powering on the board
    5.   You then power on the board
    6.   The USB-UART does not enumerate in some cases

    For an explanation why, see http://microzed.org/content/usb-uart-pzcc-fmc-wont-enumerate

     

    According to the datasheet, the RSTn pin may also be an open-drain POR status. However, from what we heard from SiLabs, the RSTn circuit is powered internally by VIO, which is why the circuit examples show pulling it up to VIO. We worried that leaving RSTn floating with VIO unpowered would also cause issues in some cases.

     

    We considered adding a small 1.8V regulator sourced by CP2104 VDD. The level translator option that we used was cheaper.

     

    As the circuit is now, the CP2104 will consistently enumerate as soon as the cable is plugged in, regardless of whether board power is on or not.

     

    Bryan

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