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Level converting for serial communication with a PC.

Dan S17
Expert 1510 points
Other Parts Discussed in Thread: MAX232

In an upcoming phase of my application I will need to use the Hercules SCI for UART communication with a PC. Will I need to level convert the signals from the PC or should they already be 3.3V?

  • 0
    TI__Guru 68920 points
    Dan,

    For a PC with a DB-9 or DB-25 legacy COM port you need a chip like MAX232. The voltages on RS-232 swing positive and negative and you need more than a simple CBT type level shifter for this job.

    But your launchpad includes an FTDI chip which should show up as both an emulator and a virtual COM port, and if you use the SCI port on the chip that is connected to the FTDI chip you don't need to do anything.
    (other than write code) If for some reason you need a different / second serial port for modern PC's a USB -> serial port chip is probably worth considering as DB9 connectors have pretty much vanished. [they are even more scarce than through-hole components ;) ]
  • 0 in reply to Anthony F. Seely
    Expert 1510 points
    So the FTDI chip (Future Technology Devices International) converts 5V to 3.3V signals? And if I use the Hercules SCI port then I am automatically using the FTDI chip?
  • 0 in reply to Dan S17
    TI__Guru 68920 points

    Dan,

    Dan S17 said:
    So the FTDI chip (Future Technology Devices International) converts 5V to 3.3V signals?

    No, not really.  It is a USB<>UART chip, and on the UART side the signals are 3.3V and will interface to the TMS570.

    On the USB side,  you just need to know that it interfaces to a standard PC's USB port and one of the functions enumerates as a virtual COM port.   You can check the schematics to see which SCI/LIN is routed to the FTDI chip and make sure that resistors (if any) are installed.  [on some boards, we have resistors that you can remove to break this connection in case you do not want the SCI/LIN to be used by the FTDI chip but instead used by a booster pack or similar expansion board].

  • 0 in reply to Anthony F. Seely
    Expert 1510 points
    Hi Anthony

    On the schematic for the XDS100v2 FT2322HL I see pins with the following labels: FTDI_TCKI, FTDI_TDI, FTDI_TDO, and FTDI_TMS.

    Those pins are also labeled as: ADBUS0, ADBUS1, ADBUS2, and ADBUS3 respectively.

    I don't see any connectors on J1-J4 bearing the ADBUSX labels but on J3 I see pins with the following labels: ADIN0, ADIN1, ADIN2, and ADIN3. Are the ADIN0-ADIN3 pins the correct pins to the FTDI IO pins?

    Dan