Serial port CN6 not working as expected

Discussion in 'Troubleshooting' started by dnulnets, Oct 6, 2014.

  1. dnulnets

    dnulnets New Member

    Jun 9, 2013
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    I'm having some problems with the serial interface to the Arduino from an external computer running Fedora 20. I have attached the external computer to the CN6 USB Port on my UDOO Quad (udoobuntu 3.0.35). The problem is staring me in the face, but I don't seem to be able to crack it. I have created a minimalistic example echoing characters on the serial port testing some setups. See below:

    Setup 1 : Simple sketch using serial read and write to echo a character. J18 in position and everything run from the Arduino-IDE terminal on the UDOO (ttymxc3). Works perfect !!!

    Setup 2 : Same as 1 but J18 removed and run from the Arduino-IDE terminal on the external Fedora 20 (ttyUSB0). Works perfect!!!

    Setup 3 : Simple program doing the exactly same thing as the sketch but using Atmel ASF on the Fedora 20 instead (CONF_BOARD_UART_CONSOLE). Remove J18. Upload with bossac. Shutdown. Put back J18 and reboot. Run from the Arduino-IDE terminal on the UDOO (ttymxc3). Works perfect.

    Setup 4 : Same as Setup 3 but I do not put back J18 and I run the Arudino-IDE-terminal on Fedora 20 (ttyUSB0). Nothing happens. Characters sent from the Arduino ends up on the external Arduino-IDE terminal as expected, but no characters seems to be received by the Arduino from the external Arduino-IDE terminal (ttyUSB0). I have seen some postings on a PIO_PULLUP but it doesn't fix this.

    Now to the strange part.

    4.1 With Setup 4 and no J18 I also starts the Arduino-IDE terminal on the UDOO linux (ttymxc3). I can type characters in the terminal, Arduino receives it and echoes it back to both the UDOO Arduino-IDE terminal (ttymxc3) and the external Arduino-IDE terminal (ttyUSB0). This I cannot do with the arduino-sketch only with the application in ASF.

    4.2 With Setup 4 and no J18 I shut down the UDOO linux (shutdown) without removing the power cord. The Arduino then restarts and everything works from the external Arduino-IDE terminal ttyUSB0.

    So somehow the "serial in" to the arduino is still connected to the UDOO ttymxc3 and not to the external CN6-USB when I use ASF. So I guess there must be something missing in the setup in my ASF-app that causes this behaviour.

    Any ideas on some pin or other thing I am obviously missing or any other suggestions.



    ASF-setup-snippet of the serial port:

    /** Enable Com Port. */
    /** Pins description corresponding to Rxd,Txd, (UART pins) */
    /* Usart Hw ID used by the console (UART0) */

    static void configure_console(void)
    usart_serial_options_t uart_serial_options;
    uart_serial_options.baudrate = CONF_UART_BAUDRATE;
    uart_serial_options.paritytype = CONF_UART_PARITY;
    uart_serial_options.charlength = 8;
    uart_serial_options.stopbits = 1;
    /* Configure console UART. */
    stdio_serial_init(CONF_UART, &uart_serial_options);

    And then I just use puts, getchar etc.
  2. fetcher

    fetcher Member

    Mar 9, 2014
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    If you check board schematics, the /dev/ttymxc3 RS-232 transmit signal (coming from the i.MX6 pin labeled "KEY_COL0", transmitting toward SAM3X end) is connected in parallel with the transmit signal from the USB-to-serial chip behind CN6. In the opposite direction of flow, two receivers being simultaneously fed from the SAM3X's RS-232 TX signal is fine, but RS-232 isn't designed to function with two transmitters in parallel-- each is always pulling the line high or low even when idle, so they'll fight each other and neither will work, or else the stronger one (capable of sourcing/sinking more current) will "win" -- not a good thing for hardware health even if it does work.

    For this to work the way you want, /dev/ttymxc3's serial transmitter needs to be turned off (tristated, set as high-impedance / input-only). There may be several ways to do this, but the first that comes to mind is to manipulate the i.MX6's pin-muxing registers, to temporarily flip that KEY_COL0 pin back to being a GPIO.

    There is a utility called "devregs", meant for another i.MX6 board, but which will work on an UDOO with some caveats -- it writes directly to hardware registers, and can easily crash your CPU if you're not careful.

    See my earlier post here (on using this for PWM control) for a link and more information on this tool:

    With that installed, and going by the table on page 290 of Freescale's i.MX6 reference manual (IMX6DQRM.pdf), I believe this may do the trick:

    # sudo devregs IOMUXC_SW_MUX_CTL_PAD_KEY_COL0 5 # mode 5=GPIO4_IO06 , should default to input state

    To turn the RS-232 transmitter back on,

    # sudo devregs IOMUXC_SW_MUX_CTL_PAD_KEY_COL0 4 # mode 4 = UART4_TX_DATA

    The raw register address, as defined in /etc/devregs_imx6x.dat is 0x020E01F8, if you want to write to it in some other way. Of course you have to be root...

    Note that since you're going behind the Linux kernel's back, it won't let you manipulate that GPIO by normal means (it won't appear in /sys/class/gpio), but it should come up in the desired input state automatically.

    Register IOMUXC_SW_MUX_CTL_PAD_KEY_COL0 (0x020E01FC) controls the UART4 receiver (also mode 4=enabled, mode 5 GPIO) which shouldn't need to be changed but might be helpful in testing.

    Maybe there's an easier, less kludgely way to do this without poking at registers directly -- perhaps the proper ioctl() on the ttymxc3 device can also tri-state the serial transmitter? Anyone know? Putting it into RS-485 mode, if that toggle exposed in the driver may work, since RS-485, unlike RS-232 does make provision for multiple transmitters on a common line.
  3. dnulnets

    dnulnets New Member

    Jun 9, 2013
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    that did it, now it works perfectly :) Thanks :D

    I also thought about shutting down or never starting ttymxc3 before posting but didn't really get to grips on how ...


  4. dnulnets

    dnulnets New Member

    Jun 9, 2013
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    Hi again,

    one side effect I have seen is that the IOMUXC_SW_MUX_CTL_PAD_KEY_COL0 seems to get reset to 4 whenever I reset the arduino through J16 or via bossac.
  5. fosuco

    fosuco New Member

    Jul 25, 2014
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    @dnulnets: It's Udoo's modified bossac utility that is returning the IOMUXC_SW_MUX_CTL_PAD_KEY_COL0 pin to an output after programming. The endcmd() method from <Win|Posix|OSXPosix>SerialPort.cpp in this commit is sending a command code of 0 into the udoo_ard.c kernel driver located here.

    Instead of blindly returning the _KEY_COL0 and _KEY_ROW0 GPIOs to their default values after programming, I think udoo_ard.c should be saving off the current states of both GPIOs when the programming command comes in and then returning them as they were when a command code of 0 is received. I'll submit a pull request after some testing.
  6. dnulnets

    dnulnets New Member

    Jun 9, 2013
    Likes Received:
    Thanks for the info, that explains it :)
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  9. delba

    delba Administrator Staff Member

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