7 Replies Latest reply on Feb 20, 2015 2:42 AM by tushar panda

    Trouble with ttl to usb interface


      Hey guys, riotboard noob here!  A local computer store had a USB to TTL serial cable based on the PL2303 chip and I can see the boot information through screen and minicom as my riotboard is booting up so that's working just fine.  However, when it comes to the prompt where I can press any key to interrupt the boot sequence, nothing works.  Neither keypresses on the host computer (Ubuntu Linux) or an attached usb keyboard will interrupt the 3 second timer.  What am I doing wrong?  I apologize in advance if this is a painfully obvious problem.  I'm really stumped, though!


      Background: I purchased the 7" LCD display and connector board for my riotboard.  I'm following the documentation where it instructs me to interrupt the boot sequence so I can enter the appropriate parameters to enable the screen.

        • Re: Trouble with ttl to usb interface

          To cancel the boot you have to do that over your serial connection.

          With an attached USB Keyboard this won´t work.


          What terminal progam are you using to connect on the serial port?

          You have to press a key in this terminal program to cancel the boot.


          I´m mainly using putty in Windows. But as far as I know "screen" should be working on the terminal in Ubuntu.

          • Re: Trouble with ttl to usb interface

            Sorry you did not specify the O/S you used. This sounds a bit more software related. Did you check your connections? Please look at @Debian on RIoTboard - After basic install, adding a GUI Desktop and other usefull stuff

              • Re: Trouble with ttl to usb interface

                I don't think this is software related. I've used minicom on PCs, Raspberry Pi, ODROID-C1, and other SBCs with no problems and I've used screen on PCs with no problems. Its

                also unlikely that minicom and screen would have the exact same problem.


                Rich, after checking out the riotboard and USB/TTL converter information from what I see they should be connected like this:


                USB/TTL                           riotboard pin #


                red wire                             not connected

                white wire  (input)               1  UART2_TXD  (output, this one must be correct for you to see the boot information coming from the riotboard)

                green wire  (output)            2  UART2_RXD (input)

                black wire                         3  GND


                If this is how you have it wired then there must be a problem with the USB/TTL converter output, or the riotboard UART2 RXD input. This could be anything from bad IC pins on

                either one (nothing you can do), bad solder connections to the IC pins on either one or the connector on the riotboard or the wires on the USB/TTL converter (may be fixable if you

                can solder), problems with the PCB on either one (probably nothing you can do), or the USB/TTL converter green and black wires are connected to the wrong place (swapped)

                although it probably wouldn't work at all because there would be no good ground reference between them.


                If you have a digital multimeter you could verify that there is 5V between the red and black wires on the USB/TTL converter when it is plugged into a USB port. This would at

                least tell you that the USB/TTL converter is probably wired properly. Just about anything else would require an oscilloscope to trace out the problem.

              • Re: Trouble with ttl to usb interface
                John Beetem

                I'm not a RIoTboard user but I've done a lot of UART.


                Here are a few things I'd try.  First, make sure the USB UART dongle is working by itself.  Unplug it from the RIoTboard and connect the RX and TX bits to each other.  Then start up your terminal emulator and see if it echoes the characters you type to the screen.


                If this doesn't work, you either have a bad USB UART dongle or you don't have the serial configuration right.  Specifically, be sure that flow control is OFF or NONE since you only have RX and TX.


                Once the USB UART echo works, make sure you have the correct serial config for RIoTboard.  You obviously have the Baud rate correct or you wouldn't be seeing the boot trace.  I think the correct setting is 115200N8 so make sure you have parity turned OFF, 8 bits per byte, and flow control OFF.


                Otherwise, as Gary says you probably need a 'scope to see the signal from USB UART TX to RIoTboard RX.


                Hope this helps!

                • Re: Trouble with ttl to usb interface

                  Thanks for all of the replies, gentlemen!  I am going to pick up a multimeter after work today and check the voltage and also try the echo test referenced above.


                  But, yes, you are correct - I can see the output as the machine is booting up.  Could it be a permissions problem maybe?  Or maybe the usb port on the laptop I've been using is bad?


                  Related question:  if I pick up a dedicated serial port card for my desktop, is it just a 1:1 straight pin mapping between the card's serial port rx/tx pins and the rx/tx pins on the riotboard?


                  by the way, it's nice to see a vibrant and helpful community for this device!