Daisy Seed to host USB bidirectional custom data transmission

Arduino’s Serial.read() and Serial.write() are such powerful tools that I can’t understand why the Daisy Seed makes bidirectional sending of arbitrary data so difficult for beginners. I did some investigations, which I will list below.

Straight up googling for “Daisy Seed serial write” might lead a curious beginner to this getting started article. This approach calls itself “Serial printing”, while a quick look into the schematic reveals that the D+ and D- lines are basically directly connected to the MCU’s pins B15 and C15 respectively. While the “S” in “USB” means “serial” as well, the term is usually used in the context of the UART protocoll, which can’t be the case with the Seed, as the USB-to-UART converter present on many other dev boards is missing here. This is also clear when looking at a device manager, as no COM-port (under windows) or RS232-emulator (under Linux) is visible.

While I can flash code via an external JTAG debugger, it remains a mystery to me how I can access the ports described in the mentioned getting started guide. Connecting the Seed to my machine doesn’t register any device associated with a COM-connection, although running dfu-util displays this:

Found DFU: [0483:df11] ver=0200, devnum=10, cfg=1, intf=0, path="1-11", alt=1, name="@Option Bytes   /0x5200201C/01*128 e", serial="200364500000"
Found DFU: [0483:df11] ver=0200, devnum=10, cfg=1, intf=0, path="1-11", alt=0, name="@Internal Flash   /0x08000000/16*128Kg", serial="200364500000"

…which tells me that a) my Seed is working and b) I installed the dfu-util correctly as described in the wiki.

Interestingly, the actual UART connection is on pins 13 and 14, which is fine as a backup solution, but requires an extra connection and a dedicated USB-to-UART converter.

After some digging, I found a very promising project which acts as a CLI for the Seed from a windows host machine. In the code on the Seed, line 83 in SeedComServer.cpp utilizes the DaisySeed’s instance’s usb_handle.TransmitInternal() function to send messages via the USB line connected to the micro-USB jack on the board. Similarly, line 210 registers a user callback for the handling of incoming data. On the host side, a COM port is opened (see line 274 in SeedCom.c) which again tells me that I should see a COM port (loosely using this term for Linux as well) from the Seed, which I don’t.

How is this handled? Should the Seed appear as COM-port as long as BOOT and RESET are not pressed and then disappear and only be visible to the dfu-util? Apart from this issue (which is probably just on my side due to driver problems and the usual stuff) I really would have loved to see some examples for stuff like this because a simple, bidirectional communication is just so helpful for all sorts of things, especially when getting started as a beginner.

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I use GNU Linux (Debian) so when using PrintLine debugging I can monitor the serial port with:

cu -l /dev/ttyACM0


screen /dev/ttyACM0

or to dump to a file:

screen -L -Logfile mylogfile.xml /dev/ttyACM0

Granted, it’s only uni-directional, but it is handy.



Thanks for the tip with logging to a file, very helpful.

I figured out the bidirectional data transfer with the help of a friend.

The crucial part is that the USB port on the Seed is configurable during runtime, meaning it is not a hardwired device, but an interchangeable one. While it is only visible to dfu-util when engaged in flashing, it will appear as COM or its Linux equivalent as soon as the line hw.usb_handle.Init(UsbHandle::FS_INTERNAL); is run. Only then will it be recognized.

Now the method hw.usb_handle.TransmitInternal((uint8_t*)buff, strlen(buff)); will act as a serial send and the callback UsbCallback set in hw.usb_handle.SetReceiveCallback(UsbCallback, UsbHandle::FS_INTERNAL); will act as serial read.

However, having to pull this from the examples labeled “USB_CDC” leaves one looking for a bidirectional serial implementation very confused. Would have loved to see a small paragraph at the bottom of the logger example that looks something linke this:

Looking for serial sending AND receiving?

Check out the USB-CDC examplewhich turns the micro-USB port of the Daisy Seed into a virtual COM port!