USB-Serial cable


#1

Hi,

I am receiving a Dragonboard 410c soon. For the serial console, can I use an Adafruit RPi usb-serial cable (PL2303TA) or is there anything special with the 96boards’ TE-C0344? Any suggestion would be useful.

Thanks,
Gijo


#2

The DragonBoard 410c is 1.8V I/O, as are all boards conforming to the 96Boards CE spec. The Adafruit PL2303TA is set up for 3.3V I/O. Not a good mix. You need a 1.8V UART serial cable, or just use one of the adapters that are linked in the ‘Products->accessories->debug’ tab above.


#3

Extract from PL2303TA datasheet: “Flexible signal level requirement on the RS232-like
serial port side also allows PL2303TA to connect directly to any 3.3V~1.8V range devices.”

This is what confused me. Sorry, am I missing some basics?


#4

The Prolific chip inside the adapter can be configured to operate at 1.8V, but the assembled unit provided by Adafruit has been configured to run at 3.3V. The datasheet on the Adafruit web site is for the Prolific chip, not for the assembled unit. In order to configure operation at 1.8V an external 1.8V regulator is required which is not in the Adafruit version. The Tectona version (listed on this site) uses the same chip and has the 1.8V regulator.

Full Disclosure: I am an employee of Qualcomm Canada. Any opinions expressed in this or any other post may not reflect the opinions of my employer.


#5

Understand. Thanks for clarifying


#6

This much cheaper cable is working out-of-the box.

[28086.033082] usb 1-1: new full-speed USB device number 12 using xhci_hcd
[28086.219306] usb 1-1: New USB device found, idVendor=067b, idProduct=2303
[28086.219319] usb 1-1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[28086.219327] usb 1-1: Product: USB-Serial Controller
[28086.219333] usb 1-1: Manufacturer: Prolific Technology Inc.
[28086.220545] pl2303 1-1:1.0: pl2303 converter detected
[28086.222134] usb 1-1: pl2303 converter now attached to ttyUSB0


page 10 : Flexible signal level requirement on the RS232-like
serial port side also allows PL-2303HX to connect directly to any 3.3V~1.8V range devices.

@96boards , could you please test the cable above and delete the advertising of the ridiculous priced cables and adapter boards?

I bought the “96Boards UART Serial Adapter” and it smoked through. It is not working anymore.


#7

I’m a little puzzled by your link. Although the PL2303 chipset is capable of running at 1V8 TTL logic I can’t see how the cable you link to can be configured for this logic level because I can’t see a reference voltage lead.

The documentation says its fixed at 3V3 which is quite enough to physically damage boards… I’m not saying that is doesn’t work for you but, unless I have missed something, I don’t think it is safe enough to recommend.


#8

It is similar unsafe as the recommended Tectona TE-C0344 cable.
There are absolutely no specs available for the Tectona TE-C0344 cable.

Could you please measure the output voltages on the cable?


#9

The Tectona cable is specifically designed to meet the 1V8 TTL logic requirement. See the app note here: https://developer.qualcomm.com/download/db410c/uart-usb-adapter-application-note.pdf


#10

That was maybe the intention, but the cable doesn’t meet the spec, there is no spec, so who knows what output levels to expect?


#11

It is similar in that both use the same USB -> TTL UART bridge chip (PL2303) but this chip is paired with voltage regulator (for 3.3v the internal regulator can be used as a voltage reference) that sets the TTL signal level. For both reliability and electrical safety it is important not to use a serial cable where the PL2303’s VDD_325 pin is connected to anything other than 1.8v.


#12

You might be interested to take a look at this device: https://www.pololu.com/product/1308

Even if you don’t fancy buying one and making an attempt at the SMD soldering needed to convert it to 1.8v operation, the description of how to do the mod is a really good illustration of what I explained in my last post.