Hi,
I am, quite simply, trying to hook up an FTDI USB-to-UART (3v3) cable to my jetson nano EVK on the /dev/ttyTHS1 port. (L4T r32.5.0)
To reproduce:
- Only connect GND + TX pin (pin 8 of the EVK) to the FTDI-UART cable. (The RX works, and I am ignoring it for the purpose of this post, and just not connect it) Log in through ethernet cable on ssh, and disabled nvgetty, installed microcom, and control /dev/ttyTHS1 through “sudo microcom -p /dev/ttyTHS1 -s 115200”. On the PC, just open putty on the virtual serial port
- After I let the jetson send a single character, it goes into an oscillation on its TX pin.
- Once I disconnect the TX pin cable, the oscillation stops.
- If I send characters from the jetson without anything connected to the pin aside from the scope, all works perfectly
- If I put a 500 ohm series resistor on the line, everything still works
- If I only put a 130 ohm resistor in series, the TX line goes into oscillation
(FYI, tested this with 2 different common FTDI-UART 3v3 cables)
- If I simply connect 130 ohms resistor to the TX line to ground, the line goes into immediate oscillation as well.
- Same with 500 ohms to ground (cant source 10mA) without going into oscillation
- Same with 1k to ground (cant source 3mA) without going into oscillation
- 5k to ground was ok. No oscillation. (it can source 660 uA)
Some images:
TX pin of jetson without anything connected:
TX pin with 500 ohm series resistor to FTDI connector:
TX pin with 130 ohm in series => starts oscillation
So, I would guess for some reason the FTDI cable is sinking too much current, which then somehow causes the jetson uart block to go into oscillation (and never recover)…
I did not bother to check the datasheet for max current sourcing on that pin, because… it’s just uart, and you’d expect it to work with such a default cable. But apparently not. (I thought I’d make this post in case some else ran into the same issue). Also, the fact that it goes into oscillation… indefinitely seriously threw me off track…
Now that I looked at the datasheet, I don’t immediately find the maximum drive strength of pins. Google tells me the drive strength is configurable though the device-tree, but I dont directly find what the range is exactly. Can somebody perhaps enlighten me on this?
Thanks for your feedback