Does the GL-X750C4 need a pullup resister on the UART?

aunixsystem · 2019-09-11T16:38:24.122Z

Hi there.

I am attempting to get a working UART on my GL-X750C4 and am having some trouble.

Attached is a screenshot of my terminal program and the settings i’ve applied to the serial port dongle. It does appear that i have the baud rate and parity / flow control settings correct (they do match the settings here: UART Serial - GL.iNet Docs)

but despite those settings, here’s what i get out on the UART:

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(coolterm won’t let me copy the HEX encoded data, so please see the attached screenshot for that)

coolterm_settings1512×2144 913 KB

( i will post more pictures in more posts… the forums are limiting me to 1 img per post… grrr

The USB/TTL dongle that i am using is a cheap one used to program the ESP8266 micro controllers; it’s based on the Cyprus 210X chip. From lsusb:

Product ID:	0x7523
Vendor ID:	0x1a86

** The reason ** why i am asking if a pull up resistor is needed is because I get 100% garbled UART except for the last line when i use the web UI to reboot the device. the last line before the device reboots is a standard kernel message log of the format:

[timestamp.microseconds] Device is restarting message here....

And then the device reboots and the uart is garbled again. My theory is that there’s something about the shutdown routine that adjusts the internal pullups on the UART so the last line is clear, but whatever config/register change is applied… it is wiped out on reboot.

I will try to capture a full UART log to show exactly what i mean.

aunixsystem · 2019-09-11T16:39:00.951Z

because i’m limited to just one pic per post, here’s the picture showing the data that i get on the UART on device boot

coolterm_data1496×2140 786 KB

aunixsystem · 2019-09-11T16:40:07.127Z

and like above… one more picture… this time, you can see how i’ve wired in the UART. For reference, the USB->TTL dongle is a cheap ESP01 programmer if you wish to google that pinout and confirm my wiring

uart_wired1830×910 305 KB

dzwghx · 2019-09-12T03:34:18.773Z

The garbled UART is generally caused by a mismatch in the baud rate. You can take the 3.3v line and try it out. You only need three wires to connect.

aunixsystem · 2019-09-12T05:03:10.447Z

I have captured a video that shows the UART behavior. I will post it here if interested.

I should have mentioned in OP that I have already tried all sorts of combinations of parity, flow and baud rate. I wouldn’t have gone straight to asking about the pull-up on UART0TXD if i hadn’t already done so.

Same with the 3v3 line, I’ve tried with and without.

As mentioned, for exactly only one line of uArt output there is no garbled output. If all lines but one are garbled, but I made no physical change to the device under test… Then it must be a software component that has changed the config.

So I am asking the community if they’ve gotten a working uArt connection on the x750 or not. Or, if a glinet engineer is reading, please let me know if I need to supply my own pull up.

here is the text that comes from the console when the device is rebooted. I have left notes in between the < $NOTE> blocks:


<The device was booted; i was connected to it's WIFI AP and using the GL web ui, not LUCI to reboot the router.  I have trimmed the data from the console up to this point. Below you see exactly what my console has about 3 seconds after i click the "yes" button on the "do you want to reboot?" prompt.>

[ 1407.620000] reboot: Restarting system
.
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< and it continues to be garbled... Until I repeat the reboot>

alzhao · 2019-09-12T06:16:40.193Z

Seems you need to use another UART adapter. This is generally caused by this.

The device IO voltage is 2.5V and sensitive compared to those devices with 5V IO.

aunixsystem · 2019-09-12T07:16:09.148Z

I have tried two. Both the same chipset. They are 3v3 devices, not 5v.

That still does not explain one un garbled line.

aunixsystem · 2019-09-17T19:43:42.968Z

I ordered a new FTDI based USB to Serial converter and am no longer having issues w/ the UART.

To answer my original question: no pullups are needed.

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