Unreadable UART output on S905 board

noob404 · October 28, 2024 at 10:34 AM

Hi,

Just got my hands on an S905 board (it says S905-X8L05-V1.10 above the eMMC) using a mini USB to TTL reader:-

The image below shows the UART pinouts I have tried (The text in black denotes header on the TTL reader).

Case 1

No output at all.

Case 2

Continuous output on Putty (Tx indicator on TTL reader constatly lit, Rx blinks every second or so), but all gibberish. But, if I were to connect the HDMI cable, the output slows down. Here's a sample of Putty output with 115200 set as baud rate:-

Things I've tried with Case 2 pinout:-

1. Tried every baud rate that I could find starting from 4800 to 921600.

2. Tried moving the jumper from 3.3 to 5V and viceversa.

3. Tried different combinations, some absurd (RX to RX, GND to RX, etc.)

4. Checked TTL reader with another device (works as expected)

5. Tried putting the board in recovery mode when booting.

My questions:

1. What am I doing wrong?

2. Is there a chance that the manufacturer has somehow made UART almost impossible on this board?

**chewitt** · October 28, 2024 at 2:33 PM

I've never seen a device that wasn't 115200,8,n,1 and I would advise against connecting the 3.3v/5v pins on the TTL device (asking for shorting issues and power isn't required). Yes it's possible that UART output is blocked in bootloaders, but that's normally only major-vendor streaming boxes (Amazon, etc.) and you're seeing stuff on-screen so that's unlikely. Not sure what to suggest as the only time I've seen something similar is when I'm connected to the wrong pins, e.g. JTAG not UART.

Tempting though large ones are, and the square one is normally GND .. Have you tried the 3x little round pads that are labelled?

noob404 · October 28, 2024 at 3:17 PM

Thank you for your comment.

Was able to fix this issue. The baud rate was 115200. But, the pins were wrong. In the image above, if you move a bit below from where the pins in the image are, there is a pin bracket with exactly 4 pins. Upon a closer inspection, I found them properly labelled RX, TX, GND and VCC. Soldered to them and Putty now properly outputs the log.

BTW, did try the smaller pins, but much like the bigger pins to their left, these were weird as well. TX to TX gave me output, which obviously was weird.

Quote from chewitt

I've never seen a device that wasn't 115200,8,n,1 and I would advise against connecting the 3.3v/5v pins on the TTL device (asking for shorting issues and power isn't required). Yes it's possible that UART output is blocked in bootloaders, but that's normally only major-vendor streaming boxes (Amazon, etc.) and you're seeing stuff on-screen so that's unlikely. Not sure what to suggest as the only time I've seen something similar is when I'm connected to the wrong pins, e.g. JTAG not UART.
Tempting though large ones are, and the square one is normally GND .. Have you tried the 3x little round pads that are labelled?

noob404 · October 28, 2024 at 5:01 PM

Check earlier post.