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avafinger / bananapi-zero-ubuntu-base-minimal

BananaPi M2 Zero - Ubuntu Focal Base Minimal Image (Experimental) - U-Boot 2017.09 / Kernel 4.18.y / Kernel 4.19.y / Kernel 4.20.y / Kernel 5.3.y / Kernel 5.6.y / Kernel 5.7.y / Kernel 5.11.y
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UART baudrate accuracy on ttyS3 #55

Closed Prototyx closed 3 years ago

Prototyx commented 3 years ago

Hi,

I am struggling with the baudrate accuracy. I need to communicate with another device at a baudrate of 921600 bauds. But it is impossible.

Normaly at 921600 bauds, the duration of a bit should be arround 1.085 us. But in my case it is around 1.44 us.

image

Here is the output of stty < /dev/ttyS3

speed 921600 baud; line = 0; min = 0; time = 0; -brkint -icrnl -imaxbel -opost -onlcr -isig -icanon -iexten -echo

The max speed that I can reach without too many error is 115200 bauds on the BPI zero M2. Are there some issues with the UART baudrate accuracy? Or maybe have I a crappy board with the bad crystal ?

I am using 5.3.4 kernel.

Thanks for you help

avafinger commented 3 years ago

I have never used UART with more than 115200 bauds. Can you test with /dev/ttyS2 ? (Extended UART 2). I can't remember If UART2 is activated, if not you need to edit the DT. Perhaps the temperature of the Board is too High?

Prototyx commented 3 years ago

I found the problem.

Actually it is written in the H2+ datasheet : image

And the serial clock frequency is at 24 MHz. The divisor is on 16 bit.

If we take the value of the divisor from 1 to 10 we have the following baud rates :

Divisor BaudRate
1 1500000
2 750000
3 500000
4 375000
5 300000
6 250000
7 214285,714
8 187500
9 166666,667
10 150000

So it is not possible to have something close to 921600 bauds.

I decrease the baudrate of my device at 500000 bauds and now the BPI M2 zero sends and receives without problem.