UART6 issue

[morind79]

Hi All,

I have made a search on the forrum concerning Uart issue, and saw that everything is working as soon as you have a recent Debian version.
Mine is 3.4.67+

I used Minicom on A20 board, and Terminte on Windows side.

Sending characters from A20 to Windows, no problem at all
But sending from Windows to A20, then I see strange characters in Minicom.

What can be the issue ?
I work at 9600 8N1

Best regards,
Denis

[MBR]

How is set the hardware flow control? Is it turned off on both sides?

[morind79]

Yes, hardware flow control is disable on both side.
One thing I did not said is this uart is connected to a TXB0104, and after some search seems this can introduce some problems.
I ordered TXS04104 instead. Wait to have them to make a test.

[dave-at-axon]

How are you connecting the UART on the board to the PC?

Are you using a USB to TTL converter on the PC or a USB to RS232?

I don't see why you need the logic level converter unless your USB to TTL is 5V.

They certainly won't work with RS232 levels.

[morind79]

Hi Dave,

I solved my issue, this was due to the TXB0104. I unsoldered it from the board and now everything work just fine.
To give you some more info whay I had was :

Uart6 -> MAX3232 -> RS232
      -> TXB0104 -> Uart of another device working at 4.3V

So as you can see Uart 6 was connected to 2 devices (MAX3232 was there for debug purpose)
I am waiting for the TXS0104 now.

Best regards
Denis

[dave-at-axon]

Hi Denis,

Ah, that would make sense. There was a contention as RS232 does not support 2 drivers on the same lines.

I am not sure about your 4.3V level translation here as the SIM900 modules, although they are powered from 4.3V, the data lines are actually at 2.8V logic levels. Feeding 4.3 to then is likely to cause damage.

Which module are you using?

Dave...

PS.. I got your files and looking at the schematics just now. Which software do you use for the PCB design?

[morind79]

Hi Dave,

I use this GSM module : http://www.open-electronics.org/small-breakout-for-sim900-gsm-module/

Denis

PS : For PCB Design I used Altium

[dave-at-axon]

OK. It looks like this module

http://www.open-electronics.org/small-breakout-for-sim900-gsm-module/

The DATASHEET for the SIM900 shows that RXD must be pulled high to VDD_EXT which is 2.8V and the MAX input on any digital interface pin is 3.1V so if you are using 4.3 you will need to fix this.

What I normally do is use a level translator that is powered from the 2.8V VDD_EXT output.

[morind79]

When I have a look at this http://www.open-electronics.org/gsm-gps-shield-for-arduino/# there is a schematic.
I see that one one line TXD it is 3.3V, on RXD it is 2.5V.

So I am a little bit lost.
I thing that I should just change the +4.3 bu +2.8V on U19 (TXS0104)

What do you think ?

[dave-at-axon]

Yes, the TXD will be 3.3V from your board and this exceeds the max input to the board. The 2.5 is the output from the modem.

To ensure that the modem works reliably I would make sure that the voltage inputs are not exceeded.

Also all of the SIM900 modules are 2.8V on interface pins but a lot of designs using them just use 3.3V and this is not good practice if you want it to be reliable.

[morind79]

Well I see from the schematic of the small breakout board that :

Power On/Off
Rst
LED

can be used with 4.3V logic level because they are using a transistor to pull down the logic level.

But for RI, high level is 2.8V and low level 0V ?
If yes, then I have to modify this pin...