(OSD3358) UART0 is receiving characters when there is nothing connected to it

[jakthree]

Some sequences of characters are very similar to characters that were recently sent out the UART0 port, some are seeming random and on some occasions it receives a serial BREAK which causes the next character to be treated as a system command character. Unfortunately, if the next character is a ‘b’ then the system will reboot.

What could be causing the unwanted characters to show up and what can I do to stop it?

Thank you,

[Aedan Cullen]

It may be that the UART0 RX signal is accidentally floating, or something is pulling it to an indeterminate logic state. To check, try connecting a strong pull-up resistor (the relevant power rail is board-dependent of course) and see if the issue still appears.

It could be helpful to know whether you’re using an off-the-shelf or custom board. (For instance, by “nothing connected”, do you mean there are absolutely no components on the board connected to the RX pin?)

[jakthree]

This is a custom board based on the Beagle Bone Black. The BBB has a 6-pin header for console output via connecting a USB to 3.3v TTL cable. UART0_TX and UART0_RX from the OSD3358 are connected to the 6-pin header through a SN74LVC2G241DCUR chip. I can’t find anywhere in the Octavo app notes where the UART0 is used as a console port. The BBB has a 100k ohm resistor connected to pin-2 (B_UARTo_RX) to ground.

Do you recommend the same?

Thank you,

[jakthree]

I looked at the OSD3358-BAS-SBC-Ref-Design-SCH.pdf and  OSD3358-SM-RED_Platform_sch.pdf. Both have a 100K resistor connecting Sn74LVC2G241 pin-2 (B_UART0_RX) to GND. I tried same on one of my custom board and it seems to have fixed the ‘receiving characters when nothing connected’ problem. I will try same on multiple boards.

Thank you

 

[jakthree]

UART0 pin out in shown on sheet 3/9 the OSD3358-BAS-SBC-Ref-Design-SCH.pdf.

Where in the OSD335x Reference Design Tutorial Series is the UART0 pin out mentioned?

Thank you,

[Neeraj Dantu]

jackthree,

The UART0 console is discussed here in the tutorial series: https://octavosystems.com/app_notes/osd335x-design-tutorial/bare-minimum-boot/peripheral-circuitry/. As you can see, there is no pull-down on RX. The pull-down on RX in the reference designs were added to make sure that the RX pin is not floating, which ensures that there is no halt during u-Boot execution, but this is not necessary to do in hardware. You can activate internal pull-up/pull-down in device tree as shown in https://github.com/octavosystems/OSD335x-Device-Tree/blob/master/OSD3358-SM-RED/osd3358-bsm-refdesign.dts#L219. Please check your device tree and try to add pull-up/pull-down configuration there. Note that internal configuration is a weak pull-up/pull-down. If you are still having issues after that, something might be interfering with UART0 comm lines.

Best,
Neeraj

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