AM3357: UART Driver Issue - AM335X

 Bin Liu and Remi Leveque Andreas Dannenberg1  we have below config changes,

&uart2 {
pinctrl-names = "default", "sleep";
pinctrl-0 = <&rs485_pins_default>;
pinctrl-1 = <&rs485_pins_sleep>;
linux,rs485-enabled-at-boot-time;
rs485-rts-delay = <2 2>;
rs485-rts-active-high;
rts-gpio = <&gpio0 22 GPIO_ACTIVE_HIGH>;
status = "okay";
};

rs485_pins_default: pinmux_rs485_pins_default {
pinctrl-single,pins = <
AM33XX_PADCONF(AM335X_PIN_MII1_TX_CLK, PIN_INPUT, MUX_MODE1)
AM33XX_PADCONF(AM335X_PIN_MII1_RX_CLK, PIN_OUTPUT, MUX_MODE1) 
>;
};

when we see in logic analyzer we are receiving data correctly when we probe TX and RX as below which comes from transviver (SN65HVD11DR),

we are reading NULL bytes in our application and also we are receiving broken data as we are reading 10 bytes of data not all the time we are suspecting UART driver is adding it as its having switched between DE/REdue to fast transition in differential signals. please help us on this 

Hi Manu,

I will review the information you provided tomorrow and get back to you.

Hi Manu,

Can you please try to add "rs485-rx-during-tx" property in the &uart2 node to see if this resolves the issue?

ok will check and get back.. for now i will change padmux to pullup as per our design

We got to our most significant "root cause" fairly quickly. We found that the RxD output from the RS-485 transceiver has no pull-up on the board to keep it from floating when DE/RE-not was raised high. (Note, as is very typical in RS-485 applications, DE and RE-not are tied together in our design.) Manu was able to program the pad mux to provide the pull-up and the original anomaly was eliminated. During qualification testing we discovered more though. We're seeing two different cases of uncommanded signals on the DE/RE-not line that are (a) sometimes causing communications errors and (b) are unexplained even when they don't.

1. We occasionally see a short positive pulse on DE/RE-not. When it occurs in the middle of a receive packet, it garbles the message, forcing an unnecessary retry. When it occurs when the RS-485 bus is idle, it does no harm, but of course shouldn't be happening.
2. We also occasionally see DE/RE-not remain high for a significant time after the transmitted packet is completed. In some case this lingering drive enable almost reaches to the reply coming in on the bus. Again, this particular anomaly is not currently causing us problems, but if our downstream module replied just a tad quicker, it would.

It appears that these anomalies are occurring at the driver level. We've looked at our application layer code very carefully and nothing is touching the DE/RE-not signal. We'd appreciate your advice and help in tracking this down.

We also had a question about the value of the selectable pull-up that the pad mux provides.

Thanks in advance for the help.

John R. Graham
Siemens

Hi John,

Bin is out of office until April 29th. Let me see if someone on the hardware team can take a look. Please expect a delayed response.

Hello John,

Please refer below FAQ and E2Es regarding the leakage current with pullup or pulldown enabled.

(+) [FAQ] AM3351 , AM3352 , AM3354 , AM3356 , AM3357 , AM3358 , AM3359 Custom board design – E2E, FAQ references and design guidelines for reference during custom board design - Processors forum - Processors - TI E2E support forums

E2Es related to pullup/pulldown

https://e2e.ti.com/support/processors-group/processors/f/processors-forum/532832/am572x-i-o-pullup-pulldown-resistor-value

https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1089444/am3354-gpmc_a9-internal-pull-down-resistor/5733402

https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1282287/am3354-am335x-internal-pull-up-pull-down-resistors-resistance/4862944

https://e2e.ti.com/support/processors-group/processors/f/processors-forum/253870/resistance-of-am335x-internal-pull-ups-pull-downs

5.7 DC Electrical Characteristics

Regards,

Sreenivasa 

Hello John,

The weak internal pulls may not be able to hod the attached device input in a known state.

Checking if you have had a chance to add an external pull and perform testing.

Regards,

Sreenivasa 

Hi Sreenivasa,

Very first thing, we installed a proper pull-up resistor. The floating Rx issue was immediately resolve. By experimentation we discovered that we got identical behavior with the pin-mux-commanded internal pull-up. The anomalies I've reported most recently here are occurring in both configurations.

Regards,
John

Hello John, 

Thank you.

Trying to understand the frequency of occurrence. Are you updating the DE/RE every time data is sent or at the start of transmission etc.

Can you please explain the sequence.

Regards,

Sreenivasa