12:03 <Siegurd> Hello! How to set internal pull-up of TX pin of uart (/dev/ttyS2)?

12:05 <Siegurd> I'm receiving leading zero when using RS-485 and it might help

12:08 <Siegurd> config-pin -l p1.10 shows uart or gpio_pu but not something like uart_tx_pu

16:42 <zmatt> Siegurd: uart txd pin is an output and continuously driven (as long as the pin is configured to uart txd function), hence pull-up/down will have no effect

16:47 <zmatt> also the uart mode actually has pull-up enabled (even though it will do nothing for the txd pin)

16:48 <zmatt> p1.10 also has pull-up enabled in its default mode

16:48 <zmatt> oh wtf, but its default mode is spi ... who thought that was a good idea

16:54 <zmatt> yiiikes why do so many pocketbeagle pins have a non-gpio default in its dts

16:55 <zmatt> some of which will be output by default (which means there's no safe way to use them as input since they'll get configured to output before software has the chance to configure them correctly)

16:56 <zmatt> this is fixable with an overlay of course, but it seems like an ill-conceived default

16:58 <zmatt> also, some are configured as gpio (default input) with no pull enabled, which is also bad

17:10 <Siegurd> thanks, yhe problem solved with pull a

17:10 <Siegurd> and b

17:10 <Siegurd> of RS-485

17:11 <Siegurd> a to +3v3 and b to GND

17:16 <Siegurd> hmm but now both tx and rx are +3v3 between transferring data

17:19 <Siegurd> rx should be in LOW when no data is coming

17:21 <zmatt> ??

17:22 <zmatt> if your bus needs a default state then yes you should use correct termination that includes pull-up/down (this is part of your bus termination since your termination resistor may need to be adjusted depending on the value of the pull-up/down

17:23 <zmatt> and is your default bus state is high (which is typical) then obviously you're going to receive high by default

17:26 <zmatt> a bus needs a default state unless either 1. there's only one node that's continuously driving the bus, everyone else is receive-only, or 2. all receivers are of a fault-tolerant type that can cope with an open bus (a == b, undriven)

17:30 <zmatt> why would you want rx to be low? that's a break, not an idle state

17:31 <Siegurd> thanks bro! now I can sleep well)

17:33 <Siegurd> I just worked with uart only and rx was always in low by default

17:34 <zmatt> rx is never low (logic 0, "space") by default with an uart, it's always been high (logic 1, "mark") by default

17:35 <zmatt> unless you mean actual RS232 which is inverted (3V to 15V for logic 0, -15V to -3V for logic 1)

17:37 <Siegurd> let me see my old oscill photos

17:40 <zmatt> https://people.ece.cornell.edu/land/courses/ece4760/PIC32/uart/rs232_bits.png

17:41 <zmatt> (the actual voltage varies of course)

17:42 <zmatt> the uart needs to be at logic-1 level when idle, otherwise you can't detect the start bit (which is a logic 0)

17:43 <zmatt> (by "logic 0"/"logic 1" I mean whatever voltage is used to communicate a 0-bit or 1-bit respectively)

17:47 <Siegurd> yep

17:48 <Siegurd> you are 100% right

17:48 <zmatt> with rs485 I _think_ a logic-1 is signalled by A-B > 0.2V and logic-0 by A-B < -0.2V

17:48 <zmatt> if I remember correctly