[OpenWrt-Devel] Booting boards with UART attached (was: Re: [PATCH] ramips: mt7621: use lzma-loader for D-Link DIR-860L B1)

[Paul Fertser]

Hello,

On Mon, Apr 20, 2020 at 11:01:46AM +0200, Szabolcs Hubai wrote:
> I have a CP2102 USB to TTL module, and used with "minicom -D /dev/ttyUSB0".
> 
> 
> Intereestingly, when I connected with serial to the router it was
> sometime unstable:
> * refuse to start
> * freeze at boot soon after: "Error applying setting, reverse things
> back" messge

I can't tell if it's exactly the problem there but when working with
UARTs it's good to keep in mind that Tx idles high and so when you
disconnect power from your target you'll have your UART module feeding
some current to the SoC's Rx from the module's Tx and that often
confuses the hardware. Proper way to fix this would be to feed USB
UART module's output buffers from target's Vcc (that's why most UART
headers have that pin); the next best option would be to not have
target's Rx connected at all (and hotplugging it just before you
actually need it); yet another possible way might be to add a 1k (or
more) resistor in series to limit this current.

-- 
Be free, use free (http://www.gnu.org/philosophy/free-sw.html) software!
mailto:fercerpav at gmail.com

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