linux-kernel - Re: Break doesn't work on a CP2105

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Date:   Mon, 5 Jun 2023 07:25:04 -0500
From:   Corey Minyard <minyard@....org>
To:     Johan Hovold <johan@...nel.org>
Cc:     Craig Shelley <craig@...rotron.org.uk>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        linux-usb@...r.kernel.org
Subject: Re: Break doesn't work on a CP2105

On Mon, Jun 05, 2023 at 09:15:28AM +0200, Johan Hovold wrote:
> On Sun, Jun 04, 2023 at 03:11:43PM -0500, Corey Minyard wrote:
> > On Sun, Jun 04, 2023 at 02:17:16PM +0200, Johan Hovold wrote:
> 
> > > I just verified break signalling on the first port of my CP2105 using a
> > > logic analyser and everything seems to work as expected.
> > > 
> > > There's also no mention of any issue with break in the errata.
> > > 
> > > Could you check which firmware revision you have by enabling debugging
> > > and reconnecting the device?
> 
> > [    4.253869] usb 2-1.2: new full-speed USB device number 3 using ehci-pci
> > [    4.342570] usb 2-1.2: New USB device found, idVendor=10c4, idProduct=ea70, bcdDevice= 1.00
> > [    4.350939] usb 2-1.2: New USB device strings: Mfr=1, Product=2, SerialNumber=5
> > [    4.358259] usb 2-1.2: Product: CP2105 Dual USB to UART Bridge Controller
> > [    4.365052] usb 2-1.2: Manufacturer: Silicon Labs
> > [    4.369765] usb 2-1.2: SerialNumber: 01070456
> > [    4.374925] cp210x 2-1.2:1.0: cp210x converter detected
> > [    4.381086] cp210x 2-1.2:1.0: cp210x_get_fw_version - 1.18.1
> > [    4.386915] usb 2-1.2: cp210x converter now attached to ttyUSB0
> > [    4.393529] cp210x 2-1.2:1.1: cp210x converter detected
> > [    4.399835] cp210x 2-1.2:1.1: cp210x_get_fw_version - 1.18.1
> > [    4.405657] usb 2-1.2: cp210x converter now attached to ttyUSB1
> 
> So we have the same firmware revision.
>  
> > This is an embedded reference board, I guess there is some probability
> > that this is just broken on this board, though I'm not quite sure how.
> 
> Is the port accessible somehow so that you could hook up a scope or
> logic analyser? Or is that what you did already when you mentioned
> seeing garbage sent after 2.5 seconds?

I didn't hook up a scope, but I did set the baud to a really low rate
and send a character that was all zeros and got a break on the other
end.  It wasn't garbage I was seeing after 2.5 seconds, it was basically
a small number of 0 bits, it appeared.

I have a scope that I can capture with, but this is all embedded onto a
board.  I looked at the schematic and there are some resistors that I
could hook to, perhaps.  I'll see what I can do, but I don't have
surface mount stuff to be able to attach easily.

Thank you,

-corey

> 
> That last bit seems to match the behaviour I see with the second, SCI
> port, where the last character sent is resent when trying to signal a
> break.
> 
> Johan