linux-kernel - Re: Debugging i2c : i2cdetect cant detect a device on i2c line

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Message-ID: <ZPoIKhytmUQqtONp@shell.armlinux.org.uk>
Date:   Thu, 7 Sep 2023 18:28:10 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Lucas Tanure <tanure@...ux.com>
Cc:     Bagas Sanjaya <bagasdotme@...il.com>,
        Linux Devicetree <devicetree@...r.kernel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Andi Shyti <andi.shyti@...nel.org>,
        kernelnewbies <kernelnewbies@...nelnewbies.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Linux I2C <linux-i2c@...r.kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Raul Piper <raulpblooper@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: Debugging i2c : i2cdetect cant detect a device on i2c line

On Thu, Sep 07, 2023 at 05:17:47PM +0100, Lucas Tanure wrote:
> On Thu, 7 Sept 2023, 14:56 Russell King (Oracle), <linux@...linux.org.uk>
> wrote:
> 
> > On Thu, Sep 07, 2023 at 08:36:32PM +0700, Bagas Sanjaya wrote:
> > > [also Cc: devicetree and ARM folks]
> > >
> > > On Thu, Sep 07, 2023 at 08:21:44AM +0530, Raul Piper wrote:
> > > > Hello ,
> > > > I am trying to detect an i2c device A on i2c1 line on  one of the Arm
> > > > Cortex A7 platform but not able to see any device on a given address (
> > > > 0x3d) .
> > > >
> > > > Is there any parameters of i2c which i can change like rise/fall time
> > > > , timeout etc in a device tree or kernel source and re test it?
> > > > I have tried changing the i2c speed from 100KHz to 400 KHz  but no
> > success.
> > > > I have even tried removing the 1.5K pull ups on the i2c lines but no
> > result.
> >
> > Honestly, from this description, I'm wondering if this posting is a joke.
> >
> > I2C is entirely _reliant_ on pull-ups. It's a wire-or bus, and the
> > logic 1 state is created by no device pulling the signal low, thereby
> > allowing the pull-up resistor to pull the line to the logic 1 state.
> >
> > The pull-up must be the correct strength for the devices on the bus.
> > If it is too strong, then a driver may not be able to pull the signal
> > sufficiently low for other devices to register it as a logic 0.
> >
> > Conversely, the pull-up must be strong enough so that the rise-time
> > of the signal is sufficient to register as a logic 1.
> >
> > If it's a problem with the rise time, then increasing the clock rate
> > will just make the situation worse.
> >
> > So, if other devices work on the bus, it could be that the Vil
> > threshold of this device is not being achieved, whereas the other
> > devices are happy. Therefore, I would suggest you study the data
> > sheets of the device that isn't being detected.
> >
> > Lastly, if the undetectable device has a reset line, it's possible
> > that the device isn't responding because it's being held in reset.
> >
> Please try to use an logic analyser like saleae logic.
> Probe the i2c bus, reset line, power lines, pins that set the i2c address
> for the device.
> Can tell us which device it is?

I assume you are replying to Bagas? Please me it clear in future.
Thanks.

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