linux-kernel - Re: [PATCH] ARM: dts: da850-lego-ev3: slow down A/DC as much as possible

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Message-ID: <fda2606b-3c7b-539a-ae07-4d9b57c34335@lechnology.com>
Date:   Tue, 25 Sep 2018 10:23:40 -0500
From:   David Lechner <david@...hnology.com>
To:     Sekhar Nori <nsekhar@...com>
Cc:     Kevin Hilman <khilman@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: dts: da850-lego-ev3: slow down A/DC as much as
 possible



On 9/25/18 1:04 AM, Sekhar Nori wrote:
> Hi David,
> 
> On Monday 24 September 2018 07:01 AM, David Lechner wrote:
>> Due to the electrical design of the A/DC circuits on LEGO MINDSTORMS EV3,
>> if we are reading analog values as fast as possible (i.e. using DMA to
>> service the SPI) the A/DC chip will read incorrect values - as much as
>> 0.1V off when the SPI is running at 10MHz. (This has to do with the
>> capacitor charge time when channels are muxed in the A/DC.)
> 
> So you believe this is an issue with the board design rather than the
> limitation of the ADC chip itself? Because if its the later, the fix
> should be in the driver instead.

It is the board design. There are external 4.7k series resistors that 
make the RC charge time so long.

> 
>>
>> Both of these patches mentioned have been picked up in the respective
>> subsystems.
> 
> Is this needed for the -rc cycle or can wait for v4.20?

v4.20