Messages in this thread |  | | Date | Thu, 21 Nov 2019 22:24:22 -0500 | From | Sasha Levin <> | Subject | Re: [PATCH AUTOSEL 4.19 087/209] ARM: dts: da850-lego-ev3: slow down A/DC as much as possible |
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On Wed, Nov 13, 2019 at 10:10:30AM -0600, David Lechner wrote: >On 11/12/19 7:48 PM, Sasha Levin wrote: >>From: David Lechner <david@lechnology.com> >> >>[ Upstream commit aea4762fb46e048c059ff49565ee33da07c8aeb3 ] >> >>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.) >> >>This patch slows down the SPI as much as possible (if CPU is at 456MHz, >>SPI runs at 1/2 of that, so 228MHz and has a max prescalar of 256, so >>we could get ~891kHz, but we're just rounding it to 1MHz). We also use >>the max allowable value for WDELAY to slow things down even more. >> >>These changes reduce the error of the analog values to about 5mV, which >>is tolerable. >> >>Commits a3762b13a596 ("spi: spi-davinci: Add support for SPI_CS_WORD") >>and e2540da86ef8 ("iio: adc: ti-ads7950: use SPI_CS_WORD to reduce >>CPU usage") introduce changes that allow DMA transfers to be used, so >>this slow down is needed now. >> > >I doubt that the commits above would be backported, so it does not >make sense to backport this patch.
I've dropped it, thanks!
-- Thanks, Sasha
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