Re: Getting alot of zero values from ads7846 using atmel_spi

["Niclas Karlsson" <Niclas.Karlsson-AM1FMPraTHU@public.gmane.org>]

tor 2011-03-10 klockan 15:41 +0100 skrev David Lamparter:
> On Tue, Mar 08, 2011 at 10:44:56AM +0100, Niclas Karlsson wrote:
> > Do anyone know why I read so many zeroes?
> 
> You're probably reading values faster than the chip supports. Check the
> chip documentation for a maximum readout frequency or a ready flag.
> > I have made some printk in ads7846.c to trace the problem. It seemed to
> > read more correct values when I added the printks.
> 
> A printk takes quite some time on an ARM like yours. The throttle makes
> it work...

I've made som further investigations. The atmel_spi.c driver utilize PDC
(Peripheral DMA Controller) and when I was tracing the problem I
discovered that the interrupt indicating the PDC has written the data to
memory is fired before the data actually is written to memory. If a
small delay is added after the interrupt (like a trace printk("The value
is\n")) the PDC have enough time to write the data to the memory and I
get correct values from the ADC connected to the SPI.
Thoughts how to solve this?
> 
> 
> David
> 
> P.S.: you're lucky to not get crap values - seems they got the chip
> design right...
> 
/Niclas
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