From: Mark Ruys <[email protected]> Reply-To: <[email protected]> Date: Wednesday, May 7, 2014 at 11:31 AM To: <[email protected]> Subject: Re: [beagleboard] AIN4 erroneous
> > > Op woensdag 7 mei 2014 05:12:21 UTC+2 schreef Charles Steinkuehler: >> On 5/6/2014 3:28 PM, Mark Ruys wrote: >>> > On two different BBB's (latest revision) I've noticed strange behavior of >>> > AIN4. I have a TMP36 (analog temperature sensor) running on 3V3. Vout >>> > measures 712 mV ((712 - 500) / 10 C = 21 degree C). If I attach Vout to >>> any >>> > AIN* except AIN4, I can use the BB and read out 712. So the ADC is >>> working, >>> > as is the TMP36 sensor. >>> > >>> > As soon as I connect Vout to AIN4, the voltage drops from 712 to 673 mV >>> and >>> > slowly decreasing. Why is this? Is another part of the BBB already using >>> > AIN4 and is it reserved? >>> > >>> > No other capes or anything else connected to P8/P9. >> >> Make sure you have the touch-screen (ADC) driver configured properly. >> >> There are internal FETs that drive voltage onto various AIN lines in >> order to sense resistive touch screens with no other external >> components. If the driver is turning any of these FETs on, they could >> be working against your external temperature sensor, which may be slowly >> decreasing it's output voltage as the output drive transistors heat up. > > Sounds perfectly reasonable to me. Thanks for the explanation. > >> >> See the Technical Reference Manual for details on the drive FETs hooked >> to the AIN pins.. > > I tried to find info on how to program (disable) the internal FETs, but > nothing useful so far. I did found > http://processors.wiki.ti.com/index.php/AM335x_ADC_Driver's_Guide: good info > on ADC from TI, but no mentioning of the internal FETs. > > So I have no clue how to disable the FET on AIN4. The funny thing is, the > BeagleBone's I tested on are fresh unmodified stock BB's. Only the > cape-bone-iio dts loaded. Then AIN 0-3, 5-7 work okay but AIN 4 pollutes my > sensor by feeding it some voltage. So I just use another AIN as a work around. Look at your boot logs and see if there is any reference to TSC or TouchScreen. Also, in your kernel configuration, is ³TI Touchscreen Interface² selected? Look at your kernel .config and see if CONFIG_TOUCHSCREEN_TI_AM335X_TSC is enabled. The touchscreen driver is typically enabled via the device tree ³arch/arm/boot/dts/am33xx.dtsi². Hope this helps. Regards, John > > Mark > >> >> -- >> Charles Steinkuehler >> [email protected] <javascript:> > > -- > For more options, visit http://beagleboard.org/discuss > --- > You received this message because you are subscribed to the Google Groups > "BeagleBoard" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/d/optout. -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
