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. 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.
