By the way, the ADC gives out a value of 0-4095. So scaling voltage is really simple. Say you get a value of 4024 for a reading. In order to get a reading from 0-1.8v from this . . .
((4024 / 4096) * 1.8) or 1.768359375v The max voltage on the pins still can not exceed 1.8v, this is why one needs an opamp, or similar device to keep the input scaled down. But, in this manner you could even track large voltages. Hundreds or even thousands of volts, at the cost of resolution. Which unfortunately can not be avoided. So going by the above example . . . ((4024 / 4096) * 300) or 294.7265625v Does this make sense ? On Fri, Oct 30, 2015 at 9:59 AM, William Hermans <yyrk...@gmail.com> wrote: > *BBB and Linux are new to me. One of the things I would like to do is >> compute a ratio between two voltages, each of which might exceed 1.8 VDC. >> One solution might be to use DACs to sent feedback voltages to operational >> amplifiers that are supplied by 1.8 VDC. The goal is to determine the >> ratio as quickly as possible and without waiting for Linux to get around to >> it (waiting for the ADC to happen).* >> > > > What sample rate do you need ? If something around only a few thousand > samples a second. This can be done from userspace without using the PRU's. > You can even use mmap() with /dev/mem/ and directly manipulate the ADC's > registers, which can sample the ADC faster than 200ksps. However, the > sample rate would not be as deterministic as using the PRU's. > > You can use a PRU to read the ADC's for sure. If you search the web for > Beaglebone PRU + ADC > , you will find many ways. People have been doing this for a long time( > since the beaglebone white ), and if you search this google group, you'll > even find full code for something someone called "ADCcollector". It's C, > and PASM assembly, but is a very short / easy read / understanding. > > As for how to physically connect to the ADC, I'd listen to evilwulfile. > He's been an EE for longer than many people on this group have been alive, > and he knows his stuff ( I know him personally ). Don't risk burning up > your board, as I've seen several people on these groups do . . . > > On Fri, Oct 30, 2015 at 9:43 AM, TJF <jeli.freih...@gmail.com> wrote: > >> Hi John! >> >> As Przemek mentioned, a simple voltage divider can scale the input >> voltage. Additionaly I use a LED (orange color) between the ADC input and >> ground in order to block overvoltage. (The LED starts to light up at 1V5 >> and consumes full power at 1V8. 1 / 6 of the ADC resolution get lost (is >> less accurate), but the burning LED as indicatior is a nice side effect.) >> >> Regarding software you should have a look at libpruio >> <http://hackaday.com/2015/02/16/library-upgrade-to-pru-gives-fast-io-on-beaglebone/#comments> >> . >> >> BR >> >> -- >> 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 beagleboard+unsubscr...@googlegroups.com. >> 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 beagleboard+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
