BeagleBone Black with a I2C Digital Analog Converter

[Ian Lepore]

On Sat, 2018-08-11 at 17:30 -0300, Dr. Rolf Jansen wrote:
> > 
> > Am 08.08.2018 um 14:05 schrieb Dr. Rolf Jansen <rj at obsigna.com>:
> > 
> > > 
> > > Am 08.08.2018 um 12:45 schrieb Ian Lepore <ian at freebsd.org
> > > <mailto:ian at freebsd.org>>:
> > > 
> > > On Wed, 2018-08-08 at 10:59 -0300, Dr. Rolf Jansen wrote:
> > > > 
> > > > 
> > > > ...
> > > > Please can somebody, check this, and perhaps give some hints in
> > > > case I got something wrong? I don't have an oscilloscope, a
> > > > multimeter only, can I check somehow if I specified the correct
> > > > pins?
> > > > 
> > > > What needs to be done next? I guess, I need to write a simple
> > > > device driver, don't I? Or can I use somehow the iicbb(4) – I2C
> > > > generic bit-banging driver, since actually that DAC chip needs
> > > > bit banging only -- repeatedly write 3 byte in row. I yes, how
> > > > can I use it for my purpose?
> > > At this point, i2c(8) is your friend, it lets you do i2c bus
> > > transactions from the commandline without a device-specific
> > > driver for
> > > each i2c device. A good starting point is a bus scan:
> > > 
> > >   i2c -f /dev/iic1 -s
> > > 
> > > If the bus is working you should see something like:
> > > 
> > >   Scanning I2C devices on /dev/iic1: 60
> > Ian, thank you very much for your response.
> > 
> > As a matter of fact, the source of all my doubts told in the
> > initial post above is that yesterday after hours in a trial&error
> > circle, I was not able to get a sensible response from the exact
> > bus scan command which suggested:
> > 
> > i2c -f  /dev/iic1 -s -v
> > 
> > Up to now, by no means I was able to see anything else than:
> > 
> > dev: /dev/iic1, addr: 0x0, r/w: r, offset: 0x00, width: 8, count: 1
> > Hardware may not support START/STOP scanning; trying less-reliable
> > read method.
> > 
> > This START/STOP thing let me to think that I perhaps need to get
> > attached a device driver.
> > 
> > I am also still in doubt, whether I really got the pinmux straight.
> > I checked very carefully the physical connections, anyway I am
> > stuck at this point.
> I got it working.
> 
> 1.) The specification of the DAC board which I attached to my BBB was
> wrong in regards to the acceptable Vcc voltage range (2.7 to 5.5 V).
> I started to use it with 5 V, and by no means I got any response from
> it. After doing a full u-turn around the block – to no avail I tried
> all possible settings with FreeBSD 11.2, then all with Debian/Linux
> 9, then all with new spare DAC board – I switched the Vcc to the 3.3
> V supply, and it works. Halleluja! I am back on FreeBSD 12-CURRENT,
> and I can live with 3.3 V, since I need to pass the DAC output
> through a scaling OpAmp anyway in order to have 0 ... 10 V. 
> 
> 2. There is still something wrong with my overlay. I need to hack in
> the same I2C1 changes into the main DT, and then the bus scan by i2c
> -f  /dev/iic1 -s gives me the expected output. For my present project
> this is a minor issue, though. My best guess here is, that there is a
> problem with the reference of the I2C1 node to the proposed pinmux
> node.
> 
> On Debian/Linux the very overlay does work.
> 
> Best regards
> 
> Rolf
> 

Ah.  The board probably does support 2.7-5.5v, but if the i2c bus has
pullups that only pull the lines up to 3.3v, that may be too close to
the VIH transition levels for the DAC chip running at 5v. Even if it's
within the VIH range, if the pulls aren't strong enough the clock
signal may not rise above the 5v VIH fast enough for reliable comms.

-- Ian