On Fri, Jul 6, 2018 at 12:22 AM Andreas Klinger <ak@xxxxxxxxxxxxx> wrote: > > Hi Rob, > > please see explanation below. Thanks. > > Rob Herring <robh@xxxxxxxxxx> schrieb am Thu, 05. Jul 15:30: > > On Wed, Jul 04, 2018 at 02:36:38PM +0200, Andreas Klinger wrote: > > > Add clock-frequency property for hx711 ADC > > > > > > This is the frequency of PD_SCK. It affects only the high value duration > > > since low value duration is not relevant and we are not able to switch > > > faster than the minimum duration specified. > > > > > > After PD_SCK goes high DOUT is read just before PD_SCK goes down again. > > > This is necessary because of parasitic capacities on the wiring. > > > > s/capacities/capacitance/ > > I'll change. > > > > > > > > > Signed-off-by: Andreas Klinger <ak@xxxxxxxxxxxxx> > > > --- > > > Documentation/devicetree/bindings/iio/adc/avia-hx711.txt | 10 ++++++++++ > > > 1 file changed, 10 insertions(+) > > > > > > diff --git a/Documentation/devicetree/bindings/iio/adc/avia-hx711.txt b/Documentation/devicetree/bindings/iio/adc/avia-hx711.txt > > > index b3629405f568..4bee51d536e1 100644 > > > --- a/Documentation/devicetree/bindings/iio/adc/avia-hx711.txt > > > +++ b/Documentation/devicetree/bindings/iio/adc/avia-hx711.txt > > > @@ -8,11 +8,21 @@ Required properties: > > > See Documentation/devicetree/bindings/gpio/gpio.txt > > > - avdd-supply: Definition of the regulator used as analog supply > > > > > > +Optional properties: > > > + - clock-frequency: Frequency of PD_SCK > > > + This setting affects the duration of the high value > > > + phase of the clock (PD_SCK). The low value phase is > > > + not affected since it is not relevant for the > > > + measurement. > > > > That's not how frequency works. The high time should be equal to the low > > time. If you have parasitic capacitance affecting the rise time, then it > > is going to affect the fall time too. > > > > Perhaps there could be some reason not to have a square wave, but I > > didn't see one in the datasheet. > > When PD_SCK goes high the ADC sets DOUT to the corresponding value. Only after a > rising edge DOUT is set. The falling edge doesn't matter. When bit-banging yes, but the simplest h/w implementation would shift data on one edge and sample on the other. > The parasitic capacitance is occuring only on DOUT line, but not on PD_SCK. The > difference is that PD_SCK is driven by the microcontroller, and DOUT by the ADC. Maybe for you, but for someone else's design? I just don't want to see the binding evolving fixing one problem at a time. > So after i set the rising edge on PD_SCK i have to wait until DOUT reaches its > value. Datasheet says this can take at maximum 0.1 us. This might be true, but > without wiring between ADC and microcontroller. With wiring of a 10 m cable i > saw 1 us on the oscilloscope. > > If required i can post a screenshot of the oscilloscope. > > The code which is executed in the driver on PD_SCK during low value takes much > more than the requested 0.2 us (measured 0.56 us). Of course i can make it a > square wave with equal low and high time, but i don't see it's needed and it'll > slow down the communication speed. > > Out of my frequency generator i can get a rectangular waveform with different > low and high times and it's still called frequency. A don't have another > expression for it. Yes, but the frequency is not 1/(high time) as you defined. But I wasn't expecting 2 orders of magnitude difference. > Do you have a suggestion how to call this attribute in the DT if frequency is > not suitable? If you really want to not have 50% duty cycle, then of course you would need to define an additional property. > > > > > + Minimum value allowed is 20 kHz because of maximum > > > + high time of 50 microseconds. > > > > So 10kHz should be minimum. > > > > Because the low time is very short (0.5 us) in comparison to the high time > (50 us) in the actual waveform i neglected it. Okay, that's not obvious. Rob -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
