AD7176-2 Overlay for RPi - Error in Selecting Matching Device

Question

Hi All,

Following up on my previous post, I've made a DT overlay for the AD7176-2 based on an overlay for the AD7124-8. But I'm still running into the same issue that the device is not able to be selected.

This is my DT Overlay

/dts-v1/;
/plugin/;

/ {
	compatible = "brcm,bcm2835", "brcm,bcm2708", "brcm,bcm2709";

	fragment@0 {
		target-path = "/";
		__overlay__ {
			vref: fixedregulator@0 {
				compatible = "regulator-fixed";
				regulator-name = "fixed-supply";
				regulator-min-microvolt = <5000000>;
				regulator-max-microvolt = <5000000>;
				regulator-boot-on;
			};
		};
	};

	fragment@1 {
		target-path = "/";
		__overlay__ {
			clocks {
				ad7176_mclk: clock@0 {
					#clock-cells = <0>;
					compatible = "fixed-clock";
					clock-frequency = <16000000>;
				};
			};
		};
	};

	fragment@2 {
		target = <&spi0>;
		__overlay__ {
			#address-cells = <1>;
			#size-cells = <0>;
			status = "okay";

			ad7176@0 {
				compatible = "adi,ad7176-2";
				reg = <0>;
				spi-max-frequency = <5000000>;
				interrupts = <25 2>;
				interrupt-parent = <&gpio>;
				refin1-supply = <&vref>;
				clocks = <&ad7124_mclk>;
				clock-names = "mclk";

				#address-cells = <1>;
				#size-cells = <0>;

				channel@0 {
					reg = <0>;
					diff-channels = <0 1>;
				};

				channel@1 {
					reg = <1>;
					diff-channels = <2 3>;
				};
			};
		};
	};

	fragment@3 {
		target = <&spidev0>;
		__overlay__ {
			status = "disabled";
		};
	};

	fragment@4 {
		target = <&spidev1>;
		__overlay__ {
			status = "disabled";
		};
	};
};

And this is the error message I get;

Traceback (most recent call last):
File "/home/analog/AD7176/AD7176_adi.py", line 4, in <module>
my_adi = adi.ad717x(uri="local:")
File "/usr/local/lib/python3.9/dist-packages/adi/ad717x.py", line 85, in __init__
raise Exception("Error in selecting matching device")
Exception: Error in selecting matching device

I have compiled using the command; $ dtc -O dtb -o rpi-ad7176-myOverlay.dtbo rpi-ad7176-myOverlay.dts in Kuiper Linux command line and moved the .dtbo file into the /boot/overlays folder. I've added the following line into the /boot/config.txt file; dtoverlay=rpi-ad7176-myOverlay.

Any insight into where I'm going wrong would be greatly appreciated.

I'd also like to make the inputs single ended (I believe you have to specify this in the .dts/.dtbo file) - if anyone has an example they can show me of how to do this that'd be awesome!

MarkThoren · Accepted Answer

Hi Jay, 
 Sorry I realized that there's a dangling question here. Regarding " shifting voltages on the rising edge of the clock (5th rising edge in byte 2). " - yes this is exactly the sort of thing I was referring to. The MISO line should only ever transition at falling SCK edges. This means that the rising edge may be ringing above VCC, then back down below the AD7176 SCK input's threshold voltage, which is being interpreted as a falling edge. Also note that while your scope may not be showing that effect, it could be due to your probing technique, scope bandwidth, etc. The only thing that matters is what the AD7176 sees. 
 If that's the case - clock edges being misinterpreted, then all bets are off on the data itself. I'd focus on further improvement to your setup - more grounds, shorter grounds, and even separating the ribbon cable conductors to reduce crosstalk may help. 
 Another anecdote - as the driver for the ADIN1110 (SPI interface to Raspberry Pi) was being developed, the setup looked similar to yours. The author had to throttle back the SPI frequency to a few MHz (one or two.) When the CN0575 boards showed up, which use the ADIN1110, and includes a full 40-pin interface connector with all ground pins connected to a proper ground plane, we were able to bump the SCK frequency all the way to the datasheet maximum: https://github.com/analogdevicesinc/linux/blob/rpi-5.10.y/arch/arm/boot/dts/overlays/rpi-cn0575-overlay.dts#L28 
 Of course it is the edge rate that is more important than the frequency, but as frequency increases, ringing edges will bleed into one another. We didn't do any signal integrity measurements on the hand wired setup, it was better to just go straight to the PC board. 
 Could you re-do your board to include the Raspberry Pi connector? There are lots of low-cost, quick-turn PC board manufacturers you could use. 
 -Mark 
 
 P.S. the MISO line wiggling AFTER the SCK has stopped might just be the DRDY# indication - the period looks a bit shorter than 1 division at 5 us per division, and the max sample rate is 250 ksps, or a period of 4 us.)