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Question asked by Lang Lin on Dec 13, 2018
Latest reply on Dec 14, 2018 by igorpadykov

We are using GPIO1_IO3, GPIO1_IO8, and GPIO1_IO9 as 3 ADC input pins to measure DC voltage in the range of 1.6V-1.8V.  The reference VREFH is DC 3.3V.   I've measure on the board, these voltages are as expected.


We are seeing that ADC read in Linux (sys/bus/iio/devices/iio:devices/in_voltage0/1/2_raw) in the range of 4050-4095.  For a 12 bit ADC, these are absolutely too high.


I've checked our device tree file.  There seems nothing suspicious for now.  There was a bug, a 3.3V applied on one ADC input.  It was fixed.

I verified with memtools on the IOMUX reg configuration.  They are all 0xB0.


I also tried to recalibrate ADC through memtools.  When CFG.ATRIG = 0, set GC.CAL = 1.  The CALF flag is 0.  It sounds that calibration is good.


Here are the related parts in our device tree file.


                regulators {

                                compatible = "simple-bus";

                                #address-cells = <1>;

                                #size-cells = <0>;


                                reg_vref_3v3: regulator@1 {

                                                compatible = "regulator-fixed";

                                                regulator-name = "vref-3v3";

                                                regulator-min-microvolt = <3300000>;

                                                regulator-max-microvolt = <3300000>;




&adc1 {

                pinctrl-names = "default";

                pinctrl-0 = <&pinctrl_adc1>;

                vref-supply = <&reg_vref_3v3>;

                num-channels = <3>;

                status = "okay";



                pinctrl_adc1: adc1grp {

                                fsl,pins = <                                          

                                                MX6UL_PAD_GPIO1_IO03__GPIO1_IO03              0xb0      /* ADC_SYS4V0_SENSE (IN) */

                                                MX6UL_PAD_GPIO1_IO09__GPIO1_IO09              0xb0      /* ADC_GEN3V6_SENSE (IN)*/

                                                MX6UL_PAD_GPIO1_IO08__GPIO1_IO08              0xb0      /* ADC_LDO3V3_SENSE (IN)*/