Analog-to-Digital Converter (ADC)

An Analog-to-Digital Converter (ADC) is a device that translates an analog voltage to a digital value that a microprocessor can understand.

The ConnectCore 6UL provides two types of ADC interfaces:

The i.MX6UL CPU ADCs (mapped to GPIO1_IO00 to GPIO1_IO09 pads)
The Micro Controller Assist (MCA) ADCs

Both ADC controllers are 12-bit resolution, right justified, unsigned format. The MCA ADC channels are suitable for low-frequency sampling (under 10 Hz). For higher frequency sampling, Digi recommends you use the CPU ADC channels are recommended. The ADC drivers only support "one-shot" mode. Continuous capture mode is not currently supported.

Available ADC pins

On the ConnectCore 6UL there are:

10 CPU pads (GPIO1_IO00..GPIO1_IO09) that can be configured as ADC channels (multiplexed with other functionality).
Five MCA pads that can be configured as ADC channels:

MCA_IO0
MCA_IO1
MCA_IO3
MCA_IO4
MCA_IO5

On the ConnectCore 6UL SBC Express:

GPIO1_IO04 (i.MX6UL ADC channel 4) is accessible on the expansion connector and optionally (requires a 0-ohm resistor) on Grove connector J12.
MCA_IO0 (MCA ADC channel 0) is accessible on Grove connector J12.
MCA_IO4 (MCA ADC channel 4) is accessible on the expansion connector and on Grove connector J11.

On the ConnectCore 6UL SBC Pro:

The following ADC channels are accessible on the GPIO expansion connector:

EXP_GPIO_1 (i.MX6UL ADC channel 5)
EXP_GPIO_2 (i.MX6UL ADC channel 3)
EXP_GPIO_3 (i.MX6UL ADC channel 2)
MCA_IO1 (MCA ADC channel 1)
MCA_IO3 (MCA ADC channel 3)
IOEXP_IO3 (I/O Expander ADC channel 3)
IOEXP_IO4 (I/O Expander ADC channel 4)
IOEXP_IO5 (I/O Expander ADC channel 5)

Formula

The value read on the ADC inputs responds to the formula:

V_READ = V_IN * 4095 / V_REF

where:

V_READ is the digital value read
V_IN is the analog input voltage
V_REF is the ADC voltage reference

Voltage reference

The result of the ADC conversion for a given input voltage is inversely proportional to the reference voltage (V_REF) of the ADC.

For i.MX6UL ADC channels the reference voltage is the output of PMIC regulator vdda_adc_3v3, which is configured by default to 3.3V.
For the MCA ADCs the reference voltage is the MCA_VCC voltage.

CAUTION! MCA_VCC is an input voltage for the ConnectCore 6UL so the user must configure this reference voltage via the device tree (property digi,adc-vref) for accurate conversion of measurements using the MCA ADC channels.

For the I/O expander ADC channels on the ConnectCore 6UL SBC Pro the reference voltage is the 3V3_IOEXP line, which is connected to a 3.3V regulator.

ADC channel mapping

The i.MX6UL ADC channels are available on the pads GPIO1_IO00 to GPIO1_IO09, which correspond to the ADC channels 0 to 9.
The indexes of MCA ADC channels correspond with their MCA IO number: MCA_IO0 is channel 0, MCA_IO4 is channel 4, and so on.

CAUTION! Not all MCA IO pins are ADC-capable.

The indexes of I/O expander ADC channels on ConnectCore 6UL SBC Pro correspond to their IO number: IOEXP_IO3 is channel 3, IOEXP_IO4 is channel 4, and so on.

CAUTION! Not all I/O expander pins are ADC-capable.

Kernel configuration

You can manage the i.MX6 ADC driver support through the following kernel configuration option:

Freescale vf610 ADC driver (CONFIG_VF610_ADC)

You can manage the MCA ADC driver support through the following kernel configuration option:

Digi ConnectCore 6UL Micro Controller Assist ADC (CONFIG_MCA_CC6UL_ADC)

You can manage the I/O Expander ADC driver support through the following kernel configuration option:

Digi IO Expander (CONFIG_MFD_MCA_IOEXP)

These options are enabled as built-in on the default ConnectCore 6UL kernel configuration file.

Platform driver mapping

The ADC drivers are located at:

File	Description
drivers/iio/adc/vf610_adc.c	i.MX6UL ADC driver
drivers/iio/adc/mca-cc6ul-adc.c	ConnectCore 6UL MCA ADC driver
drivers/iio/adc/mca-ioexp-adc.c	Digi I/O expander ADC driver
drivers/iio/adc/mca-common-adc.c	Common code for MCA and I/O expander ADC driver

Device tree bindings and customization

The i.MX6UL ADC device tree binding is documented at Documentation/devicetree/bindings/iio/adc/vf610-adc.txt.

The ConnectCore 6UL MCA ADC device tree binding is documented at Documentation/devicetree/bindings/iio/adc/digi,mca-cc6ul-adc.txt.

The I/O Expander ADC device tree binding is documented at Documentation/devicetree/bindings/iio/adc/digi,mca-ioexp-adc.txt.

The device tree must contain entries for:

The ADC interface
The enabled channels
The IOMUX (only for i.MX6UL ADC channels)

i.MX6UL ADC interface

Common i.MX6UL device tree

            adc1: adc@02198000 {
                compatible = "fsl,imx6ul-adc", "fsl,vf610-adc";
                reg = <0x02198000 0x4000>;
                interrupts = <GIC_SPI 100 IRQ_TYPE_LEVEL_HIGH>;
                clocks = <&clks IMX6UL_CLK_ADC1>;
                num-channels = <2>;
                clock-names = "adc";
                status = "disabled";
            };

ConnectCore 6UL MCA ADC interface

Common ConnectCore 6UL device tree

    mca_cc6ul: mca@7e {
        mca_adc: adc {
            compatible = "digi,mca-cc6ul-adc";
        };
    };

I/O expander ADC interface (ConnectCore 6UL SBC Pro only)

Common ConnectCore 6UL device tree

    mca_ioexp: mca_io@6e {
        mca_ioexp_adc: adc {
            compatible = "digi,mca-ioexp-adc";
            digi,adc-vref = <3300000>;
        };
    };

i.MX6UL enabled ADC channels and IOMUX configuration

This example describes how to enable i.MX6UL ADC channel 4 (corresponding to pad GPIO1_IO04) by using property adc-ch-list to specify the channels you want to enable.

The i.MX6UL pads must have the IOMUX configuration to properly behave as ADC (this is not needed for MCA ADC channels).

&adc1 {
    pinctrl-names = "default";
    pinctrl-0 = <&pinctrl_adc1>;
    adc-ch-list = <4>;
    status = "okay";
};
 
&iomuxc {
    imx6ul-ccimx6ul {
        /* Uncomment specific pins of the ADC channels enabled in 'adc-ch-list' */
        pinctrl_adc1: adc1grp {
            fsl,pins = <
                /* GPIO1_4/ADC1_IN4 (pin 7 of the expansion header) */
                MX6UL_PAD_GPIO1_IO04__GPIO1_IO04        0xb0
            >;
        };
    };
};

MCA-enabled ADC channels and voltage reference

This example describes how to enable MCA ADC channels 0 and 4 (corresponding to pads MCA_IO0 and MCA_IO4) by using property digi,adc-ch-list to specify the channels you want to enable.

For accurate measurements conversion, the input voltage MCA_VCC must be measured and passed to the kernel via property digi,adc-vref.

&mca_adc {
    digi,adc-ch-list = <0 4>;
    digi,adc-vref = <3000000>;
};

Customize the ADCs for your platform

Follow these steps to configure ADC channels on your custom design:

Identify the pins that you want configured as ADC and verify that they are ADC-capable (either MCA or i.MX6UL).
In the device tree of your platform, create or uncomment the nodes for the ADC interfaces you plan to use (mca_adc for MCA and adc1 for i.MX6UL).
Enable the channels indexes that you want to enable by using the adc-ch-list property (for i.MX6UL ADC channels) or digi,adc-ch-list (for MCA ADC channels).
For i.MX6UL ADC channels, configure the iomux of the pins to operate as ADC and configure the pad settings.
Compile the device tree and install it in your platform.

Using the ADCs

The ADC drivers are designed as standard Industrial I/O (IIO) device drivers that can be accessed from the sysfs or from user applications.

Sysfs access

When enabled, the ADC drivers (MCA and i.MX6UL) will create the corresponding device entries in the IIO sysfs directory (/sys/bus/iio/devices). To determine which entry corresponds to which driver read the name descriptor with:

~# cat /sys/bus/iio/devices/iio\:device0/name
mca-cc6ul-adc
~# cat /sys/bus/iio/devices/iio\:device1/name
2198000.adc

In this example, device0 corresponds to the MCA ADC and device1 corresponds to the i.MX6UL ADC.

The driver creates a file entry named in_voltageX_raw for each ADC channel, where X corresponds to the channel number. Read the input value of a channel with:

Reading an ADC through the sysfs

# cat /sys/bus/iio/devices/iio\:device0/in_voltage0_raw
768

The returned value is a decimal number with the result of the conversion. In the example:

V_READ = V_IN * 4095 / V_REF → V_IN = V_READ * V_REF / 4095 → V_IN = 768 * 3 / 4095 = 0.563 V

To help with the calculation you can simply multiply the read value by the value contained in the in_voltage_scale descriptor:

Reading an ADC through the sysfs

# cat /sys/bus/iio/devices/iio\:device0/in_voltage_scale
0.732421875

V_IN = V_READ * Scale = 768 * 0.732421875 = 0.563 V

Sample application

An example application called adc_sample is included in the dey-examples package of meta-digi layer. This application shows how to access the ADCs on the ConnectCore 6UL platform (both i.MX6UL and MCA ADCs).

To learn the syntax of the application run:

Sample application

~# adc_sample --help

Find the source code of the application under ${DEY_INSTALL_DIR}/sources/meta-digi/meta-digi-dey/dey-examples/files/adc_sample/.