An Analog-to-Digital Converter (ADC) is a device that translates an analog voltage to a digital value that a microprocessor can understand.
The MCA ADC controllers are 12-bit resolution, right justified, unsigned format. The MCA ADC channels are suitable for low-frequency sampling (under 10 Hz).
Available ADC pins
On the ConnectCore 8M Mini system-on-module:
-
Five MCA pads can be configured as ADC channels:
-
MCA_IO1
-
MCA_IO2
-
MCA_IO3
-
MCA_IO4
-
MCA_IO14
-
On the ConnectCore 8M Mini Development Kit:
-
The following ADC channels are accessible on the XBee Cellular socket J39-J40:
-
MCA_IO1 (MCA ADC channel 1)
-
MCA_IO2 (MCA ADC channel 2)
-
MCA_IO3 (MCA ADC channel 3)
-
MCA_IO4 (MCA ADC channel 4)
-
Formula
The value read on the ADC inputs responds to the formula:
\$V_(READ) = (V_(IN) * 4095) / V_(REF)\$
where:
-
VREAD is the digital value read
-
VIN is the analog input voltage
-
VREF is the ADC voltage reference
Voltage reference
-
For the MCA ADCs the reference voltage is configurable through one of the following device tree properties:
-
digi,adc-vref = <value_in_mV>
: This device tree entry makes MCA ADCs use MCA_VCC as reference voltage. This is an input voltage for the ConnectCore 8M Mini so you must configure it to the voltage value for your board. -
digi,internal-vref
: This boolean device tree entry makes MCA ADCs use a steady temperature-compensated 1.2 V as reference voltage.-
Pad MCA_VREF_OUT will output the 1.2 V to be used as a voltage reference for external peripherals.
-
The value in
digi,adc-vref
property will be ignored.
-
-
ADC channel mapping
-
The indexes of MCA ADC channels correspond to their MCA IO number: MCA_IO1 is channel 1, MCA_IO2 is channel 2, and so on.
Not all MCA IO pins are ADC-capable. See MCA I/O pads for a list of all available MCA IOs and their capabilities.
Kernel configuration
You can manage the ADC drivers through the following kernel configuration options:
-
Digi ConnectCore SOMs Micro Controller Assist ADC (
CONFIG_MCA_ADC
)
These options are enabled as built-in on the default ConnectCore 8M Mini kernel configuration file.
Kernel drivers
-
ConnectCore 8M Mini MCA ADC driver:
drivers/iio/adc/mca-adc.c
Device tree bindings and customization
The MCA ADC device tree binding is documented at
Documentation/devicetree/bindings/iio/adc/digi,mca-adc.txt
.
The device tree must contain entries for:
-
The ADC interface
-
The enabled channels
ConnectCore 8M Mini MCA ADC interface
mca_cc8m: mca@63 {
mca_adc: adc {
compatible = "digi,mca-cc8m-adc";
};
};
Example: enable MCA ADC channels 1, 2, 3 and 4 on ConnectCore 8M Mini Development Kit
For example, MCA ADC channels 1, 2, 3 and 4 are available on the ConnectCore 8M Mini Development Kit XBee Cellular socket. To enable them, you must:
-
Disable the
mca_uart2
node on the device tree. -
Add channels 1, 2, 3 and 4 to
digi,adc-ch-list
property on the ADC node. -
Enable the
mca_adc
node.
For accurate measurement conversion, you must measure the input voltage MCA_VCC and pass it to the kernel via property digi,adc-vref
.
//&mca_uart {
// status = "okay";
/* UART connected to XBee cellular socket */
// mca_uart2: serial@0x940 {
// iopins-names = "rx", "tx", "cts", "rts";
// iopins = <2 1 3 4>;
// };
//};
&mca_adc {
digi,adc-ch-list = <1 2 3 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 you want configured as ADC and verify that they are ADC-capable.
-
In the device tree of your platform, create or uncomment the nodes for the ADC interfaces you plan to use (
mca_adc
for MCA). -
Enable the channel indexes you want to enable by using
digi,adc-ch-list
. -
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 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-cc8m-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:
# cat /sys/bus/iio/devices/iio\:device0/in_voltage1_raw
37
The returned value is a decimal number with the result of the conversion. In the example for the {cpu-family} ADC:
\$V_(READ) = (V_(IN) * 4095) / V_(REF) -> V_(IN) = (V_(READ) * V_(REF)) / 4095 -> V_(IN) = (37 * 1.8) / 4095 = 0.016 V\$
To help with the calculation you can simply multiply the read value by the value contained in the in_voltage_scale
descriptor:
# cat /sys/bus/iio/devices/iio\:device0/in_voltage_scale
0.439453125
\$V_(IN) = V_(READ) * "<in_voltage_scale>" = 37 * 0.439453125 = 16.259 mV = 0.016 V\$
Sample application
An example application called apix-adc-example
is included in the dey-examples-digiapix recipe (part of dey-examples package) of meta-digi layer.
This application shows how to access the ADCs using Digi APIx library on the ConnectCore 8M Mini platform.
Go to GitHub to see the application instructions and source code.
See ADC API for more information about the ADC APIx.