ADC

Introduction

C40PL There are two AD interfaces on the development board:
temperature sensor (Temperature Sensor) and successive approximation
ADC (Successive Approximation Register). Among them:

TS-ADC (Temperature Sensor): supports 2 channels.

SAR-ADC (Successive Approximation Register): supports six-channel
single-ended 10-bit SAR-ADC, with a maximum conversion rate of 1MSPS
and a 20MHz A/D converter clock.

The core uses an industrial I/O subsystem to control the ADC, which is
mainly designed for sensors with AD conversion or DA conversion. The
following takes SAR-ADC as an example to introduce the basic
configuration method of ADC.

DTS configuration

Configure DTS nodes

The SAR-ADC nodes of C40PL defined in
"kernel/arch/arm/boot/dts/rv1126.dtsi" file, as showm below:

   saradc: saradc@ff5e0000 {
   	compatible = "rockchip,rk3399-saradc";
   	reg = <0xff5e0000 0x100>;
   	interrupts = <GIC_SPI 40 IRQ_TYPE_LEVEL_HIGH>;
   	#io-channel-cells = <1>;
   	clocks = <&cru CLK_SARADC>, <&cru PCLK_SARADC>;
   	clock-names = "saradc", "apb_pclk";
   	resets = <&cru SRST_SARADC_P>;
   	reset-names = "saradc-apb";
   	status = "disabled";
   };

Drive instructions

Get AD channel

   struct iio_channel *chan;                    #Defines the IIO channel structure
   chan = iio_channel_get(&pdev->dev, NULL);    #Get AD channel structure

Note:

 "iio_channel_get" gets the IIO channel structure through the
parameter pdev passed in by the probe function. The probe function is
as follows:

   static int XXX_probe(struct platform_device *pdev);

Read the original data collected by AD

   int val,ret;
   ret = iio_read_channel_raw(chan, &val);

Call the "iio_read_channel_raw" function to read the raw data
collected by AD and store it in "val".

Calculate the collected voltage

The standard voltage is used to convert the value of AD to the voltage
value required by the user. The calculation formula is as follows:

   Vref / (2^n-1) = Vresult / raw

Note:

Vref is the standard voltage

n is the number of bits converted to AD

Vresult is the collection voltage required by the user

raw is the original data that AD collects

For example, if the standard voltage is 1.8V, the AD acquisition bit
number is 12 bits, and the raw data collected by AD is 445, then:

   Vresult = (1800mv * 445) / 4095;

Interface specification

   struct iio_channel *iio_channel_get(struct device *dev, const char *consumer_channel);

Function : Gets the iio channel description.

Parameters :  dev: Using the device description pointer of this
channel.  consumer_channel: The IIO channel description pointer used
by the device.

   void iio_channel_release(struct iio_channel *chan);

Function : Release the channel obtained by the iio_channel_get
function.

Parameters :  chan ：The channel description pointer to be released.

   int iio_read_channel_raw(struct iio_channel *chan, int *val);

Function : Read the original data collected by chan channel AD.

Parameters :  chan：Collection channel pointer to read.  val：The
pointer to the result of reading.

Gets all ADC values

There is a convenient way to query the value of each SARADC:

   cat /sys/bus/iio/devices/iio\:device0/in_voltage*_raw

FAQs

Follow the steps above to apply for SARADC. Why is there an
application error?

When the driver needs to get the ADC channel to use, it needs to
control the load time of the driver, which must be after saradc
initialization. Saradc uses "module_platform_driver()" for platform
device driver registration and ultimately calls "module_init()".
Therefore, the driver loading function of the user only needs to use
one with lower priority than "module_init()", such as
"late_initcall()", so as to ensure that the loading time of the driver
is later than the initialization time of saradc and avoid errors.