ADC

Introduction

The ROC-RK3506B-CC development board has two kinds of AD interface,
respectively: Temperature Sensor and Successive Approximation
Register. Among them:

TS-ADC (Temperature Sensor): Supports one channel.

SAR-ADC (Successive Approximation Register): Supports eight-channel
single-ended 10-bit SAR-ADC, The maximum conversion rate is 1MS/s.

The kernel uses the industrial I/O subsystem to control the ADC, which
is mainly designed for AD conversion or DA conversion sensor. The
following is an example of SAR-ADC using ADC fan to introduce the
basic configuration method of ADC.

Hardware connection

The ADC interface connected to the pin header is as follows


DTS configuration

Configure DTS nodes

The SAR-ADC nodes of ROC-RK3506B-CC defined in
"kernel/arch/arm/boot/dts/rk3502.dts" file, as showm below:

   saradc: adc@ff4e8000 {
   	compatible = "rockchip,rk3506-saradc", "rockchip,rk3562-saradc";
   	reg = <0xff4e8000 0x8000>;
   	interrupts = <GIC_SPI 57 IRQ_TYPE_LEVEL_HIGH>;
   	#io-channel-cells = <1>;
   	clocks = <&cru CLK_SARADC>, <&cru PCLK_SARADC>;
   	clock-names = "saradc", "apb_pclk";
   	resets = <&cru SRST_P_SARADC>;
   	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, the standard voltage is 1.8V, the collection bits of AD
is 10, and the original data collected by AD is 568, then:

   Vresult = (1800mv * 568) / 1023;

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.

Debug method

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.