drivers: power: ltc2378: Add README documentation for LTC2378

Add README.rst documentation file for LTC2378 alongside other
documentation related files.

Signed-off-by: Cherrence Sarip <cherrence.sarip@analog.com>

Author: csarip

doc/sphinx/source/drivers/adc/ltc2378.rst

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+.. include:: ../../../../../drivers/adc/ltc2378/README.rst

drivers/adc/ltc2378/README.rst

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+LTC2378-20 no-OS driver
+=======================
+
+.. no-os-doxygen::
+
+Supported Devices
+-----------------
+
+- :adi:`LTC2378-20`
+
+Overview
+--------
+
+The LTC2378-20 is a high-speed, low-power, and low-noise 20-bit SAR
+(Successive Approximation Register) analog-to-digital converter (ADC). It
+operates from a 2.5V supply and supports both unipolar (0 to VREF) and
+bipolar (±VREF) input ranges, with VREF values ranging from 2.5V to 5.1V.
+Despite its high resolution, the device consumes only 21mW and delivers
+excellent performance with a maximum ±2ppm INL and a signal-to-noise ratio
+(SNR) of 104dB, ensuring no missing codes at 20-bit resolution.
+
+The ADC features a high-speed SPI-compatible serial interface that supports
+multiple logic levels —1.8V, 2.5V, 3.3V, and 5V—and includes a daisy-chain
+mode for connecting multiple devices. Its fast 1Msps throughput and zero
+cycle latency make it ideal for high-speed applications. An internal
+oscillator simplifies timing by setting the conversion time automatically,
+and the device powers down between conversions to reduce power consumption,
+which scales with the sampling rate.
+
+The LTC2378-20 supports both unipolar and bipolar input modes. In unipolar
+mode, the input range is from 0V to VREF with 20-bit resolution (2^20 codes).
+In bipolar mode, the input range is from -VREF to +VREF with effective
+19-bit resolution (2^19 codes for each polarity). The device uses a BUSY
+signal to indicate conversion completion and a CNV signal to initiate
+conversions.
+
+Applications
+------------
+
+* Medical Imaging
+* High Speed Data Acquisition
+* Portable or Compact Instrumentation
+* Industrial Process Control
+* Low Power Battery-Operated Instrumentation
+* Automated Test Equipment (ATE)
+
+LTC2378-20 Device Configuration
+-------------------------------
+
+Driver Initialization
+---------------------
+
+In order to be able to use the device, you will have to provide the support
+for the communication protocol (SPI) alongside GPIO pins for conversion
+control (CNV) and busy indication (BUSY).
+
+The first API to be called is **ltc2378_init**. Make sure that it returns 0,
+which means that the driver was initialized correctly.
+
+The initialization API uses the device descriptor and an initialization
+parameter. The initialization parameter contains the SPI configuration,
+GPIO parameters for CNV and BUSY pins, reference voltage (VREF) in microvolts,
+and input mode (unipolar or bipolar). These are defined in the header file
+of the driver.
+
+ADC Configuration
+-----------------
+
+The LTC2378-20 can be configured for either unipolar or bipolar input modes
+using the **input_mode** parameter during initialization.
+
+In **unipolar mode** (LTC2378_UNIPOLAR):
+- Input range: 0V to VREF
+- Resolution: 20 bits (2^20 = 1,048,576 codes)
+- Code 0 represents 0V, code 1048575 represents VREF
+
+In **bipolar mode** (LTC2378_BIPOLAR):
+- Input range: -VREF to +VREF
+- Resolution: 19 bits effective (2^19 = 524,288 codes per polarity)
+- Two's complement format with proper sign extension
+
+ADC Data Acquisition
+--------------------
+
+Data acquisition is performed using the **ltc2378_read_raw** API, which:
+
+1. Initiates a conversion using the CNV signal
+2. Waits for the BUSY signal to indicate conversion completion
+3. Reads the 20-bit result via SPI
+4. Returns the raw digital code
+
+The **ltc2378_raw_to_uv** API converts the raw digital code to voltage
+in microvolts, taking into account the configured input mode and reference
+voltage.
+
+Power Management
+----------------
+
+The LTC2378-20 automatically powers down between conversions to minimize
+power consumption. The **ltc2378_power_down** API can be used to manually
+put the device into power-down mode by holding the CNV signal low.
+
+LTC2378-20 Driver Initialization Example
+----------------------------------------
+
+.. code-block:: bash
+
+	struct ltc2378_dev *ltc2378_dev;
+	struct no_os_spi_init_param ltc2378_spi_ip = {
+		.device_id = SPI_DEVICE_ID,
+		.max_speed_hz = 1000000,
+		.chip_select = SPI_CS,
+		.mode = NO_OS_SPI_MODE_0,
+		.bit_order = NO_OS_SPI_BIT_ORDER_MSB_FIRST,
+		.platform_ops = SPI_OPS,
+		.extra = SPI_EXTRA,
+	};
+
+	struct no_os_gpio_init_param ltc2378_gpio_cnv = {
+		.port = GPIO_CNV_PORT_NUM,
+		.number = GPIO_CNV_PIN_NUM,
+		.platform_ops = GPIO_OPS,
+		.extra = GPIO_EXTRA
+	};
+
+	struct no_os_gpio_init_param ltc2378_gpio_busy = {
+		.port = GPIO_BUSY_PORT_NUM,
+		.number = GPIO_BUSY_PIN_NUM,
+		.platform_ops = GPIO_OPS,
+		.extra = GPIO_EXTRA
+	};
+
+	struct ltc2378_init_param ltc2378_ip = {
+		.spi_init = &ltc2378_spi_ip,
+		.gpio_cnv_init = &ltc2378_gpio_cnv,
+		.gpio_busy_init = &ltc2378_gpio_busy,
+		.vref_uv = 2500000,  // 2.5V reference
+		.input_mode = LTC2378_BIPOLAR
+	};
+
+	ret = ltc2378_init(&ltc2378_dev, &ltc2378_ip);
+	if (ret)
+		goto error;
+
+	// Read ADC value
+	uint32_t raw_data;
+	int32_t voltage_uv;
+
+	ret = ltc2378_read_raw(ltc2378_dev, &raw_data);
+	if (ret)
+		goto error;
+
+	ret = ltc2378_raw_to_uv(ltc2378_dev, raw_data, &voltage_uv);
+	if (ret)
+		goto error;
+
+LTC2378-20 no-OS IIO support
+---------------------------
+
+The LTC2378-20 IIO driver comes on top of the LTC2378-20 driver and offers support
+for interfacing IIO clients through libiio.
+
+LTC2378-20 IIO Device Configuration
+-----------------------------------
+
+Voltage Channel Attributes
+--------------------------
+
+The LTC2378-20 IIO device provides a single voltage input channel with the
+following attributes:
+
+* ``raw`` - The raw digital code from the ADC (0 to 1048575 for 20-bit)
+* ``scale`` - The scale factor to convert raw values to millivolts
+* ``processed`` - The processed voltage value in millivolts (raw * scale)
+
+The scale factor is automatically calculated based on the configured reference
+voltage and input mode:
+
+- **Unipolar mode**: scale = VREF / 2^20
+- **Bipolar mode**: scale = VREF / 2^19
+
+For example, with VREF = 2.5V:
+- Unipolar scale = 2500000 µV / 1048576 ≈ 2.384 µV/LSB
+- Bipolar scale = 2500000 µV / 524288 ≈ 4.768 µV/LSB
+
+LTC2378-20 IIO Driver Initialization Example
+--------------------------------------------
+
+.. code-block:: bash
+
+	int ret;
+
+	struct ltc2378_iio_desc *ltc2378_iio_desc;
+	struct ltc2378_iio_desc_init_param ltc2378_iio_ip = {
+		.ltc2378_init_param = &ltc2378_ip,
+	};
+
+	struct iio_app_desc *app;
+	struct iio_app_init_param app_init_param = { 0 };
+
+	ret = ltc2378_iio_init(&ltc2378_iio_desc, &ltc2378_iio_ip);
+	if (ret)
+		goto exit;
+
+	struct iio_app_device iio_devices[] = {
+		{
+			.name = "ltc2378-20",
+			.dev = ltc2378_iio_desc,
+			.dev_descriptor = ltc2378_iio_desc->iio_dev,
+		},
+	};
+
+	app_init_param.devices = iio_devices;
+	app_init_param.nb_devices = NO_OS_ARRAY_SIZE(iio_devices);
+	app_init_param.uart_init_params = uip;
+
+	ret = iio_app_init(&app, app_init_param);
+	if (ret)
+		goto remove_iio_ltc2378;
+
+	ret = iio_app_run(app);
+
+	iio_app_remove(app);
+
+remove_iio_ltc2378:
+	ltc2378_iio_remove(ltc2378_iio_desc);
+exit:
+	if (ret)
+		pr_info("Error!\n");
+	return ret;