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Table of Contents
Digital Potentiometer Linux Driver
Supported Devices
Reference Circuits
Evaluation Boards
- PMOD-DPOT
Description
The ad525x_dpot driver exports a simple sysfs interface. This allows you to work with the immediate resistance settings as well as update the saved startup settings. Access to the factory programmed tolerance is also provided, but interpretation of this settings is required by the end application according to the specific part in use.
Files
Each dpot device will have a set of eeprom, rdac, and tolerance files. How many depends on the actual part you have, as will the range of allowed values.
The eeprom files are used to program the startup value of the device.
The rdac files are used to program the immediate value of the device.
The tolerance files are the read-only factory programmed tolerance settings and may vary greatly on a part-by-part basis. For exact interpretation of this field, please consult the datasheet for your part. This is presented as a hex file for easier parsing.
Source Code
Status
Files
| Function | File |
|---|---|
| driver | drivers/misc/ad525x_dpot.c |
| i2c bus support | drivers/misc/ad525x_dpot-i2c.c |
| spi bus support | drivers/misc/ad525x_dpot-spi.c |
| include | drivers/misc/ad525x_dpot.h |
Example platform device initialization
For compile time configuration, it’s common Linux practice to keep board- and application-specific configuration out of the main driver file, instead putting it into the board support file.
For devices on custom boards, as typical of embedded and SoC-(system-on-chip) based hardware, Linux uses platform_data to point to board-specific structures describing devices and how they are connected to the SoC. This can include available ports, chip variants, preferred modes, default initialization, additional pin roles, and so on. This shrinks the board-support packages (BSPs) and minimizes board and application specific #ifdefs in drivers.
Example Platform / Board file (I2C Interface)
Declaring I2C devices
Unlike PCI or USB devices, I2C devices are not enumerated at the hardware level. Instead, the software must know which devices are connected on each I2C bus segment, and what address these devices are using. For this reason, the kernel code must instantiate I2C devices explicitly. There are different ways to achieve this, depending on the context and requirements. However the most common method is to declare the I2C devices by bus number.
This method is appropriate when the I2C bus is a system bus, as in many embedded systems, wherein each I2C bus has a number which is known in advance. It is thus possible to pre-declare the I2C devices that inhabit this bus. This is done with an array of struct i2c_board_info, which is registered by calling i2c_register_board_info().
So, to enable such a driver one need only edit the board support file by adding an appropriate entry to i2c_board_info.
For more information see: Documentation/i2c/instantiating-devices
static struct i2c_board_info __initdata bfin_i2c_board_info[] = { #if defined(CONFIG_AD525X_DPOT) || defined(CONFIG_AD525X_DPOT_MODULE) { I2C_BOARD_INFO("ad5245", 0x2c), }, { I2C_BOARD_INFO("ad5245", 0x2d), }, #endif }
Example Platform / Board file (SPI Interface)
Declaring SPI slave devices
Unlike PCI or USB devices, SPI devices are not enumerated at the hardware level. Instead, the software must know which devices are connected on each SPI bus segment, and what slave selects these devices are using. For this reason, the kernel code must instantiate SPI devices explicitly. The most common method is to declare the SPI devices by bus number.
This method is appropriate when the SPI bus is a system bus, as in many embedded systems, wherein each SPI bus has a number which is known in advance. It is thus possible to pre-declare the SPI devices that inhabit this bus. This is done with an array of struct spi_board_info, which is registered by calling spi_register_board_info().
For more information see: Documentation/spi/spi-summary
static struct spi_board_info bfin_spi_board_info[] __initdata = { #if defined(CONFIG_AD525X_DPOT) || defined(CONFIG_AD525X_DPOT_MODULE) { .modalias = "ad5291", .max_speed_hz = 5000000, /* max spi clock (SCK) speed in HZ */ .bus_num = 0, .chip_select = 1, }, #endif };
Old Method
static struct spi_board_info bfin_spi_board_info[] __initdata = { #if defined(CONFIG_AD525X_DPOT) || defined(CONFIG_AD525X_DPOT_MODULE) { .modalias = "ad_dpot", .platform_data = "ad5291", .max_speed_hz = 5000000, /* max spi clock (SCK) speed in HZ */ .bus_num = 0, .chip_select = 1, }, #endif };
Adding Linux driver support
Configure kernel with “make menuconfig” (alternatively use “make xconfig” or “make qconfig”)
The ad525x_dpot driver depends on CONFIG_SPI or CONFIG_I2C
Device Drivers --->
[*] Misc devices --->
<*> Analog Devices Digital Potentiometers
<*> support I2C bus connection
<*> support SPI bus connection
Hardware configuration
Driver testing
Locate the device in your sysfs tree. This is probably easiest by going into the common i2c directory and locating the device by the i2c slave address.
# ls /sys/bus/i2c/devices/ 0-0022 0-0027 0-002fSo assuming the device in question is on the first i2c bus and has the slave address of 0x2f, we descend (unrelated sysfs entries have been trimmed).
# ls /sys/bus/i2c/devices/0-002f/ eeprom0 rdac0 tolerance0You can use simple reads/writes to access these files:
# cd /sys/bus/i2c/devices/0-002f/ # cat eeprom0 0 # echo 10 > eeprom0 # cat eeprom0 10 # cat rdac0 5 # echo 3 > rdac0 # cat rdac0 3
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