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AD7873 Input Touch Screen Digitizer Linux Driver

Supported Devices


The AD7843/AD7873 is a 12-bit successive-approximation ADC with a synchronous serial interface and low on resistance switches for driving touch screens. The parts operates from a single 2.2 V to 5.25 V power supply and features throughput rates greater than 125 kSPS. The external reference applied to the AD7843 can be varied from 1 V to +VCC, while the analog input range is from 0 V to VREF. The device includes a shutdown mode that reduces the current consumption to less than 1 µA.

The AD7873 is similar to the AD7843 but has added functionality such as an on-chip temperature sensor (-40°C to + 85°C), on-chip 2.5 V reference and direct battery and touch-pressure measurement.

See also: AD7877 Touchscreen Device Driver See also: AD7879 Touchscreen Device Driver


Software configurable features

Source Code


Source Mainlined?
git Yes


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.

21 Oct 2010 16:10

Touchscreen characteristics vary between boards and models. The platform_data for the device's “struct device” holds this information.

<source trunk/include/linux/spi/ads7846.h c linux-kernel>

These snippets are all from the same file. arch/blackfin/mach-bf537/boards/stamp.c:

#include <linux/spi/ads7846.h>
static struct bfin5xx_spi_chip ad7873_spi_chip_info = {
	.bits_per_word	= 8,
static int ads7873_get_pendown_state(void)
	return gpio_get_value(GPIO_PF6);
static struct ads7846_platform_data __initdata ad7873_pdata = {
	.model		= 7873,		/* AD7873 */
	.x_max		= 0xfff,
	.y_max		= 0xfff,
	.x_plate_ohms	= 620,
	.debounce_max	= 1,
	.debounce_rep	= 0,
	.debounce_tol	= (~0),
	.get_pendown_state = ads7873_get_pendown_state,

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.rst

21 Oct 2010 16:10
static struct spi_board_info bfin_spi_board_info[] __initdata = {
		.modalias = "ads7846",
		.max_speed_hz = 2000000,     /* max spi clock (SCK) speed in HZ */
		.bus_num = 0,
		.irq = IRQ_PF6,
		.chip_select = GPIO_PF10 + MAX_CTRL_CS,	/* GPIO controlled SSEL */
		.controller_data = &ad7873_spi_chip_info, /* needed only on Blackfin */
		.platform_data = &ad7873_pdata,
		.mode = SPI_MODE_0,

Adding Linux driver support

Configure kernel with “make menuconfig” (alternatively use “make xconfig” or “make qconfig”)

The AD7873 Driver depends on CONFIG_SPI
Input device support
  -*- Generic input layer (needed for keyboard, mouse, ...)
  < >   Support for memoryless force-feedback devices
  < >   Polled input device skeleton
  < >   Sparse keymap support library
        *** Userland interfaces ***
  < >   Mouse interface
  < >   Joystick interface
  <*>   Event interface
  < >   Event debugging
        *** Input Device Drivers ***
  [ ]   Keyboards  --->
  [ ]   Mice  --->
  [ ]   Joysticks/Gamepads  --->
  [ ]   Tablets  --->
  [*]   Touchscreens  --->
 	 --- Touchscreens
 	 <*>   ADS7846/TSC2046/AD7873 and AD(S)7843 based touchscreens (NEW)
 	 < >   AD7877 based touchscreens
 	 < >   Analog Devices AD7879-1/AD7889-1 touchscreen interface (NEW)
  [ ]   Miscellaneous devices  --->
      Hardware I/O ports  --->

Hardware configuration

Driver testing

Driver compiled as a module

root:~> modprobe evdev
root:~> modprobe ads7846
ads7846 spi0.18: touchscreen, irq 56
input: ADS7873 Touchscreen as /devices/platform/bfin-spi.0/spi0.18/input/input0

Driver compiled into the kernel

Your kernel startup messages should include something like this

ads7846 spi0.18: touchscreen, irq 56
input: ADS7873 Touchscreen as /devices/platform/bfin-spi.0/spi0.18/input/input0

Checking for proper installation

After the kernel boot your device folder should include at least one device node for the touchscreen

root:/> ls -al /dev/input/
drw-r--r--    2 root     root            0 Jan  1 00:03 .
drwxr-xr-x    5 root     root            0 Jan  1 00:03 ..
crw-rw-r--    1 root     root      13,  64 Jan  1 00:03 event0

Check that the interrupt is registered.

root:~> cat /proc/interrupts | grep ads7846
56:         16      GPIO  ads7846

root:~> cat /sys/class/input/input0/name
ADS7873 Touchscreen

Use the event_test utility to test proper function

root:/> event_test /dev/input/event0                                                                           
Input driver version is 1.0.0                                                                                  
Input device ID: bus 0x0 vendor 0x0 product 0x0 version 0x0                                                    
Input device name: "ADS7873 Touchscreen"                                                                       
Supported events:                                                                                              
  Event type 0 (Reset)                                                                                         
    Event code 0 (Reset)                                                                                       
    Event code 1 (Key)                                                                                         
    Event code 3 (Absolute)                                                                                    
  Event type 1 (Key)                                                                                           
  Event type 3 (Absolute)                                                                                      
    Event code 0 (X)                                                                                           
      Value    760                                                                                             
      Min        0                                                                                             
      Max     4095                                                                                             
    Event code 1 (Y)                                                                                           
      Value   1973                                                                                             
      Min        0                                                                                             
      Max     4095                                                                                             
    Event code 24 (Pressure)                                                                                   
      Value      0                                                                                             
      Min        0                                                                                             
      Max        0                                                                                             
Testing ... (interrupt to exit)                                                                                
Event: time 258292.823196, type 1 (Key), code 330 (Touch), value 1
Event: time 258292.823209, type 3 (Absolute), code 0 (X), value 776
Event: time 258292.823214, type 3 (Absolute), code 1 (Y), value 505
Event: time 258292.823220, type 3 (Absolute), code 24 (Pressure), value 491
Event: time 258292.823225, type 0 (Reset), code 0 (Reset), value 0
Event: time 258292.828643, type 3 (Absolute), code 0 (X), value 789
Event: time 258292.828653, type 3 (Absolute), code 1 (Y), value 489
Event: time 258292.828659, type 3 (Absolute), code 24 (Pressure), value 485
Event: time 258292.828664, type 0 (Reset), code 0 (Reset), value 0
Event: time 258292.834085, type 3 (Absolute), code 0 (X), value 790
Event: time 258292.834095, type 3 (Absolute), code 1 (Y), value 499
Event: time 258292.834101, type 3 (Absolute), code 24 (Pressure), value 482
Event: time 258292.834106, type 0 (Reset), code 0 (Reset), value 0
Event: time 258292.839527, type 3 (Absolute), code 0 (X), value 789
Event: time 258292.839538, type 3 (Absolute), code 1 (Y), value 502
Event: time 258292.839544, type 3 (Absolute), code 24 (Pressure), value 481
Event: time 258292.839549, type 0 (Reset), code 0 (Reset), value 0
In case you touch the surface and don't receive events, it's likely that something with your /PENIRQ Interrupt is wrong.
check irq number in your platform device file

More Information

resources/tools-software/linux-drivers/input-touchscreen/ad7873.1346847165.txt.gz · Last modified: 05 Sep 2012 14:12 by Michael Hennerich