This version (11 Feb 2016 21:10) was approved by Lars-Peter Clausen.The Previously approved version (05 Sep 2012 14:12) is available.
Table of Contents
AD7873 Input Touch Screen Digitizer Linux Driver
Supported Devices
Description
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
Configuration
Software configurable features
Source Code
Status
Files
| Function | File |
|---|---|
| driver | drivers/input/touchscreen/ads7846.c |
| include | include/linux/spi/ads7846.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.
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:
#if defined(CONFIG_TOUCHSCREEN_ADS7846) || defined(CONFIG_TOUCHSCREEN_ADS7846_MODULE) #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, }; #endif
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
static struct spi_board_info bfin_spi_board_info[] __initdata = { #if defined(CONFIG_TOUCHSCREEN_ADS7846) || defined(CONFIG_TOUCHSCREEN_ADS7846_MODULE) { .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, }, #endif };
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 root:/>
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.txt · Last modified: 11 Feb 2016 21:10 by Lars-Peter Clausen