• AD7873
• AD7843

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

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

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 
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

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
};

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

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  --->

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

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

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

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