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This version (05 Sep 2012 14:11) was approved by mhennerich.The Previously approved version (09 Nov 2011 17:08) is available.Diff

AD7877 Input Touch Screen Controller Linux Driver

Supported Devices

Description

The AD7877 is a 12-bit, successive approximation ADC with a synchronous serial interface and low on resistance switches for driving touch screens.

To reduce the effects of noise from LCDs, the acquisition phase of the on-board ADC is controlled via the STOPACQ pin. User-programmable conversion controls include variable acquisition time and first conversion delay. Up to 16 averages can be taken per conversion. There is also an on-board DAC for LCD back-light or contrast control. The AD7877 runs in either slave or master mode using a conversion sequencer and timer. It is ideal for battery-powered systems such as personal digital assistants with resistive touch screens and other portable equipment.

The AD7877 operates from a single 2.7 V to 5.25 V power supply, and features throughput rates of 125 kSPS. The AD7877 features direct battery measurement on two inputs, temperature and touch-pressure measurement.

The AD7877 also has an on-board reference of 2.5 V. When not in use, it can be shut down to conserve power. An external reference can also be applied and varied from 1 V to +VCC, with an analog input range of 0 V to VREF. The device includes a shutdown mode that reduces its current consumption to less than 1 μA.

See also: uclinux-dist:tslib

See also: AD7879 Touchscreen Device Driver

Software configurable features

Software configurable:

  • STOPACQ Polarity (LOW, HIGH)
  • First Conversion Delay (0.5us, 128us, 1ms, 8ms)
  • Acquisition Time (2us, 4us, 8us, 16us)
  • Number of Averages (1,4,8,16)
  • Pen Down Acquisition Interval Timer (convert once, every 0.5ms, 1ms, 8ms)

Source Code

Status

Source Mainlined?
git Yes

Files

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 · Michael Hennerich

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

file: include/linux/spi/ad7877.h

/* linux/spi/ad7877.h */
 
/* Touchscreen characteristics vary between boards and models.  The
 * platform_data for the device's "struct device" holds this information.
 *
 * It's OK if the min/max values are zero.
 */
struct ad7877_platform_data {
	u16	model;			/* 7877 */
	u16	vref_delay_usecs;	/* 0 for external vref; etc */
	u16	x_plate_ohms;
	u16	y_plate_ohms;
 
	u16	x_min, x_max;
	u16	y_min, y_max;
	u16	pressure_min, pressure_max;
 
	u8	stopacq_polarity;	/* 1 = Active HIGH, 0 = Active LOW */
	u8	first_conversion_delay;	/* 0 = 0.5us, 1 = 128us, 2 = 1ms, 3 = 8ms */
	u8	acquisition_time;	/* 0 = 2us, 1 = 4us, 2 = 8us, 3 = 16us */
	u8	averaging;		/* 0 = 1, 1 = 4, 2 = 8, 3 = 16 */
	u8	pen_down_acc_interval;	/* 0 = covert once, 1 = every 0.5 ms,
					   2 = ever 1 ms,   3 = every 8 ms,*/
};

These snippets are all from the same file. arch/blackfin/mach-bf537/boards/pnav10.c:

#include <linux/spi/ad7877.h>

file: arch/blackfin/mach-bf537/boards/pnav10.c

static const struct ad7877_platform_data bfin_ad7877_ts_info = {
	.model			= 7877,
	.vref_delay_usecs	= 50,	/* internal, no capacitor */
	.x_plate_ohms		= 419,
	.y_plate_ohms		= 486,
	.pressure_max		= 1000,
	.pressure_min		= 0,
	.stopacq_polarity 	= 1,
	.first_conversion_delay = 3,
	.acquisition_time 	= 1,
	.averaging 		= 1,
	.pen_down_acc_interval 	= 1,
};

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

21 Oct 2010 16:10 · Michael Hennerich

file: arch/blackfin/mach-bf537/boards/pnav10.c

static struct spi_board_info bfin_spi_board_info[] __initdata = {
#if defined(CONFIG_MTD_M25P80) \
	|| defined(CONFIG_MTD_M25P80_MODULE)
	{
		/* the modalias must be the same as spi device driver name */
		.modalias = "m25p80", /* Name of spi_driver for this device */
		.max_speed_hz = 25000000,     /* max spi clock (SCK) speed in HZ */
		.bus_num = 0, /* Framework bus number */
		.chip_select = 1, /* Framework chip select. On STAMP537 it is SPISSEL1*/
		.platform_data = &bfin_spi_flash_data,
		.controller_data = &spi_flash_chip_info,
		.mode = SPI_MODE_3,
	},
#endif
 
#if defined(CONFIG_SND_BF5XX_SOC_AD183X) \
	|| defined(CONFIG_SND_BF5XX_SOC_AD183X_MODULE)
	{
		.modalias = "ad183x",
		.max_speed_hz = 3125000,     /* max spi clock (SCK) speed in HZ */
		.bus_num = 0,
		.chip_select = 4,
	},
#endif
#if defined(CONFIG_MMC_SPI) || defined(CONFIG_MMC_SPI_MODULE)
	{
		.modalias = "mmc_spi",
		.max_speed_hz = 25000000,     /* max spi clock (SCK) speed in HZ */
		.bus_num = 0,
		.chip_select = 5,
		.controller_data = &mmc_spi_chip_info,
		.mode = SPI_MODE_3,
	},
#endif
#if defined(CONFIG_TOUCHSCREEN_AD7877) || defined(CONFIG_TOUCHSCREEN_AD7877_MODULE)
{
	.modalias		= "ad7877",
	.platform_data		= &bfin_ad7877_ts_info,
	.irq			= IRQ_PF2,
	.max_speed_hz		= 12500000,     /* max spi clock (SCK) speed in HZ */
	.bus_num		= 0,
	.chip_select  		= 5,
},
#endif
 
};

Adding Linux driver support

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

The AD7877 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

The AD7877 Linux Driver only uses the /DAV Data Available Interrupt. /PENIRQ is not used. STOPACQ can be used to reduce the effect of noise from an LCD screen on the touch screen measurements.

Driver testing

Driver compiled as a module

root:~> modprobe evdev
root:~> modprobe ad7877
input: AD7877 Touchscreen as /class/input/input2
root:~>

Driver compiled into the kernel

Your kernel startup messages should include something like this

input: AD7877 Touchscreen as /class/input/input2

Use the evtest/event_test utility to test proper function

root:/> event_test /dev/input/event2
Input driver version is 1.0.0
Input device ID: bus 0x0 vendor 0x0 product 0x0 version 0x0
Input device name: "AD7877 Touchscreen"
Supported events:
  Event type 0 (Sync)
  Event type 1 (Key)
    Event code 330 (Touch)
  Event type 3 (Absolute)
    Event code 0 (X)
      Value   1268
      Min        0
      Max     4095
    Event code 1 (Y)
      Value   1250
      Min        0
      Max     4095
    Event code 24 (Pressure)
      Value      0
      Min        0
      Max     1000
Testing ... (interrupt to exit)
Event: time 67435.364000, type 1 (Key), code 330 (Touch), value 1
Event: time 67435.364000, type 3 (Absolute), code 0 (X), value 1624
Event: time 67435.364000, type 3 (Absolute), code 1 (Y), value 1514
Event: time 67435.364000, type 3 (Absolute), code 24 (Pressure), value 349
Event: time 67435.364000, -------------- Report Sync ------------
Event: time 67435.408000, type 3 (Absolute), code 0 (X), value 1626
Event: time 67435.408000, type 3 (Absolute), code 1 (Y), value 1516
Event: time 67435.408000, type 3 (Absolute), code 24 (Pressure), value 342
Event: time 67435.408000, -------------- Report Sync ------------
Event: time 67435.452000, type 3 (Absolute), code 1 (Y), value 1514
Event: time 67435.452000, type 3 (Absolute), code 24 (Pressure), value 339
Event: time 67435.452000, -------------- Report Sync ------------
Event: time 67435.496000, type 3 (Absolute), code 0 (X), value 1625
Event: time 67435.496000, type 3 (Absolute), code 24 (Pressure), value 343
Event: time 67435.496000, -------------- Report Sync ------------
Event: time 67435.596000, type 3 (Absolute), code 24 (Pressure), value 0
Event: time 67435.596000, type 1 (Key), code 330 (Touch), value 0
Event: time 67435.596000, -------------- Report Sync ------------

Common Problems

In case you see a message like this

spi0.2: Failed to probe AD7877 Touchscreen

This means that the SPI communication and initilaization with the AD7877 touchscreen controller/digitizer failed. check bus_num and chip_select in your platform device file

#if defined(CONFIG_TOUCHSCREEN_AD7877) || defined(CONFIG_TOUCHSCREEN_AD7877_MODULE)
{
	.modalias		= "ad7877",
	.platform_data		= &bfin_ad7877_ts_info,
	.irq			= IRQ_PJ11,
	.max_speed_hz		= 12500000,     /* max spi clock (SCK) speed in HZ */
	.bus_num		= 0,
	.chip_select  		= 2,
	.controller_data = &spi_ad7877_chip_info,
},
#endif

Testing

See the uclinux-dist:tslib page for information on testing the touchscreen from userspace.

Interrupt

Check that the interrupt is registered.

root:~> grep ad7877 /proc/interrupts
258:          0   ad7877

In case you touch the surface and don't receive events, it's likely that something with your /DAV Interrupt is wrong.
check irq number in your platform device file

In case you get a message like: evtest: No such device, it's likely that you have not install the necessary modules

Aux functions

Under the device folder there are auxiliary functions such as temperature, GPIO, DAC and ADC read backs and controls.

root:/sys/class/input/input2> cd device/
root:/sys/devices/platform/bfin-spi.0/spi0.2> ls -al
drwxr-xr-x    3 root     root            0 Jan  1 00:15 .
drwxr-xr-x    4 root     root            0 Jan  1  2007 ..
-r--r--r--    1 root     root         4096 Jan  1 00:15 aux1
-r--r--r--    1 root     root         4096 Jan  1 00:15 aux2
-r--r--r--    1 root     root         4096 Jan  1 00:15 aux3
-r--r--r--    1 root     root         4096 Jan  1 00:15 bat1
-r--r--r--    1 root     root         4096 Jan  1 00:15 bat2
lrwxrwxrwx    1 root     root            0 Jan  1 00:15 bus -> ../../../../bus/spi
-rw-rw-r--    1 root     root         4096 Jan  1 00:15 dac
-rw-rw-r--    1 root     root         4096 Jan  1 00:15 disable
lrwxrwxrwx    1 root     root            0 Jan  1 00:15 driver -> ../../../../bus/spi/drivers/ad7877
-rw-rw-r--    1 root     root         4096 Jan  1 00:15 gpio4
lrwxrwxrwx    1 root     root            0 Jan  1 00:15 input:event2 -> ../../../../class/input/input2/event2
lrwxrwxrwx    1 root     root            0 Jan  1 00:15 input:input2 -> ../../../../class/input/input2
-r--r--r--    1 root     root         4096 Jan  1 00:15 modalias
drwxr-xr-x    2 root     root            0 Jan  1  2007 power
lrwxrwxrwx    1 root     root            0 Jan  1 00:15 subsystem -> ../../../../bus/spi
-r--r--r--    1 root     root         4096 Jan  1 00:15 temp1
-r--r--r--    1 root     root         4096 Jan  1 00:15 temp2
-rw-r--r--    1 root     root         4096 Jan  1 00:15 uevent
root:/sys/devices/platform/bfin-spi.0/spi0.2> cat temp1
963
root:/sys/devices/platform/bfin-spi.0/spi0.2> cat temp1
960
root:/sys/devices/platform/bfin-spi.0/spi0.2> cat temp2
1167
root:/sys/devices/platform/bfin-spi.0/spi0.2> cat temp2
1167
root:/sys/devices/platform/bfin-spi.0/spi0.2> cat aux1
1929
root:/sys/devices/platform/bfin-spi.0/spi0.2> cat aux1
1929

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