• AD7877

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:

• 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)

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/pnav10.c:

#include 

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

Your kernel startup messages should include something like this

input: AD7877 Touchscreen as /class/input/input2

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 ------------

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

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

Check that the interrupt is registered.

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

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