AD7877

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.

https://wiki.analog.com/_media/software/driver/linux/ad7877_block_dia.png..figure::https://wiki.analog.com/_media/software/driver/linux/ad7877_input_structure.png

See also: tslib - Touchscreen Access Library

See also: AD7879

Driver features and limitations

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

Function

File

driver

drivers/input/touchscreen/ad7877.c

include

include/linux/spi/ad7877.h

Hardware configuration

Tip

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.

https://wiki.analog.com/_media/software/driver/linux/ad7877_example_schematic.png

Adding Linux driver support

Kernel configuration

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

Note

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  --->

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

Platform-data configuration

The AD7877 driver has no devicetree binding; it is configured entirely through struct ad7877_platform_data (include/linux/spi/ad7877.h), supplied as the SPI device’s platform data:

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 = convert once, 1 = every 0.5ms,
                                   2 = every 1ms, 3 = every 8ms */
};

The board code registers the SPI device with a pointer to this structure in spi_board_info.platform_data, together with the IRQ, chip select and bus number.

Driver testing

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

Testing

See the tslib - Touchscreen Access Library page for information on testing the touchscreen from userspace.

Interrupt

Check that the interrupt is registered.

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

Tip

In case you touch the surface and don’t receive events, it’s likely that something with your /DAV Interrupt is wrong. Check the IRQ in the board’s platform data.

Tip

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, temp1/temp2, aux1/aux2/aux3 and bat1/bat2 return raw ADC conversion codes, not degrees Celsius or millivolts. The dac file accepts an unsigned decimal value and writes its low eight bits (use 0–255). gpio4 is available; the gpio3 module option replaces aux3 with gpio3. disable controls touchscreen IRQ handling.

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

Troubleshooting

Common Problems

In case you see a message like this

spi0.2: Failed to probe AD7877 Touchscreen

This means that the SPI communication and initialization with the AD7877 touchscreen controller/digitizer failed. Check the bus_num and chip_select in the board’s spi_board_info entry:

{
    .modalias     = "ad7877",
    .platform_data    = &ad7877_ts_info,
    .irq          = irq,
    .max_speed_hz     = 12500000,  /* max spi clock (SCK) speed in HZ */
    .bus_num      = 0,
    .chip_select      = 2,
},