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:
Function | File |
---|---|
driver | drivers/input/touchscreen/ad7877.c |
include | include/linux/spi/ad7877.h |
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.
These snippets are all from the same file. arch/blackfin/mach-bf537/boards/pnav10.c
:
#include
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
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 --->
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.
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
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
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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