Commit 51cb21a9 authored by Marc Singer's avatar Marc Singer Committed by Russell King

[ARM] 3407/1: lpd7x: documetation update

Patch from Marc Singer

New documentation for the touchscreen controllers and LCD panels.
Signed-off-by: default avatarMarc Singer <elf@buici.com>
Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
parent d4adcffb
README on the ADC/Touchscreen Controller
========================================
The LH79524 and LH7A404 include a built-in Analog to Digital
controller (ADC) that is used to process input from a touchscreen.
The driver only implements a four-wire touch panel protocol.
The touchscreen driver is maintenance free except for the pen-down or
touch threshold. Some resistive displays and board combinations may
require tuning of this threshold. The driver exposes some of it's
internal state in the sys filesystem. If the kernel is configured
with it, CONFIG_SYSFS, and sysfs is mounted at /sys, there will be a
directory
/sys/devices/platform/adc-lh7.0
containing these files.
-r--r--r-- 1 root root 4096 Jan 1 00:00 samples
-rw-r--r-- 1 root root 4096 Jan 1 00:00 threshold
-r--r--r-- 1 root root 4096 Jan 1 00:00 threshold_range
The threshold is the current touch threshold. It defaults to 750 on
most targets.
# cat threshold
750
The threshold_range contains the range of valid values for the
threshold. Values outside of this range will be silently ignored.
# cat threshold_range
0 1023
To change the threshold, write a value to the threshold file.
# echo 500 > threshold
# cat threshold
500
The samples file contains the most recently sampled values from the
ADC. There are 12. Below are typical of the last sampled values when
the pen has been released. The first two and last two samples are for
detecting whether or not the pen is down. The third through sixth are
X coordinate samples. The seventh through tenth are Y coordinate
samples.
# cat samples
1023 1023 0 0 0 0 530 529 530 529 1023 1023
To determine a reasonable threshold, press on the touch panel with an
appropriate stylus and read the values from samples.
# cat samples
1023 676 92 103 101 102 855 919 922 922 1023 679
The first and eleventh samples are discarded. Thus, the important
values are the second and twelfth which are used to determine if the
pen is down. When both are below the threshold, the driver registers
that the pen is down. When either is above the threshold, it
registers then pen is up.
README on the LCD Panels
========================
Configuration options for several LCD panels, available from Logic PD,
are included in the kernel source. This README will help you
understand the configuration data and give you some guidance for
adding support for other panels if you wish.
lcd-panels.h
------------
There is no way, at present, to detect which panel is attached to the
system at runtime. Thus the kernel configuration is static. The file
arch/arm/mach-ld7a40x/lcd-panels.h (or similar) defines all of the
panel specific parameters.
It should be possible for this data to be shared among several device
families. The current layout may be insufficiently general, but it is
amenable to improvement.
PIXEL_CLOCK
-----------
The panel data sheets will give a range of acceptable pixel clocks.
The fundamental LCDCLK input frequency is divided down by a PCD
constant in field '.tim2'. It may happen that it is impossible to set
the pixel clock within this range. A clock which is too slow will
tend to flicker. For the highest quality image, set the clock as high
as possible.
MARGINS
-------
These values may be difficult to glean from the panel data sheet. In
the case of the Sharp panels, the upper margin is explicitly called
out as a specific number of lines from the top of the frame. The
other values may not matter as much as the panels tend to
automatically center the image.
Sync Sense
----------
The sense of the hsync and vsync pulses may be called out in the data
sheet. On one panel, the sense of these pulses determine the height
of the visible region on the panel. Most of the Sharp panels use
negative sense sync pulses set by the TIM2_IHS and TIM2_IVS bits in
'.tim2'.
Pel Layout
----------
The Sharp color TFT panels are all configured for 16 bit direct color
modes. The amba-lcd driver sets the pel mode to 565 for 5 bits of
each red and blue and 6 bits of green.
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