RELEASE
Revision B1
©Serious Integrated, Inc.
26
LCD DISPLAY
The LCD display (or “glass”) on the SIM535 is a 7” diagonal active area 800x480 TFT with optional 4-wire resistive touch
layer. Various LCD options are available for the SIM535; see
Ordering Information
for more details.
The LCD display has no on-glass frame buffer or memory. The MCU is responsible for delivering pixel data at a specific
frequency to the LCD display as well as various clock signals otherwise the display will not function correctly and will not
display a stable image. No valid image is possible unless the MCU is operating and, under software control, the MCU is
delivering pixel and timing data to the LCD display continuously. The LCD display, in absence of a valid signal from the
MCU, may automatically enter self-test mode and display various cycling test patterns.
The LCD backlight is enabled when
PJ8/PWM2A-BLEN
is driven high, which turns on the backlight power boost circuit
driving a constant current at approximately 9.6-18V (depending on the LCD) into the LCD’s backlight LEDs. This
BLEN
signal has a weak pull-down, so the backlight is off until the MCU pin is initialized, including during and directly after
system
RESET#
. This pin can be pulse-width-modulated (“PWM’d”) to enable backlight dimming. A PWM driven by a
typical 1 KHz clock with 16 PWM steps for a PWM net frequency of 64 Hz is generally sufficient and flicker free with duty
cycles from 0 to 100%. In no circumstances should the PWM clock exceed 16 KHz.
The
Serious Human Interface™ Platform
has the backlight driver included; setting the platform glass backlight value to 0 to
100% automatically modulates the dimming circuit.
GRAPHICS CONTROLLER
Since the LCD display has no on-glass frame buffer, pixel data must be held in memory and streamed continuously to the
display hardware. This memory must also be MCU accessible in order for software to “draw” into the frame buffers and
transfer images and drawings to the screen.
On the SIM535, the
SH7269
has sufficient internal RAM to contain two 16-bit frame buffers. One can be used as the
currently displaying frame, the other as a hidden frame where updates and drawing can happen. Then the two can be
swapped (the display controller pointed at the hidden buffer) to expose the new frame and allow the prior-displayed to
be used as the new working buffer. When using 24 bit color modes, one frame buffer can held in internal RAM and the
other in external RAM, or both can be in external RAM. External RAM has slower performance than internal RAM, so
the location of these frame buffers is a matter of performance evaluation and tuning by the software designer.
The
Serious Human Interface™ Platform
software system has fully-integrated and optimized drivers and frame buffer
management, making the graphics controller details transparent to the GUI designer.
TOUCH CONTROLLER
Some SIM535 family members include a resistive touch layer bonded to the LCD display. The layer can return an analog
voltage in two dimensions to be read by analog-to-digital converters and translated with a software algorithm into a
pixel hit position.
Resistive touch layers are made from a highly resilient Polyethylene Terephthalate (PET) film, and have the advantage of
being robust and usable with a stylus, finger, or any blunt object. Unlike typical capacitive touch screens, resistive touch
screens do not require the bare finger and can be used through gloves – important for certain medical, industrial, and
automotive applications. They also work well in wet conditions, although appropriate caution must be taken to ensure
liquids do not flow onto the SIM535 or other circuitry.
Serious
application note
AN0201: Resistive Touch Bezel Guidelines
is a good resource for understanding how to mount a touch screen behind a bezel.
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