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Direct Drive LCD Design Guide
entire panel to fill with valid pixels is the maximum refresh rate. Displays in existing media systems usually
have refresh rates between 48Hz and 120Hz to avoid visible flicker.
HSync, VSync and Dot Clock are all generated using TPU channels of the H8S or H8SX microcontrollers.
The TPU allows timer compare actions which synchronize these signals to the ExDMA request line. This
ensures that the clocks are generated when valid data is available on the bus.
3.1.5 Data Enable
Many panels require an additional signal to frame the valid data – this enable signal is sent at the time the
valid data is latched onto the bus – the panel will then clock in the data on the next edge of Dot Clock. It
provides added synchronization for the timing of data, but this signal can often be left in the active state if
the Dot Clock synchronization to the data is predictable.
3.1.6 RGB (Red Green Blue) Data
The data is presented to the panel in parallel. LCD panels have connections for 6 or 8bits of data for each
color totaling 18 or 24bits of color resolution. Our solution uses a 16bit data bus, so the most significant
bits of the data are presented to the panel. RGB 5-6-5 is the most common 16 bit solution. The least
significant bits can be tied low, or better still, to the MSB which would give a slightly improved range over
tying to ground.
Specifically with respect to Renesas H8S and H8SX devices, we use the ExDMA module to control the
data bus transfers.
3.1.7 Touch Screen
Our support is currently for popular resistive touch screen panels which have 4 connections (endpoints of
an X axis resistance and Y axis resistance). These inputs provide resistances proportional to the touched
location particular X and Y coordinates on the panel.
The host system (microcontroller plus logic) drives the resistive endpoints with a known reference voltage,
and the level on the channel is read into an analog to digital converter (ADC). With calibration and scaling
in the microcontroller driver code, it is possible to pinpoint the area of the panel that was touched. Action
can be taken accordingly.
An example of interface circuitry between the MCU and touch-screen is shown in Figure 6 below.
VCC
Y_DRIVE
X_D R I VE
TOUCH_YU
TOUCH_XL
TOUCH_YL
TOUCH_XR
R2
1K
C4
10nF
C2
10nF
U1D
HC126A
12 11
13
U1C
HC126A
9 8
10
C5
10nF
R4
1K
R9
1K
U1A
HC126A
2 3
1
R11
1K
R1
DNF
C1
10nF
R3
DNF
J4
MOLEX_52044
1
2
3
4
R8
DNF
R10
DNF
4
U1B
HC126A
5 6
Figure 8 Touch Screen Circuit
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