Specifications

EPC-33/34 Hardware Reference
Page 30
3 3
The VGA controller provides the following set of output connectors:
1. A 15-pin D-sub connector on the AT bracket to connect to a VGA monitor.
2. A 10-pin .100” pitch shrouded header which carries the same signals as the 15-
pin D connector.
3. A 26-pin .100” pitch shrouded header to provide the feature connector interface.
4. A 44-pin 2.0mm pitch shrouded header to connect flat panel displays.
The VGA controller includes power-up and power-down sequence control to protect
the flat panel. This includes separate outputs to control the panel logic voltage, the
panel high-voltage, and the panel backlight voltage. The BIOS includes space for
several different types of panels to be supported. At power-up, a set of jumpers is
read by the VGA controller to select which of the six options is used. Refer to
Appendix D for more information.
LCD Contrast Control
An adjustable DC-DC converter and associated circuitry is provided to allow
software control of the contrast setting of an LCD flat panel. The output from this
circuit are two variable voltages (positive and negative) which are available on the
flat panel connector.
The contrast adjustment is done by writing an 8-bit value to the I/O Port at address
0x38D. The value written to this port is used to drive a Digital to Analog converter
which controls the output voltage of the DC-DC converter. The output voltage
ranges linearly from a minimum of approximately ±4V at a register value of 0xFF, to
a maximum of +/-30V at a register value of 0x00. Positive and negative values
change together when this register value changes.
LCD Backlight Control
A software addressable on/off FET switch is provided to allow application software
control of the LCD backlight. This output on the flat panel connector is derived from
the SVGA chip’s backlight control, which is controlled by the power sequencing
built into the chip, or it can be overridden by writing directly to the CR23 register in
the SVGA chip. Writing to the I/O port at 3D4 sets up the index register so that the
desired VGA register (CR23) can be accessed through I/O port 3D5. Writing a 1 to
bit 3 of CR23 enables the override of the default backlight control. Once this bit is
set, the backlight is controlled by bit 2 of CR23 no matter what is going on elsewhere
in the VGA controller. Bit 2 of CR23 can then be used to directly turn the backlight
on and off. Writing a 1 to this bit turns the backlight on, and writing a 0 to this bit
turns the backlight off.