SPC8106 S-MOS Systems, SPC8106 Datasheet - Page 127

• Manufacturer Part Number: SPC8106
• Description: LCD/CRT VGA CONTROLLER
• Manufacturer: S-MOS Systems

10.4 Background
There are two ways in which the SPC8106 differs from the VGA standard.
Firstly, the CRTC timing registers differ since the SPC8106 is optimized to support 640x480 sin-
gle/dual panel LCD's, with some flexibility to support other size panels. In LCD mode, the panel
timing is determined by two special registers (horizontal and vertical panel size), so the CRTC
registers of the VGA standard which would normally be used to vary the display monitor timings
are ignored in LCD mode.
Secondly, there are some functions provided in the VGA register set that are never used in com-
mon VGA applications. The register bits for these functions have been removed from the register
set supported in SPC8106 in order to optimize the design for VGA operation on a 16 gray-scale
640x480 dot LCD panel. In their place are read/writable registers bits that have no effect.
General Discussion
In the SPC8106, the non-timing related CRTC registers are the same as in the VGA standard.
However, in LCD modes the SPC8106 does not use any of the timing registers that provide full
monitor timing programmability since much of the timing circuitry has been optimized in hardware
to support 640x480 panels.
For LCD mode, the SPC8106 timing circuitry is programmed by a Horizontal Panel Size register
and a Vertical Panel Size register - all other LCD timing parameters are fixed in hardware, so the
functions of all the regular timing registers is ignored in LCD mode. The functions of registers 00h,
02h to 06h,10h, 15h, 16h, and certain bits in registers 07h, 09h and 11h are ignored in LCD mode
- in their place dummy read/write registers are provided. Note that CRTC registers 01h and 12h
(Horizontal/Vertical Display Enable End, respectively) have full functionality in both LCD and CRT
modes.
For CRT mode, full CRTC register support is provided, with some minor exceptions (see next
item).
Shift-Load and Count-by-2 are normally used only in modes 0Fh and 10h when run on 64kbyte
VGA cards. Since the SPC8106 is exclusively a 256kbyte device, this option is not useful. The
Shift-4 and Count-by-4 functions are never used in any standard VGA mode or by standard soft-
ware. No problems are expected arising from the deletion of these unused features. These bits
have been removed from the register set.
The 720 dot text modes (0+,1+,2+,3+) must be reduced to 640 dot modes due to the fixed panel
size. To simplify the design, the logic to select a 9-dot character clock has been removed for LCD
display modes. The associated register bit in the Sequencer functions as normal in CRT modes,
but for LCD modes this bit has no effect and only 8 dot modes are displayed on the LCD.
HRTC/2 mode has been omitted. In standard VGA, this mode was provided to support vertical
resolutions greater than 1024.
SPC8106
CRTC Timing Registers
Shift-Load, Shift-4, Count-by-2, Count-by-4 modes not supported
Support for Maximum 640 Dot LCD Panel Only
411-1.0
S-MOS Systems, Inc. • Tel: [phone] • Fax: [phone] • http://www.smos.com
X12-SP-001-07
Hardware Functional Specification
SP1-85