LM1253A National Semiconductor, LM1253A Datasheet - Page 33

• Manufacturer Part Number: LM1253A
• Description: Monolithic Triple 180 MHz I2C CRT Pre-amp With Integrated Analog On Screen Display (OSD) Generator
• Manufacturer: National Semiconductor

Display Page RAM
Row End Code
To signify the end of a row of characters, a special ‘Row-End’ (RE) code is used in place of a character code.
Bits 11–4 Row-End Code: A special character code of 01h
Bits 3–0:
The RE character tells the OSD generator that the character codes following must be placed on a new row in the displayed win-
dow.
Skipped Line Parameters
Each displayed row of characters may have up to 15 skipped (i.e., blank) lines beneath it in order to allow finer control of the ver-
tical spacing of character rows. (Each skipped line is treated as a single auto-height character pixel line, so multiple scan lines
may actually be displayed in order to maintain accurate size relative to the character cell — see section Constant Character
Height Mechanism ).
To specify the number of skipped lines, the first character in each new row of characters to be displayed is interpreted differently
than the other characters in the row. Instead of interpreting the data in the location as a character code, the information of the
12 bit word is defined as follows:
Bits 11–8 Reserved.
Bits 7–4:
Bits 3–0:
Each new line MUST start with an SL code, even if the number of skipped lines to follow is zero. An SL code MUST always follow
an RE control code.
An RE code may follow an SL code if several ‘transparent’ lines are required between sections of the window (see example 3 be-
low). In this case, skipped lines of zero characters are displayed, causing a break in the window.
Window-End Code
To signify the end of the window, a special ‘Window-End’ (WE) code is used in place of a Row-End code.
Bits 11–4 Window-End Code: A special character code of 00h
Bits 3–0:
The WE control code tells the OSD generator that the character codes following belong to another displayed window at the next
window location. A WE control code may follow normal characters or an SL parameter, but never an RE control code.
Writing To The Page RAM
The Display Page RAM can contain up to 512 of the above listed characters and control codes. Each character, or control code
will consume one of the possible 512 locations. For convenience, a single write instruction to bit 3 of the Frame Control Register
(8400h) can reset the page RAM value to all zero.
Display Window 1 will also start at the first location (corresponding to the I
the Skip-Line (SL) parameters associated with the first row of Display Window 1. Subsequent locations should contain the char-
acters to be displayed on row 1 of Display Window 1, until the RE character code or WE character code is written into the Display
Page-RAM.
Don’t care
Skipped Lines. These four bits determine how many blank pixel lines will be inserted between the present row of dis-
play characters and the next row of display characters. A range of 0–15 may be selected.
Attribute code. The pixels in the skipped lines will normally be Color 1 of the addressed 2-Color Attribute Table entry.
Note that the pixels in the first line immediately below the character may be overwritten by the pixel override system
that creates the button box. (Refer to the Button Box Formation Section for more information.)
Don’t care
(Continued)
33
2
C address 8000h). This location must always contain
DS101265-48
DS101265-49
DS101265-50
www.national.com