SDA5250 Datasheet, PDF (22/143 Pages) Siemens Semiconductor Group – TVTEXT 8-Bit Microcontroller

SDA 525x
6.2.7
Character Generator
The character generator includes the character and control code decoder, the RAM
interface and the RGB-, BLAN- and COR-signal generator. The display generator reads
data from the display RAM and calculates appropriate data which drives the RGB output
pins. The pixel clock is generated by a start-stop-oscillator. The synchronization of
display and pixel clock is done via external sandcastle or HS and VS signals. For 60 Hz
display the number of lines per character can be reduced to 9 or 8. In this case pixel
information of line 10 or 9 plus 10 are rejected. With this mode combined with the
variable vertical offset it is possible to generate NTSC displays with 25 rows.
Characters with a binary value < 32 are interpreted as control characters. For binary
values ≥ 32 a ROM character is selected through the addition of the character address,
the language setting in SFR, the europe designation and the graphics control bits
delivered from the control bit decoder.
A total of 64 OSD characters and 64 mosaic graphics characters are available. OSD
characters with addresses 80...SF_H can be displayed together with 60 lower case
characters because there is no memory overlapping with any other characters. OSD
characters with addresses 60...7F_H can only be displayed if bit OSD in SFR LANGC is
set (see diagrams: Physical Address Space and Vertical Address Space).
Figures 6 - 13 shows the character ROM contents.
The control byte decoder analyses the serial attributes from the display memory and
generates control clocks for the RGB logic and the character address decoder. The
interpretation of control characters is corresponding to World Standard Teletext norm.
Table 5 shows the characters and the appearance on the screen.
The RGB logic combines data from the character address decoder, control byte decoder
and settings from the SFR registers and generates signal R, G, B, BLAN and COR.
6.2.8
On Screen Display (OSD)
A display page in the display memory can also be used for on screen displays. It should
be recognized that all serial attributes of a normal text page are also valid for an OSD
display. Therefore if double height is selected anywhere in a normal text page, row n and
row n-1 (upper row) should be saved and overwritten by OSD data in order to generate
a correct display. Switching back to text display is accomplished by rewriting the text
data to the page. The same procedure is needed for the “erase row bits” in row 25. By
means of enable box bits, transparent control bits and the serial attribute “OSD”, the
OSD screen can be controlled fully independent of the normal text page. The serial
OSD-bit toggles the screen between normal display and OSD.
Semiconductor Group
22
1998-04-08