AL300 datasheet(23/64 Pages) ETC

23 / 64 page

AL300

November 28, 2001

6.11.1 RAM mode

If external ROM is not used, OSD data can be stored in the internal 1K Byte of RAM. In this mode,

the OSD data is stored in bitmap form, which is written by the host through the I

2C bus. Two

independent OSD RAM address generators generate addresses for indexing the OSD data stored in

RAM. The data written from the RAM are unpacked to either 1-bit or 2-bit data depending if 2-color

mode or 4-color mode is selected. MSB bits are read out as OSD data before the LSB bits. The

following diagram shows how the OSD key generator works in RAM mode.

The RAMADDRST1 (Reg. #92h) and RAMADDRST2 (Reg. #A2h) are the starting address pointers

indicating the starting addresses of the bitmaps of OSD1 and OSD2. RAMSTRIDE1 (Reg. #93h and

#8Bh) and RAMSTRIDE2 (Reg. #A3h and #8Ch) are the line offsets in the RAM for OSD1 bitmap

and OSD2 bitmap, respectively.

6.11.2 ROM mode

If external ROM is used to store font tables or bitmaps, the internal RAM is used to store the font

code or index for assessing a specific font (a font can be viewed as a pre-defined bitmap). The

addresses of the font indexes are generated first to retrieve the index address to a specific font or

bitmap. The following ROM address generators then calculate the address of the font stored in ROM

based on the font index read from RAM. The 8-bit data read from ROM is then unpacked into 1-bit

or 2-bit data depending if 2-color mode or 4-color mode is selected. MSB bits are read out as OSD

data before the LSB bits. This is illustrated as follows:

RAM

Address

Generator 1

RAM

Address

Generator 2

MUX

512K x 16

RAM

Unpacker

1/2

AL300-10 OSD Key Generator in RAM Mode

OSDKEY

RAM Address

Generator 1

RAM Address

Generator 2

MUX

512K x 16

RAM

ROM Address

Generator 1

ROM Address

Generator 2

MUX

ROM

Unpacker

OSDKEY

1/2

RAMADDR

RAMDATA

ROMADDR

ROMDATA

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