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ADVANCE INFORMATION
VCT 38xxA
Micronas
57
3.8. Character Generator
Characters are addressed using a 10-bit character
code. The 2 MSBs of the character code define 1 of 4
character sets. Character set selection is done using
level 2 parallel attributes (see Table 3–7 on page 56).
Each character set contains 224 characters. The first
32 characters in each character set are reserved for
control codes (see Table 3–6 on page 56). On a single
screen, 896 different characters can be displayed.
Characters can be displayed in several pixel resolu-
tions provided that the according font is available. The
character generator supports horizontal resolution of
8 or 10 pixel/char and vertical resolution of 8, 10, or
13 lines/char. Characters can be combined without
separating borders to create more complex character
definitions (e.g. kanji or icons).
The pixel clock can be either 10.125 MHz or
20.25 MHz. To get 10-bit pixel information from the
character font, two memory cycles are needed. The
character font is part of the mask-programmable ROM,
but supplied with its own bus structure (see Fig. 3–2 on
page 49). By this means the data transfer between
character ROM and teletext display controller does not
stop the CPU.
Both bus structures are connected via a memory inter-
face which allows cross-connections using DMA or
wait cycles. If the character font size exceeds
12 kBytes, part of the character font can be shifted into
the program ROM which causes DMA cycles. There-
fore only less frequently used characters should be
placed into the program ROM. Vice versa seldom used
CPU code can be put into the character ROM.
The WST specification defines a number of 7-bit code
tables, which are filled with 96 characters only (the
MSB is used for parity check). In the G0 code table
some characters have several language dependent
variations. Additionally characters from the G0 code
table can be combined with diacritical marks from the
G2 code table (row 26). Furthermore different code
tables are defined for languages like cyrillic, greek or
arabic. Thus it is not possible to simply transform the
code tables into a continuous character font ROM with-
out getting unused ROM space and multiple defined
character fonts.
This problem is solved by implementing a character
code mapping (see Fig. 3–11 on page 58). The
5 MSBs of each character code are mapped into
another 5-bit code which is then used to address the
character font ROM. By this means the whole charac-
ter font is subdivided into 32 blocks of 32 characters
which can freely be distributed over the 4 character
sets.
The character code mapping is implemented as RAM
and can be programmed by software. After reset the
TPU initializes the mapping RAM for standard WST
latin code tables.
The TV controller can select predefined mappings for
latin, cyrillic and arabic teletext via the command
DISPLAY_MODE (see Table 3–16 on page 70). The
same command allows selection of a user defined
mapping which has to programmed in advance using
command USER_MAPPING.
Table 3–9:
Character resolutions
matrix
(h x v)
char/sc
reen
(PAL)
char/sc
reen
(NTSC)
osd
width
# char in
12k font
# char in
20k font
single character
8 x 8
40 x 32
40 x 28
32
μ
s
1600
2560
10 x 8
40 x 32
40 x 28
40
μ
s
1280
2048
8 x 10
40 x 26
40 x 22
32
μ
s
1280
2048
10 x 10
40 x 26
40 x 22
40
μ
s
1024
1638
8 x 13
40 x 20
40 x 17
32
μ
s
800
1280
10 x 13
40 x 20
40 x 17
40
μ
s
640
1024
combined character (2 x 2)
16 x 16
20 x 16
20 x 14
32
μ
s
400
640
20 x 16
20 x 16
20 x 14
40
μ
s
320
512
16 x 20
20 x 13
20 x 11
32
μ
s
320
512
20 x 20
20 x 13
20 x 11
40
μ
s
256
409
16 x 26
20 x 10
20 x 8.5
32
μ
s
200
320
20 x 26
20 x 10
20 x 8.5
40
μ
s
160
256
combined character (2 x 1)
16 x 10
20 x 26
20 x 22
32
μ
s
640
1024
16 x 13
20 x 20
20 x 17
32
μ
s
400
640
20 x 13
20 x 20
20 x 17
40
μ
s
320
512
combined character (1x 2)
10 x 16
40 x 16
40 x 14
40
μ
s
640
1024