H-SYNC

012

3

n + 1

n + 2

n + 3

n + 4

LD - 1

LD

0

1

Character

Period

R, G, B

LD[5:0]

Fixed

DD[7:0]

= 4 (min)

= 4n + 2

Given by

number

of characters

of the strips

Figure 5 : Horizontal Timing

FUNCTIONAL DESCRIPTION (continued)

IV.2 - D to A Timing (STV9421)

The D to A converters of the STV9421 are pulse

width modulater converter.

The frequency of the output signal is :

256

and the duty cycle is :

V1[7:0]

256

per cent.

After a low pass filter, the average value of the

output is :

V1 [7:0]

256

0

1

128

255

V1[7:0]

256 . t

XTAL

P WM1 S igna l

t

XTAL

Figure 6 : PWM Timing

V - Display Control

A screen is composed of successive scanlines gath-

ered in several strips. Each strip is defined by a

descriptor stored in memory. A table of descriptors

allows screen composition and different tables can

be stored in memory at the page addresses (8

possible

≠ addresses).

Two types of strips are available :

- Spacing strip : its descriptor (see II) gives the

number of black (FBK = 1 in DISPLAY CONTROL

register) or transparent (FBK = 0) lines.

- Character strip : its descriptor gives the memory

address of the character codes corresponding to

the 1

attributes (see code format III) are defined by a

succession of codes stored in the RAM at ad-

dresses starting from the 1

descriptor. A character strip can be displayed or

not by using the DE bit of its descriptor. A zoom

can be made on it by using the ZY bit.

After the falling edge on V-SYNC, the first strip

descriptor is read at the top of the current table of

descriptors at the address given by P[8:6] (see

DISPLAY CONTROL register) ; if it is a spacing

strip, SL[7:0] black or transparent scan lines are

displayed ; if it is a character strip, during CH[5:0] x

(I + ZY) scan lines (CH[5:0] given by the CHARAC-

TER HEIGHT register), the character codes are

read at the addresses starting from the 1

given by the descriptor until a end of line character

or the end of the scan line ; the next descriptor is

then read and the same process is repeated until

the next falling edge on V-SYNC.

STV9420 - STV9421

9/16