FUNCTIONAL DESCRIPTION (continued)

H-SYNC

0

123

n + 1

n + 2

n + 3

n + 4

LD - 1

LD

01

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

IV - Clock and Timing

The whole timing is derived from the XTALIN and the

SYNCHRO (horizontal and vertival) input frequen-

cies. The XTALIN input frequencycan be an external

clock or a crystal signal thanks to XTALIN/XTALOUT

pins. The value of this frequency can be chosen

between 8 and 15MHz, it is available on the CKOUT

pin and is used by the PLL to generate a pixel clock

locked on the horizontal synchro input signal.

IV.1 - Horizontal Timing (see Figure 5)

The number of pixel periods is given by the LINE

DURATION register and is equal to :

[LD[5:0] + 1 ] x 12.

(LD[5:0] : value of the LINE DURATION register).

This value allows to choose the horizontal size of the

characters. The horizontal left margin is given by the

HORIZONTAL DELAY register and is equal to :

(DD[7:0] : value of the DISPLAY DELAY register

This value allows to choose the horizontal position

of the characters on the screen. The value of

DD[7:0] must be equal or greater than 4 (the mini-

character periods). The length of the active area,

where R, G, B are different from 0, depends on the

number of characters of the strips.

IV.2 - D to A Timing (STV9422)

The D to A converters of the STV9422 are pulse

width modulater converter.

The frequency of the output signal is :

256

and the duty cycle is :

Vi[7:0]

256

per cent.

After a low pass filter, the average value of the

output is :

Vi [7:0]

256

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 pos-

sible

≠ 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 1st displayed character. The characters and

attributes (see code format III) are defined by a

succession of codes stored in the RAM at ad-

dresses starting from the 1st one given by the

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.

0

1

128

255

V1[7:0]

256 . t

XTAL

P WM1 Signa l

t

XTAL

Figure 6 : PWM Timing

STV9422 - STV9424

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