STV9425 datasheet(9/15 Pages) STMICROELECTRONICS

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FUNCTIONAL DESCRIPTION (continued)

H-SYNC

0123

n + 1

n + 2

n + 3

n + 4

LD - 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

IV - Clock and Timing

The whole timing is derived from the XTALIN and

the SYNCHRO (horizontal and vertival) input fre-

quencies. The XTALIN input frequency can be an

exte rnal clock or a crystal signal thanks to

XTALIN/XTALOUT pins. The value of this fre-

quency 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] + 8] x 3 x TPXCK

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

and TPXCK : pixel period).

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-

mum value of the horizontal delay is 36 x TPXCK =3

character periods). The length of the active area,

where R, G, B are different from 0, dependson the

number of characters of the strips.

IV.2 - D to A Timing

The D to A converters of the STV9425/25B/26are

pulse width modulater converter. The frequency of

the output signal is :

FXTAL

256

and the duty cycle is :

Vi[7:0]

256

per cent.

After a low pass filter, the average value of the

output is :

[7:0]

256

⋅ VDD

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 thepage addresses(8 pos-

sible

≠ addresses).Two typesof stripsareavailable :

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

number of black (FBK = 1 in DISPLAYCONTROL

- Character strip : its descriptor gives the memory

address of the character codes corresponding to

the 1

st 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 1

st 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.

128

255

V1[7:0]

TXTAL

256 . TXTAL

PWM1 Signal

Figure 6 : PWM Timing

STV9425 - STV9425B - STV9426

9/15

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