which is stored by an external leakage capacitor

to be used later as a reference for the cut-off

current measurement.

- When a current due to a cut-off pulse occurs, the

voltage appearing on the pin 34 is compared

within the IC to the voltage present on the leakage

memory. An appropriate externalcapacitor is then

charged or discharged in such a way that the

difference between each measured current and

the leakage current is kept constant, and thus the

quasi cut-off current is kept constant.

Average Beam Current Limitation

The total current of the three guns is integrated by

means of an internal resistor and an external ca-

pacitor (pin 36) and then compared with a program-

mable voltage reference (pin 38). When 70% of the

maximum permitted beam current is reached, the

drive gain begins to be reduced ; to do so, the

amplitude of the inserted pulse is increased.

In order to keep enough contrast, the maximum

drive reduction is limited to 6dB. If it is not sufficient,

the brightness is suppressed.

SPECIFICATION FOR THE THOMSON BI-DI-

RECTIONAL DATA BUS

This is a bi-directional 3-wire (ENABLE, CLOCK,

DATA) serial bus. The DATA line transmission is

bi-directional whereas ENABLE and CLOCK lines

are only microprocessor controlled. The ENABLE

and CLOCK lines are only driven by the microcom-

puter.

µP

IC

I

IC

II

IC

III

Figure 1

It is possible to select several IC from the micro-

processor via the bus. The identification of each

particular IC is achieved by the length of the word

(number of data bits/clock pulses), meaning that

each IC responds with its own particular word

length.

The number is determined while ENABLE is low

and by counting the negative clock edges. As soon

as the high edge of the ENABLE signal is applied,

the number is fixed (see Figure 2).

The reply word lenght from any of the IC on the

bi-directional line is four bits. If it is found insufficient

then the reply word can be expanded to include two

repetitive reply sequences one after the other.

The bi-directional transmission is enabled if :

- the IC has been previously addressed at the

positive going edge of the enable pulse.

- ENABLE remains high,

and DATAis available only during the period when

the clock remains low.

- number of identification bits : n

1...n : data from the microcomputer

- number of bi-directional clocks : 4

1...M : data to the microcomputer

The four bit reply word (synchronized with the clock

coming from the microcontroller) from the ad-

dressed IC to the microcontroller is sent only once.

Subsequent clock pulses present on the clock line

will be ignored by the IC in question. The data sent

to the microcontroller can generally be suppressed

completely or partially, but in the case of the video-

processor, a minimum reply word lenght of 1 has

to be maintained (see Figure 3).

This implies that a bi-directional bus that incorpo-

rates other IC’s together with a videoprocessor IC

is then also limited by the minimum reply word

restriction of 1.

The data word from the microcompter is divided

into :

- addresses within the IC

- data

The data word to the microcomputer is divided into

- two data bits,

- two address bits

After the operating voltage is applied, the first

transmission will be used as a reset command, i.e.

the data word will not be detected.

- number of identification bits : n

1...n : data from the microcomputer

- number of bi-directional clocks : 1

1 : data the microcomputer (which is the minimum

number for the videoprocessor)

TEA5040S

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