AN3352 datasheet(7/26 Pages) STMICROELECTRONICS

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AN3352

Block diagram and pin function description

Doc ID 018526 Rev 3

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1.5

DiSEqC 1.X implementation through VoTX and EXTM

If an external 22 kHz tone source is not available, it is possible to use the internal 22 kHz

tone generator signal available through the VoTX pin to drive the EXTM pin. In this way the

VoTX 22 kHz signal is superimposed to the VoRX DC voltage to generate the LNB output 22

kHz tone (see Figure 6). The internal 22 kHz tone generator, available through the VoTX pin,

must be activated during the 22 kHz transmission by the DSQIN pin or by the TEN bit. The

DSQIN internal circuit activates the 22 kHz tone on the VoTX output with 0.5 cycles ± 25 µs

delay from the TTL signal present on the DSQIN pin, and it stops with 1 cycle ± 25 µs delay

after the TTL signal has expired. The VoTX pin internal circuit must be preventively set ON by

the TTX function. This can be controlled both through the TTX pin and the I²C bit. As soon

as the tone transmission has expired, the VoTX must be disabled by setting the TTX to LOW.

The 13/18 V power supply is always provided to the LNB from the VoRX pin.

1.6

PDC optional circuit for DiSEqC 1.X applications using VoTX

signal on to EXTM pin and 22 kHz tone controlled by DSQIN

In some applications, at light output current (< 50 mA), having a heavy LNB output

capacitive load, the 22 kHz tone can be distorted. In this case it is possible to add the

“Optional” external components described on Section 2.7.

1.7

22 kHz oscillator

The internal 22 kHz tone generator is factory-trimmed in accordance with current standards

and can be selected by the I²C interface TTX bit (or TTX pin) and controlled by the DSQIN

pin (TTL compatible), which allows immediate DiSEqC data encoding. If the 22 kHz tone

presence is requested in continuous mode, the internal oscillator can be activated by the I²C

Figure 3.

DiSEqC 1.X tone burst with 22 kHz IC controller

VoRx OUTPUT

Vtone signal

High-Z

STATE

High-Z

STATE

High-Z

STATE

Push -pull

Action

Push -pull

Action

VoRx OUTPUT

Vtone signal

High-Z

STATE

High-Z

STATE

High-Z

STATE

Push -pull

Action

Push -pull

Action

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