AN2713 datasheet(7/40 Pages) STMICROELECTRONICS

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AN2713

Block diagram description

Doc ID 14431 Rev 1

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1.4

22 kHz oscillator and EXTM function

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

and can be controlled by the DSQIN pin (TTL-compatible), which allows immediate

DiSEqC™ data encoding. The rising and falling edges are kept within the 5 µs to 15 µs

range, 8 µs (typ) for 22 kHz. The duty cycle is 50% (typ), and modulates the DC output with

a 0.650 Vpp (typ) amplitude as well as the DSQIN pin.

The EXTM is a logic input to allow the activation of the 22 kHz tone output, on the VoTx pin,

by using the device’s integrated tone generator. If the EXTM pin is used, the internal 22 kHz

generator must be kept ON (TTX pin or TTX bit set HIGH). When a TTL-compatible 22 kHz

signal is applied (for example, a 22 kHz square wave from the demodulator), the EXTM

internal circuit detects the 22 kHz TTL signal code and activates the internal 22 kHz tone on

the VoTx output. The 22 kHz tone on the output is activated after a delay from the TTL signal

presence on the EXTM pin. The tone output starts with about a 1.5 T delay after the 1st

cycle of the TTL signal and stops after about a 2 T delay after the TTL signal on the EXTM

has expired (see Figure 2 below). The tone output can also be activated via the DSQIN pin.

It starts with a 1.5 T ± 25 µs maximum delay and stops after 2 T ± 25 µs maximum delay

with 20~24 kHz tolerance for EXTM input pin.

Figure 2.

EXTM timing control

1.5

Tone detector

This block provides a complete circuit to decode the 22 kHz burst code present on the

DETIN pin in a digital signal by the DSQOUT pin where an open drain MOSFET is

connected. The tone is also monitored and a dedicated bit (TMON) provides the diagnostic

function described in the Section 1.3.

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