STV0056 datasheet(23/26 Pages) STMICROELECTRONICS

23 / 26 page

FM DEMODULATION SOFTWARE ROUTINE (continued)

nd STEP (LEFT) :

VCO FREQUENCY CHECKING (VCO)

This second step is actually a waiting loop in which

the actual running frequency of the VCO is meas-

ured.

To exit of this loop is allowed when : Subcarrier

Frequency - 10kHz

≤ Measured Frequency ≤ Sub-

carrier Frequency + 10kHz (

± 10kHz is the maxi-

mum dispersion of the frequency synthesis

function).

In practice, R8 B2 B3 and R9 are read and com-

pared to the value loaded in R6 B6 B7 and R7

±1 bit.

Note :

The duration of this step depends on how large is

frequency difference between the start frequency

and the targeted frequency. Typically :

- the rate of change of the VCO frequency is about

3.75MHz/s (Cpump =10

µF)

- In addition to this settling time, 100ms must be

added to take into account the sampling period of

the watchdog.

rd STEP (LEFT)

The FM demodulationcan be started by connecting

the VCO to the phase locked loop (PLL).

In practice :

- SW3 closed

⇒ R6 B4 = H

- SW4 opened

⇒ R6 B2 = L

After this sequence of 3 steps for left channel,

a similar sequence is needed for the right channel.

Note :

In the sequence for the right, there is no need to

again select the FM deviation (once is enough for

the pair).

General Remark

Before to enable the demodulated signal to the

audio output, it is recommanded to keep the muting

and to check whether a subcarrier is present at the

wanted frequency. Such an information is available

in R8 B0 and R8 B4 which can be read.

Two different strategies can be adopted when ena-

bling the output :

- Either both left and right demodulated signals are

simultaneously authorized when both channel

are ready.

- Or while the right channel sequenceis running, the

already ready left signal is sent to the left and right

outputs and the real stereo sound L/R is output

when both channels are ready. This second option

gives sound a few hundreds of ms before the first

one.

STV0056A

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