TDA7513T datasheet(21/59 Pages) STMICROELECTRONICS

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TDA7513T

(160us - 320

µs ... 20.48ms for FM, 1ms - 2ms … 128ms for AM).

5.3.9 Intermediate Frequency Main Counter

This counter is a 11 - 21-bit synchronous autoreload down counter. Five bits (CF) are programmable to

allow the adjustment to the peak of the IF-filter response. The counter length is automatically adjusted to

the chosen sampling time and counter mode (FM, AM).

The IF counter is also used to automatically perform the stereo decoder 456kHz VCO frequency adjust-

ment.

At the start the counter will be loaded with a value equivalent to the expected number of zero-crossing in

the sampling time window (tSample x fIF ). If the correct frequency is applied to the IF counter input, at the

end of the sampling time the main counter will have either a 0h state or a 1FFFFFh state stored.

A deviation from the expected IF will result in a difference of the counter final state from either of these

values. The counter final state is then compared to either 0h or 1FFFFFh minus a number of LSB's deter-

mined by the acceptable frequency window programming (EW).

If the comparison result is good the IF counter output changes from LOW to HIGH and is made available

outside at the pin #SD (after a NAND operation with the signal strength evaluation circuit). The following

relationships apply:

tTIM = (IRC + 1) / fOSC

tCNT = (CF + 1697) / fIF

(FM mode)

tCNT = (CF + 448) / fIF

(AM mode)

where

tTIM = IF timer cycle time (sampling time)

tCNT = IF counter cycle time

Counting succesful:

tCNT - tERR = tTIM = tCNT + tERR

Count failed:

tTIM > tCNT + tERR

tTIM < tCNT - tERR

where

tERR = discrimination window (controlled by the EW registers)

The IF counter can be started only by inlock information from the PLL, and it is enabled by software

(IFENA).

5.3.10Adjustment of the Measurement Time and Frequency window

The measurement precision is adjustable by controlling the width of the frequency discrimination window

through control registers EW0 to EW2. The center frequency of the discrimination window is adjustable by

the control register CF0 to CF4. The measurement time per cycle is adjustable by setting the registers

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