TDA7513T Datasheet, PDF (21/59 Pages) STMicroelectronics – SINGLE-CHIP FM/AM TUNER WITH STEREO DECODER AND AUDIO PROCESSOR

TDA7513T
(160us - 320µs ... 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 (t_Sample x f_IF ). 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:
t_TIM = (IRC + 1) / f_OSC
t_CNT = (CF + 1697) / f_IF
t_CNT = (CF + 448) / f_IF
(FM mode)
(AM mode)
where
t_TIM = IF timer cycle time (sampling time)
t_CNT = IF counter cycle time
Counting succesful:
t_CNT - t_ERR = t_TIM = t_CNT + t_ERR
Count failed:
t_TIM > t_CNT + t_ERR
t_TIM < t_CNT - t_ERR
where
t_ERR = 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
21/59