tda7421n STMicroelectronics, tda7421n Datasheet - Page 19

• Manufacturer Part Number: tda7421n
• Description: Am/fm Tuner For Car Radio And Hi-fi Applications
• Manufacturer: STMicroelectronics

4.1 Intermediate Frequency Main Counter (IFMC)
This counter is a 13-21 bit synchronous autoreload down-counter. Four bits are programmable to have the pos-
sibility for an adjust to the frequency of the IF filter.The counter length is automatically adjusted to the chosen
sampling time and the counter mode. At the start the counter will be loaded with a defined value which is an
equivalent to the divider value (tsample fIF).If a correct frequency is applied to the IF counter frequency inputs
IF-AM and IF-FM, at the end of the sampling time the main counter is changing its state from 0 to 1FFFFFH.This
is detected by a control logic. The frequency range inside which a successful count results is detected is adjust-
able setting bits EW 0, 1, 2.
4.2 Up-down counter filter
The information coming from the IF main counter control logic is shifted into a 5 bit up down counter circuit
clocked by the sampling time signal. At the start (rising edge of the IFENA signal) the counter is set to 10H and
the SSTOP signal is forced to "1".Only when the counter reaches the value 10H - step, SSTOP goes to "0".SS-
TOP will be "1" again, if the counter reaches the value 10h + step.
Figure 3. Charge Punp Logic
Figure 4. FM and AM operation (swallow mode)
REF OSC IN
AM IN
FM IN
CURR HIGH
LOCKENA
LOCK
fosc
PRESCALER
REGISTER
REGISTER
PC0 ...PC4
COUNTER
R0 ...R15
DIVIDER
I
I
2
2
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C bus
C bus
: R
A
(O/I)
CHARGE PUMP
D96AU548
CURRENT
fsyn
D96AU545
REGISTER
PC5 ... P15
fref
DIVIDER
I
2
C bus
: B
PD
TDA7421N
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