TDA7505 datasheet(33/38 Pages) STMICROELECTRONICS

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TDA7505

Functional description

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5.2.14

Clock scheme

Due to the programmable PLL oscillator, the clock scheme is very flexible. The customer

may choose the clock frequency according to the application needs. However one should

take into account several constraints:

●

The RDS module needs a crystal frequency of 8.55 MHz or alternative an external

74.1MHz Oscillator. However the PLL may be supplied by an external clock reference

and the crystal in parallel may drive the RDS module.

●

The CODEC (A/D and D/A) module needs a clock of 512 times the audio sample rate

(Fs).

●

The audio sample rate (Fs) should be close to 44.1 kHz. This allows CD quality. Higher

sample rates will reduce the number of DSP clock cycles per Fs and hence will reduce

the available MIPS.

●

The DSP core clock frequency may not exceed 76 MHz

●

In a car radio system the second and third system clock harmonics (DSP clock and

CODEC clock) should be outside the radio frequency bands.

Two examples of convenient clock schemes are shown in the following table:

Table 28.

Examples of convenient clock schemes

Clock scheme

Alternative(1)

External clock oscillator used

Fxtal

8.55 MHz

74.1 MHz

Fcomp

Fxtal / 4

2.14 MHz

Fxtal/21

3.53 MHz

Fvco

Fcomp * 106

226.58 MHz

Fcomp * 64

225.8 MHz

Fdsp

Fvco / 3

75.53 MHz

Fvco / 3

75.28 MHz

Fcodec

Fvco / 10

22.66 MHz

Fvco / 10

22.58 MHz

44.25 kHz

44.11 kHz

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