SAA7712 datasheet(18/44 Pages) PHILIPS

18 / 44 page

1999 Aug 05

Philips Semiconductors

Preliminary speciﬁcation

Sound effects DSP

SAA7712H

Fig.8 Configurations of the equalizer sections.

handbook, full pagewidth

MGS213

OUT0

IN0

2 channel

OUT1

IN1

OUT2

IN2

OUT3

IN3

8.3.3

OVERFLOW DETECTION

The equalizer has an overflow flag. This flag is fed to

output pin EQOV. If an overflow is detected in one of the

filter sections, the signal is clipped to the maximum

allowed level. The overflow flag is immediately set.

It remains at a HIGH-level during the remaining part of the

current audio sample period and for the whole next sample

period. If no overflow is detected during this next sample

period, the overflow flag goes to a LOW-level at the

beginning of the sample period after that. Otherwise, the

overflow flag remains at a HIGH-level for at least one other

audio sample period.

8.4

Clock circuit and oscillator

8.4.1

GENERAL DESCRIPTION

The chip has a crystal clock oscillator. It can use a crystal

at either fxtal = 16.384 MHz = 512 × 32 kHz or

fxtal = 18.432 MHz = 576 × 32 kHz in fundamental mode.

The block diagram of this Pierce oscillator is shown in

Fig.9. The active element needed to compensate for the

loss resistance of the crystal is the block Gm. This block is

placed between the external pins OSC_IN

and OSC_OUT.

The gain of the oscillator is internally controlled by the

AGC block. A sine wave with a peak-to-peak voltage close

to the oscillator power supply voltage is generated.

The AGC block prevents clipping of the sine wave and

therefore the higher harmonics are as low as possible.

At the same time, the voltage of the sine wave is as high

as possible so reducing the jitter going from sine wave to

clock signal. The sinusoidal output is converted into a

CMOS compatible clock by the comparator.

The second mode of operation shown in Fig.10, is the

slave mode which is driven by a master clock directly.

The signal to pin OSC_IN can be driven to the power

supply voltages VDD_OSC and VSS_OSC.

8.4.2

SUPPLY OF THE CRYSTAL OSCILLATOR

The power supply connections of the oscillator are

separate from the other supply lines. This is to minimize

the feedback from the ground bounce of the chip to the

oscillator circuit. Pin VSS_OSC is used as the ground supply

and pin VDD_OSC as the positive supply.

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