PCM1808
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SLES177A–APRIL 2006–REVISED AUGUST 2006
SYSTEM CLOCK
The PCM1808 supports 256 fS, 384 fS and 512 fS as system clock, where fS is the audio sampling frequency.
The system clock must be supplied on SCKI (pin 6).
The PCM1808 has a system clock detection circuit which automatically senses if the system clock is operating
at 256 fS, 384 fS, or 512 fS in slave mode. In master mode, the system clock frequency must be controlled
through the serial control port, which uses MD1 (pin 111) and MD0 (pin 10). The system clock is divided down
automatically to generate frequencies of 128 fS and 64 fS, which are used to operate the digital filter and the
delta-sigma modulator, respectively.
Table 1 shows some typical relationships between sampling frequency and system clock frequency, and
Figure 17 shows system clock timing.
Table 1. Sampling Frequency and System Clock Frequency
SAMPLING FREQUENCY (kHz)
SYSTEM CLOCK FREQUENCY (fSCLK) (MHz)
256 fS
2.048
384 fS
3.072
512 fS
4.096
8
16
4.096
6.144
8.192
32
8.192
12.288
16.9344
18.432
24.576
33.8688
36.864
16.384
22.5792
24.576
32.768
45.1584
49.152
44.1
48
11.2896
12.288
16.384
22.5792
24.576
64
88.2
96
t
t
w(SCKL)
w(SCKH)
SCKI
2 V
SCKI
0.8 V
T0005B07
MAX
SYMBOL
PARAMETER
MIN
8
UNIT
tw(SCKH)
tw(SCKL)
System clock pulse duration, HIGH
System clock pulse duration, LOW
System clock duty cycle
ns
ns
8
40%
60%
Figure 17. System Clock Timing
FADE-IN AND FADE-OUT FUNCTIONS
The PCM1808 has fade-in and fade-out functions on DOUT (pin 9) to avoid pop noise, and the functions come
into operation in some cases as described in several following sections. The level changes from 0 dB to mute or
mute to 0 dB are performed using calculated pseudo S-shaped characteristics with zero-cross detection.
Because of the zero-cross detection, the time needed for the fade in and fade out depends on the analog input
frequency (fin). It takes 48/fin until processing is completed. If there is no zero cross during 8192/fS, DOUT is
faded in or out by force during 48/fS (TIME OUT). Figure 18 illustrates the fade-in and fade-out operation
processing.
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