Datasheet


SLES071B − MARCH 2003 − REVISED NOVEMBER 2006
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15
SYSTEM CLOCK AND RESET FUNCTIONS
System Clock Input
The PCM1791A requires a system clock for operating the digital interpolation filters and advanced segment DAC
modulators. The system clock is applied at the SCK input (pin 5). The PCM1791A has a system clock detection circuit
that automatically senses which frequency the system clock is operating. Table 1 shows examples of system clock
frequencies for common audio sampling rates. If the oversampling rate of the delta-sigma modulator is selected as
128 f
S
, the system clock frequency is over 256 f
S
.
Figure 24 shows the timing requirements for the system clock input. For optimal performance, it is important to use
a clock source with low phase jitter and noise. One of the Texas Instruments’ PLL1700 family of multiclock generators
is an excellent choice for providing the PCM1791A system clock.
Table 1. System Clock Rates for Common Audio Sampling Frequencies
SAMPLING FREQUENCY
SYSTEM CLOCK FREQUENCY (f
SCK
) (MHz)
SAMPLING FREQUENCY
128 f
S
192 f
S
256 f
S
384 f
S
512 f
S
768 f
S
32 kHz 4.096
(1)
6.144
(1)
8.192 12.288 16.384 24.576
44.1 kHz 5.6488
(1)
8.4672 11.2896 16.9344 22.5792 33.8688
48 kHz 6.144
(1)
9.216 12.288 18.432 24.576 36.864
96 kHz 12.288 18.432 24.576 36.864 49.152
(1)
73.728
(1)
192 kHz 24.576 36.864 49.152
(1)
73.728
(1) (2) (2)
(1)
This system clock rate is not supported in I
2
C fast mode.
(2)
This system clock rate is not supported for the given sampling frequency.
t
(SCKH)
System Clock (SCK)
t
(SCKL)
2 V
0.8 V
H
L
t
(SCY)
PARAMETERS MIN MAX UNITS
t
(SCY)
System clock pulse cycle time 13 ns
t
(SCKH)
System clock pulse duration, HIGH 5 ns
t
(SCKL)
System clock pulse duration, LOW 5 ns
Figure 24. System Clock Input Timing
Power-On and External Reset Functions
The PCM1791A includes a power-on reset function. Figure 25 shows the operation of this function. With V
DD
> 2 V,
the power-on reset function is enabled. The initialization sequence requires 1024 system clocks from the time
V
DD
> 2 V. After the initialization period, the PCM1791A is set to its default reset state, as described in the MODE
CONTROL REGISTERS section of this data sheet.
The PCM1791A also includes an external reset capability using the RST input (pin 6). This allows an external
controller or master reset circuit to force the PCM1791A to initialize to its default reset state.
Figure 26 shows the external reset operation and timing. The RST pin is set to logic 0 for a minimum of 20 ns. The
RST pin is then set to a logic 1 state, thus starting the initialization sequence, which requires 1024 system clock
periods. The external reset is especially useful in applications where there is a delay between the PCM1791A power
up and system clock activation.