Cirrus Logic CS4953xx, Hardware User Manual

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Clocking
CS4953xx Hardware Users Manual
DS732UM7 Copyright 2008 Cirrus Logic, Inc 9-12
9.2.3 PLL
The internal phase locked loop (PLL) of the CS4953xx requires an external current reference resistor. The resistor is
used to calibrate the PLL and must meet the tolerances specified below. The layout topology is shown in the typical
connection diagrams. Care should be taken when laying out the current sense circuitry to minimize trace lengths
between the DSP and resistor, and to keep high-frequency signals away from the resistor. Any noise coupled onto the
these traces will be directly coupled into the PLL, which could affect performance. Please see tables below for pin
numbers and external component values.
Figure 9-9. PLL Filter Topology
9.3 Clocking
The CS4953xx incorporates a programmable phase locked loop (PLL) clock synthesizer. The PLL takes an input
reference clock and produces all the clocks required to run the DSP and peripherals.
In A/V Receiver designs that require low-jitter clocks, the XTI pin is typically connected to an external 12.288 MHz
or 24.576 Mhz oscillator that is used throughout the system.
The CS4953xx has a built-in crystal oscillator circuit. A parallel resonant-type crystal is connected between the XTI
and XTO pins as shown in
Figure 9-10 . The value of C1 is specific to each crystal. The CS4953xx data sheet
specifies acceptable crystal parameters (including C
L
and ESR). When a crystal is used, XTAL_OUT is used to
clock other devices in the system such as the S/PDIF receiver.
The PLL is controlled by the clock manager in the DSP O/S application software. AN288, “CS4953xx Firmware
User’s Manual” should be referenced regarding what CLKIN input frequency and PLL multiplier values are
supported.
Table 9-5. PLL Filter Pins
LQFP-144
Pin #
LQFP-128
Pin #
Pin Name Pin Type Pin Description
128 20 PLL_REF_RES Input
Current Reference Resistor for
PLL filter
Table 9-6. Reference PLL Component Values
Symbol Reference Value
R1 5.1 k
Ω,
1%
PLL_REF_RES
GNDA
CS497xx
R1
VDDA
3.3V
Bead
10u 0.1u