Cirrus Logic CS4953xx - Manual (Page 100)

Clocking
CS4953xx Hardware User’s Manual
DS732UM10 Copyright 2010 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