CS2000-OTP_09 Datasheet, PDF (11/30 Pages) Cirrus Logic – Fractional-N Clock Synthesizer & Clock Multiplier

5. APPLICATIONS
CS2000-OTP
5.1 One Time Programmability
The one time programmable (OTP) circuitry in the CS2000-OTP allows for pre-configuration of the device
prior to use in a system. There are two types of parameters that are used for device pre-configuration: modal
and global. The modal parameters are features which, when grouped together, create a modal configuration
set (see Figure 17 on page 22). Up to four modal configuration sets can be permanently stored and then
dynamically selected using the M[1:0] mode select pins (see Table 1). The global parameters are the re-
maining configuration settings which do not change with the mode select pins. The modal and global pa-
rameters can be pre-set at the factory or user programmed using the customer development kit, CDK2000;
Please see “Programming Information” on page 27 for more details.
Parameter Type
Modal
Global
M[1:0] pins = 00
Configuration Set 0
Ratio 0
M[1:0] pins = 01 M[1:0] pins = 10
Configuration Set 1
Ratio 1
Configuration Set 2
Ratio 2
Configuration settings set once for all modes.
M[1:0] pins = 11
Configuration Set 3
Ratio 3
Table 1. Modal and Global Configuration
5.2 Timing Reference Clock Input
The low jitter timing reference clock (RefClk) can be provided by either an external reference clock or an
external crystal in conjunction with the internal oscillator. In order to maintain a stable and low-jitter PLL out-
put the timing reference clock must also be stable and low-jitter; the quality of the timing reference clock
directly affects the performance of the PLL and hence the quality of the PLL output.
5.2.1
Internal Timing Reference Clock Divider
The Internal Timing Reference Clock (SysClk) is limited to a lower maximum frequency than that allowed
on the XTI/REF_CLK pin. The CS2000-OTP supports the wider external frequency range by offering an
internal divider for RefClk. The RefClkDiv[1:0] global parameter should be configured such that SysClk,
the divided RefClk, then falls within the valid range as indicated in “AC Electrical Characteristics” on
page 7.
XTI/REF_CLK
Timing Reference Clock
8
MHz
<
RefClk
<
50 MHz (XTI)
58 MHz (REF_CLK)
Timing Reference
Clock Divider
÷1
÷2
÷4
Internal Timing
Reference Clock
8 MHz < SysClk < 14 MHz
Fractional-N
Frequency
Synthesizer
PLL Output
RefClkDiv[1:0]
N
Figure 8. Internal Timing Reference Clock Divider
It should be noted that the maximum allowable input frequency of the XTI/REF_CLK pin is dependent
upon its configuration as either a crystal connection or external clock input. See the “AC Electrical Char-
acteristics” on page 7 for more details.
For the lowest possible output jitter, attention should be paid to the absolute frequency of the Timing Ref-
erence Clock relative to the PLL Output frequency (CLK_OUT). To minimize output jitter, the Timing Ref-
erence Clock frequency should be chosen such that f_RefClk is at least +/-15 kHz from f_{CLK_OUT}*N/32
where N is an integer. Figure 9 shows the effect of varying the RefClk frequency around f_{CLK_OUT}*N/32.
It should be noted that there will be a jitter null at the zero point when N = 32 (not shown in Figure 9). An
DS758F1
11