CS2000-OTP

DS758F1

11

5. APPLICATIONS

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.

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.

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-

It should be noted that there will be a jitter null at the zero point when N = 32 (not shown in Figure 9). An

Parameter Type

M[1:0] pins = 00

M[1:0] pins = 01

M[1:0] pins = 10

M[1:0] pins = 11

Modal

Configuration Set 0

Ratio 0

Configuration Set 1

Ratio 1

Configuration Set 2

Ratio 2

Configuration Set 3

Ratio 3

Global

Configuration settings set once for all modes.

Figure 8. Internal Timing Reference Clock Divider

N

Internal Timing

Reference Clock

PLL Output

Fractional-N

Frequency

Synthesizer

Timing Reference

Clock Divider

XTI/REF_CLK

RefClkDiv[1:0]

8 MHz < SysClk < 14 MHz

8 MHz < RefClk <

Timing Reference Clock

50 MHz (XTI)

58 MHz (REF_CLK)