CS2300-OTP_09 Datasheet, PDF (13/26 Pages) Cirrus Logic – Fractional-N Clock Multiplier with Internal LCO

5.4 Output to Input Frequency Ratio Configuration
CS2300-OTP
5.4.1
User Defined Ratio (R_UD)
The User Defined Ratio, R_UD, is a 32-bit un-signed fixed-point number which determines the basis for the
desired input to output clock ratio. Up to four different ratios, Ratio_0-3, can be stored in the CS2300’s one
time programmable memory. Selection between the four ratios is achieved by the M[1:0] mode select
pins. The 32-bit R_UD can be expressed in either a high resolution (12.20) or high multiplication (20.12)
format selectable by the LFRatioCfg global parameter.
The R_UD for high resolution (12.20) format is encoded with 12 MSBs representing the integer binary por-
tion with the remaining 20 LSBs representing the fractional binary portion. The maximum multiplication
factor is approximately 4096 with a resolution of 0.954 PPM in this configuration. See “Calculating the
User Defined Ratio” on page 22 for more information.
The R_UD for high multiplication (20.12) format is encoded with 20 MSBs representing the integer binary
portion with the remaining 12 LSBs representing the fractional binary portion. In this configuration, the
maximum multiplication factor is approximately 1,048,575 with a resolution of 244 PPM. It is recommend-
ed that the 12.20 High-Resolution format be utilized whenever the desired ratio is less than 4096 since
the output frequency accuracy of the PLL is directly proportional to the accuracy of the timing reference
clock and the resolution of the R_UD.
Referenced Control
Parameter Definition
Ratio 0-3................................“Ratio 0 - 3” on page 20
LFRatioCfg ............................“Low-Frequency Ratio Configuration (LFRatioCfg)” on page 20
M[1:0] ....................................“M1 and M0 Mode Pin Functionality” on page 16
5.4.2
Ratio Modifier (R-Mod)
The Ratio Modifier is used to internally multiply/divide the currently addressed R_UD (Ratio_0-3 stored in the
register space remain unchanged). The available options for R-Mod are summarized in Table 2 on
page 13. R-Mod is enabled via the M2 pin in conjunction with the appropriate setting of the M2Config[2:0]
global parameter (see Section 5.7.2 on page 17).
RModSel[1:0]
00
01
10
11
R Modifier
0.5
0.25
0.125
0.0625
Table 2. Ratio Modifier
Referenced Control
Parameter Definition
Ratio 0-3................................“Ratio 0 - 3” on page 20
RModSel[1:0] ........................“R-Mod Selection (RModSel[1:0])” section on page 19
M2Config[2:0]........................“M2 Pin Configuration (M2Config[2:0])” on page 21
5.4.3
Effective Ratio (R_EFF)
The Effective Ratio (R_EFF) is an internal calculation comprised of R_UD and the appropriate modifiers, as
previously described. R_EFF is calculated as follows:
R_EFF = R_UD • R-Mod
DS844F1
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