CS2200-OTP_09 Datasheet, PDF (11/24 Pages) Cirrus Logic – Fractional-N Frequency Synthesizer

5.3 Output to Input Frequency Ratio Configuration
CS2200-OTP
5.3.1
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 CS2200’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 is represented in a 12.20 format where the 12 MSBs represent the integer binary
portion while the remaining 20 LSBs represent 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 20 for more information.
The status of internal dividers, such as the internal timing reference clock divider, are automatically taken
into account. Therefore R_UD is simply the desired ratio of the output to input clock frequencies.
Referenced Control
Parameter Definition
Ratio 0-3................................“Ratio 0 - 3” on page 18
M[1:0] ....................................“M1 and M0 Mode Pin Functionality” on page 14
5.3.2
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 11. R-Mod is enabled via the M2 pin in conjunction with the appropriate setting of the M2Config[2:0]
global parameter (see Section 5.6.2 on page 15).
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 18
RModSel[1:0] ........................“R-Mod Selection (RModSel[1:0])” section on page 17
M2Config[2:0]........................“M2 Pin Configuration (M2Config[2:0])” on page 19
5.3.3
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
To simplify operation the device handles some of the ratio calculation functions automatically (such as
when the internal timing reference clock divider is set). For this reason, the Effective Ratio does not need
to be altered to account for internal dividers.
Ratio modifiers which would produce an overflow or truncation of R_EFF should not be used. In all cases,
the maximum and minimum allowable values for R_EFF are dictated by the frequency limits for both the
input and output clocks as shown in the “AC Electrical Characteristics” on page 7.
DS842F1
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