CS2200-OTP Datasheet, PDF (11/22 Pages) Cirrus Logic – Fractional-N Frequency Synthesizer

5.3.2
CS2200-OTP
Manual Ratio Modifier (R-Mod)
The manual 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 14).
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 17
RModSel[1:0] ........................“R-Mod Selection (RModSel[1:0])” section on page 16
M2Config[2:0]........................“M2 Pin Configuration (M2Config[2:0])” on page 18
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.
Selection of the user defined ratio from the four stored ratios is made by using the M[1:0] pins.
Referenced Control
Parameter Definition
M[1:0] pins.............................“M1 and M0 Mode Pin Functionality” on page 13
5.3.4
Ratio Configuration Summary
The R_UD is the user defined ratio for which up to four different values (Ratio_0-3) can be stored in the one
time programmable memory. The M[1:0] pins then select the user defined ratio to be used as well as the
modal configuration set. R-Mod is applied accordingly. The user defined ratio and ratio modifier make up
the effective ratio R_EFF, the final calculation used to determine the output to input clock ratio. The effective
ratio is then corrected for the internal dividers. The conceptual diagram in Figure 5 summarizes the fea-
DS842PP1
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