Cirrus Logic CS2000-OTP - Manual (Page 15)

CS2000-OTP
DS758F2 15
5.4.3 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 15 . 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 19 ).
Table 2. Ratio Modifier
5.4.4 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 unless auto
clock switching is enabled in which case the LockClk[1:0] modal parameter also selects the ratio (see
“Fractional-N Source Selection” on page 15 ).
5.4.5 Fractional-N Source Selection
To select between the static ratio based Frequency Synthesizer Mode and the dynamic ratio based Hybrid
PLL Mode, the source for the fractional-N value for the Frequency Synthesizer must be changed. The
Fractional-N value can either be sourced directly from the Effective Ratio (static ratio) or from the output
of the Digital PLL (dynamic ratio) (see
Figure 13 on page 17 ). The setting of this function can be made
manual or automatically depending on the presence of CLK_IN.
RModSel[1:0] R Modifier
00 0.5
01 0.25
10 0.125
11 0.0625
Referenced Control Parameter Definition
Ratio 0-3................................ “Ratio 0 - 3” on page 23
RModSel[1:0] ........................ “R-Mod Selection (RModSel[1:0])” section on page 22
M2Config[2:0]........................ “M2 Pin Configuration (M2Config[2:0])” on page 25
Referenced Control Parameter Definition
M[1:0] pins............................. “M1 and M0 Mode Pin Functionality” on page 19
LockClk[1:0] .......................... “Lock Clock Ratio (LockClk[1:0])” section on page 23