![](http://datasheet.mmic.net.cn/10000/CS2300P-DZZR_datasheet_1433651/CS2300P-DZZR_13.png)
CS2300-OTP
DS844F2
13
5.4
Output to Input Frequency Ratio Configuration
5.4.1
User Defined Ratio (RUD)
The User Defined Ratio, RUD, 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, Ratio0-3, 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 RUD can be expressed in either a high resolution (12.20) or high multiplication (20.12)
format selectable by the LFRatioCfg global parameter.
The RUD 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 theThe RUD 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 RUD.
5.4.2
Ratio Modifier (R-Mod)
The Ratio Modifier is used to internally multiply/divide the currently addressed RUD (Ratio0-3 stored in the
register space remain unchanged). The available options for R-Mod are summarized in
Table 2 onpage 13. R-Mod is enabled via the M2 pin in conjunction with the appropriate setting of the M2Config[2:0] Table 2. Ratio Modifier
5.4.3
Effective Ratio (REFF)
The Effective Ratio (REFF) is an internal calculation comprised of RUD and the appropriate modifiers, as
previously described. REFF is calculated as follows:
REFF = RUD R-Mod
Referenced Control
Parameter Definition
RModSel[1:0]
R Modifier
00
0.5
01
0.25
10
0.125
11
0.0625
Referenced Control
Parameter Definition