CS2200-CP_09 Datasheet, PDF (23/26 Pages) Cirrus Logic – Fractional-N Frequency Synthesizer

CS2200-CP
9. CALCULATING THE USER DEFINED RATIO
Note: The software for use with the evaluation kit has built in tools to aid in calculating and converting the User
Defined Ratio. This section is for those who are not interested in the software or who are developing their
systems without the aid of the evaluation kit.
Most calculators do not interpret the fixed point binary representation which the CS2200 uses to define the output
to input clock ratio (see Section 5.2.1 on page 12); However, with a simple conversion we can use these tools to
generate a binary or hex value which can be written to the Ratio register.
9.1 12.20 Format
To calculate the User Defined Ratio (R_UD) to store in the register(s), divide the desired output clock frequen-
cy by the given input clock (RefClk). Then multiply the desired ratio by the scaling factor of 2²⁰ to get the
scaled decimal representation; then use the decimal to binary/hex conversion function on a calculator and
write to the register. A few examples have been provided in Table 2.
Desired Output to Input Clock Ratio
(output clock/input clock)
12.288 MHz/10 MHz=1.2288
11.2896 MHz/44.1 kHz=256
Scaled Decimal
Representation =
(output clock/input clock) • 2²⁰
1288490
268435456
Hex Representation of
Binary R_UD
00 13 A9 2A
10 00 00 00
Table 2. Example 12.20 R-Values
DS759F1
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