CS2200-OT Datasheet, PDF (19/23 Pages) Cirrus Logic – Fractional-N Frequency Synthesizer

CS2200-OTP
7. 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 would like to know more about how the User Defined Ratio is
calculated and stored.
Most calculators do not interpret the fixed point binary representation which the CS2200-OTP uses to define the
output to input clock ratio (see Section 5.3.1 on page 11); However, with a simple conversion we can use these tools
to generate a binary or hex value for Ratio_0-3 to be stored in one time programmable memory. Please see “Program-
ming Information” on page 21 for more details on programming.
7.1 12.20 Format
To
cy
calculate the
by the given
User
input
Defined Ratio (R_UD) to store in the register(s), divide the desired output
clock (RefClk). Then multiply the desired ratio by the scaling factor of
clock frequen-
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 3.
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 3. Example 12.20 R-Values
DS842F3
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