Cirrus Logic CS2100-CP - Manual (Page 30)

CS2100-CP
30 DS840F2
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 CS2100 uses to define the output
to input clock ratio (see
Section 5.3.1 on page 17 ); 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 High Resolution 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 (CLK_IN). Then multiply the desired ratio by the scaling factor of 2
20
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 .
Table 2. Example 12.20 R-Values
9.2 High Multiplication 20.12 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 (CLK_IN). Then multiply the desired ratio by the scaling factor of 2
12
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 .
Table 3. Example 20.12 R-Values
Desired Output to Input Clock Ratio
(output clock/input clock)
Scaled Decimal
Representation =
(output clock/input clock)
•
2
20
Hex Representation of
Binary R
UD
12.288 MHz/10 MHz=1.2288 1288490 00 13 A9 2A
11.2896 MHz/44.1 kHz=256 268435456 10 00 00 00
Desired Output to Input Clock Ratio
(output clock/input clock)
Scaled Decimal
Representation =
(output clock/input clock)
•
2
12
Hex Representation of
Binary R
UD
12.288 MHz/60 Hz=204,800 838860800 32 00 00 00
11.2896 MHz/59.97 Hz =188254.127... 771088904 2D F5 E2 08