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CDCM6208V1FRGZR Datasheet(PDF) 38 Page - Texas Instruments

Part No. CDCM6208V1FRGZR
Description  2:8 Clock Generator, Jitter Cleaner with Fractional Dividers
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
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CDCM6208V1FRGZR Datasheet(HTML) 38 Page - Texas Instruments

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SCAS943 – MAY 2015
When the output frequency plan calls for the use of some output dividers as fractional values, the following steps
are needed to calculate the closest achievable frequencies for those using fractional output dividers and the
frequency errors (difference between the desired frequency and the closest achievable frequency).
Based on system needs, decide the frequencies that need to have best possible jitter performance.
Once decided, these frequencies need to be placed on integer output dividers.
Then a frequency plan for these frequencies with strict jitter requirements can be worked out using the
common divisor algorithm.
Once the integer divider plans are worked out, the PLL settings (including VCO frequency, feedback divider,
input divider and prescaler divider) can be worked out to map the input frequency to the frequency out of the
prescaler divider.
Then calculate the fractional divider values (whose values must be greater than 2) that are needed to support
the output frequencies that are not part of the common frequency plan from the common divisor algorithm
already worked out.
For each fractional divider value, try to represent the fractional portion in a 20 bit binary scheme, where the
first fractional bit is represented as 0.5, the second fractional bit is represented as 0.25, third fractional bit is
represented as 0.125 and so on. Continue this process until the entire 20 bit fractional binary word is
Once exhausted, the fraction can be calculated as a cumulative sum of the fractional bit x fractional value of
the fractional bit. Once this is done, the closest achievable output frequency can be calculated with the
mathematical function of the frequency out of the prescaler divider divided by the achievable fractional
The frequency error can then be calculated as the difference between the desired frequency and the closest
achievable frequency.
10.5 Programming
10.5.1 Writing to the CDCM6208V1F
To initiate a SPI data transfer, the host asserts the SCS (serial chip select) pin low. The first rising edge of the
clock signal (SCL) transfers the bit presented on the SDI pin of the CDCM6208V1F. This bit signals if a read
(first bit high) or a write (first bit low) will transpire. The SPI port shifts data to the CDCM6208V1F with each
rising edge of SCL. Following the W/R bit are 4 fixed bits followed by 11 bits that specify the address of the
target register in the register file. The 16 bits that follow are the data payload. If the host sends an incomplete
message, (i.e. the host de-asserts the SCS pin high prior to a complete message transmission), then the
CDCM6208V1F aborts the transfer, and device makes no changes to the register file or the hardware. Figure 32
shows the format of a write transaction on the CDCM6208V1F SPI port. The host signals the CDCM6208V1F of
the completed transfer and disables the SPI port by de-asserting the SCS pin high.
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Product Folder Links: CDCM6208V1F

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