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AR# 34937 Clocking Wizard - When the output CLK is the same as the input, the Requested and Actual can be slightly different

Description

When using the Clocking Wizard and the Output clock frequency as the input Clock, sometimes the Request and Actual Frequencies are different in the Wizard. Why?

Solution

1) The Clocking Wizard performs the calculations to find M, D and CLKOUTx_DIVIDE rounding to 3 decimal places, which is consistent with TRCE. This can cause minor rounding error.

For example, if the input frequency is 61.44MHz and the request CLKOUT frequency = 44.8MHz, this will result in an Actual CLKOUT frequency =44.807MHz

The first step the Wizard does is convert the input frequency to period and round to 3 decimal places.

1000/61.44 = 16.276041666666666666666666666667 =~ 16.276
VCO = (1000/16.276) * 33 / 2 = 1013.7625952322437945441140329319 =~ 1013.763
CLKOUT frequency =1013.763/22.625 = 44.807204419889502762430939226519 =~ 44.807

CLKFBOUT_MULT_F = 33, CLKOUT0_DIVIDE_F = 22.625, DIVCLK_DIVIDE = 2


2) The rounding error may also cause a minor discrepancy between the displayed Actual frequency and what is output.

For example if you input 510MHz andrequest 510MHz.

Here the CLKIN is entered as 510 MHz:

When selecting 510 MHz as the output Frequency, you can see that instead of getting 510 MHz, the Actual is listed as 510.204 MHz:

The reason for this is how the calculations are done; they only use 3 decimal places. The following is how the 510.204 MHz is created:

Computed Clock Period = 1/510 MHz = 1.960784314ns with 3 decimal places = 1.960 ns Computed Output Frequency = 1/1.960nS = 510.204 MHz

In Hardware, the output frequency will be 510 MHz as Requested. In order to verify this, the output frequency can be calculated using the settings on page 4 of the wizard:


So CLKOUT0 = ((CLKIN * CLKFBOUT_MULT_F) / DIVCLK_DIVIDE ) / CLKOUT0_DIVIDE_F

((510 MHz * 6) / 3 ) / 2 = 510 MHz.

Linked Answer Records

Associated Answer Records

Answer Number Answer Title Version Found Version Resolved
46790 Spartan-6 FPGA Design Assistant - Troubleshooting Common Clocking Problems N/A N/A

Actions

AR# 34937
Date Created 06/15/2010
Last Updated 03/06/2013
Status Active
Type General Article
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