NI 6587/6589 Frequency Inaccuracy

Updated Nov 3, 2017

Issue Details

Clock frequency is not accurate based on what I requested using the sample project

Solution

  • This behaviors is expected
  • To improve accuracy, you will need to determine the exact crystal frequency and then use this information to adjust the Si570 configuration coefficients
  • In order to measure the crystal frequency for a specific module, you may download the attached files which contain a tool for calculating the crystal frequency for your particular module. The following are steps you may use to measure this value:
  1. Download the 6587_6589 Si570 Measure Frequency.zip folder below
  2. Open the NI 6587_6589 Si570 Fxtal Calculation Tool LabVIEW project (files saved in LabVIEW 2014)
  3. Once in the project, change the FPGA Target to the FlexRIO Target you are using with your 6587 or 6589; The original project was compiled for the 7951R and a bitfile is provided
    1. For a different target, create a new FPGA Target based on your FlexRIO backend, copy/paste items from the 7951R target to your new target, and configure your new target to your IO Module of choice (6587 or 6589)
    2. Copy the build specification over from the 7951R FPGA Target into your new FPGA target, and build a new bitfile
  4. With your new bitfile for your FPGA Target, open the 6587_6589 Si570 Measure Frequency (Host).vi
  5. On the block diagram, right click the Open FPGA Reference.vi at the beginning of the code, and change the reference to your newly created bitfile
  6. On the front panel, select the Resource Name of your FlexRIO backend, and start the VI
  7. While the VI is running, click Start Si570 Measure Frequency on the front panel
  8. To measure the crystal frequency, click Count? on the front panel
  9. Record the value in the indicator marked Xtal Freq (Hz). This value will be what you can use in your project.
Using the newly calculated value in the Clock Frequency Setup.vi as the Xtal Freq (Hz) input will allow your device to properly configure the frequency to within specified frequency for the Si570 chip.

6587_6589 Si570 Measure Frequency6587_6589 Si570 Measure Frequency.zip
 

Additional Information

The NI 6587/6589 Sample Projects do not calculate the crystal (Xtal) frequency for the Si570 Clocking Chips, but rather use the nominal crystal frequency. Since each Si570 device has a unique crystal the accuracy of the onboard clock can vary up to +/- 2,000 ppm. The crystal frequency is an input to the Clock Frequency Setup.vi provided in all sample projects for the adapter modules.

The block diagram in the sample projects (found in Example Finder) allows the user to input the Desired New Frequency, while sending the array of Si570 registers to the chip, but the Xtal Freq (Hz) input is left unwired. This is where the nominal default value is used when calculating settings for the Si570 chip.


Once you have determined the crystal frequency value (mentioned in the solution), you can save this value in the EEPROM for further usage, or hard-coded into your code since it will not change for that specific module. 

WAS THIS ARTICLE HELPFUL?

Not Helpful