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Summary of Contents for Xilinx Pico Blaze Frequency Generator
- Page 1 Frequency Generator Spartan-3E Starter Kit Ken Chapman Xilinx Ltd July 2006 With special thanks to Peter Alfke and Alireza Kaviani. Rev.1...
- Page 2 Limitation of Liability. In no event will Xilinx or its licensors be liable for any loss of data, lost profits, cost or procurement of substitute goods or services, or for any special, incidental, consequential, or indirect damages arising from the use or operation of the designs or accompanying documentation, however caused and on any theory of liability.
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Page 3: Design Overview
‘install_frequency_generator.bat’. Assuming you have the Xilinx software installed, your board connected with the USB cable and the board powered (don’t forget the switch), then this should open a DOS window and run iMPACT in batch mode to configure the Spartan-3E with the design. -
Page 4: Operating Instructions
Operating Instructions Press and release knob to toggle between frequency editing modes Edit cursor position mode In this mode rotating the knob to the left or right will cause the display cursor to move in the corresponding direction on the upper line of the display. - Page 5 PicoBlaze Design Size The images and statistics on this page show that the design occupies just 172 slices, 1 BRAM and 2 DCMs. This is only 3.7% of the slices available in an XC3S500E device. More significantly, this slice count can be reduced to less than 32 when implementing a fixed frequency version. MAP report Number of occupied Slices: 172 out of...
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Page 6: Design Files
PicoBlaze download. Please visit the PicoBlaze Web site for your free copy of PicoBlaze, assembler, JTAG_loader and documentation. www.xilinx.com/picoblaze Hint – The JTAG_Loader utility supplied with PicoBlaze has been included in this design. This enables the new programs to be written for PicoBlaze using the configuration file provided. - Page 7 Direct Digital Synthesis (DDS) Circuit Diagram The phase accumulator is a standard 32-bit accumulator operating at 200MHz. This accumulator is really the heart of the DDS as it is the most significant bit of the accumulator that produces the variable frequency being synthesized. The remaining circuits only multiply, divide and clean this synthesized frequency or are involved with selecting and generating the DDS control words.
- Page 8 PicoBlaze Circuit Diagram PicoBlaze provides the user interface and performs the calculations required to generate the 32-bit DDS control word ‘N’ and 5-bit DDS scaling word ‘D’. Hint – The ‘fg_ctrl.psm’ file contains significant comments to explain the operations and calculations that the PicoBlaze program is performing to generate ‘N’...
- Page 9 Phase Accumulator Waveforms The following waveforms were obtained by monitoring the output of the phase accumulator presented on stake pin ‘J4-IO9’. In each case the digital storage oscilloscope was set to infinite persistence in order capture any fluctuations over time and therefore observe the ‘envelope’ of operation. 5ns of cycle to cycle jitter is often acceptable when synthesizing lower frequencies especially if the waveform is only used as a digital clock for control and timing of slower events.
- Page 10 Phase Accumulator Spectrum An alternative way to observe the quality of the waveforms synthesised by the phase accumulator is to look at the frequency spectrum. I was lucky enough to have a 2048-point FFT feature on my oscilloscope which allows some simple observations to be made. Once again I have set the display to infinite persistence in order capture the spectrum over a long period of time (>15 seconds).
- Page 11 Final Output Waveforms These waveforms were obtained from stake pin ‘J4-IO12’ and reflect the final output of the frequency generator. Once again the digital storage oscilloscope was set to infinite persistence in order capture any fluctuations over time and therefore observe the ‘envelope’ of operation. Hint –...
- Page 12 Final Output Spectrum Observing the frequency spectrum of the final output reveals that you can not get something for nothing and helps us to understand when the frequency aligned mode should and should not be used. I have used the infinite persistence display again and this time it was even more useful to do so. As before, plots on the left cover up to up to 50MHz and on plots on the right show 5MHz centred on 12.5MHz.
- Page 13 Setting DCM Frequency Aligned Mode To set the DCM into the frequency aligned mode of operation a special option must be used during configuration file generation. This can be set in the ISE tools as shown in these screen shots from an ISE v8.1i project for this reference design. Hint –...
- Page 14 Exercises, Experiments and Suggestions Here are some exercises, experiments and suggestions for you to consider based on this reference design. Although several are specific to the Spartan- 3E Starter Kit, most are portable to your own boards and designs where I hope you will find the design concept useful. Turn it off! Probably the best way to convince yourself that the frequency aligned mode of the DCM is really doing something special is to turn it off (remove the special BITGEN option) and then look at the output waveform on an oscilloscope particularly at frequencies >50MHz.
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