I have a 10MHz clock but needed a 20MHz clock speed. I used two asynchronous clear flip flops with a series of buffers to add delay to the signal.
Is this a bad practice? Will it fail with time or temperature? It works fine on a PCB, but I am concerned! It does exactly what I want, increment the counter on both rising and falling edges.
Above is a link to a picture the Xilinx schematic.
Thanks
Grant
If this is NOT recommended, then would 2 two bit counters (one with an inverted clock) and a 4 bit adder be the best solution? I'd like to keep my clock at 10MHz.
That's almost certainly not the 'best' solution. A better solution is to use a DCM to double the frequency. At 10MHz input frequency, you'll need to use its CLKFX output.
No you wouldn't. You'd like to keep your logic clock _enabled_ at 10MHz, but clocked by your newly DCMed 20MHz clock.
HTH., Syms.
p.s. Designing with schematics? How quaint! ;-)
Grant, years ago I published a reliable clock doubler circuit, part of the "six easy pieces" that seem to be lost. In words: Run your 10 MHz clock through a 2-input XOR. Generate a toggling flip-flop by feeding Q back through an inverting LUT to the D input. Route the signal driving D also to the second XOR input. Use the XOR output to clock the flip-flop, and also use it as your 20 MHz clock.
Disadvantage: If your 10 MHz doesn't have 50/50 duty cycle, your 20 MHz will have frequency modulation. And the High (or Low depending on XOR or XNOR) time of your 20 MHz clock will be short but you can lengthen it by adding delay to the Q- to-D path. Anyhow, it's self-adaptive to the device speed. Use this trick only when no PLL or DLL is available. Peter Alfke
I repeat my request that the Xilinx marketing and/or web people put all the old stuff that they unceremoniously removed back into an archive section of the web or FTP site.
The "six easy pieces" article is exactly the sort of thing that I was worried would be lost. :-(
Just because application notes and white papers are old does NOT mean that they aren't of any use to Xilinx customers.
Eric
I will learn Verilog or VHDL soon! : ) I promise!
I spent a few months pouring over the Homebrewcpu.com schematics and now find it easy to visualize what I want to do. : )
Hi Grant, :-) I tell you what, if more people adopted your learning approach, there would be fewer folks on comp.arch.fpga struggling to understand why their 'System C' code works in ModelSIM but doesn't work in their Spartan2e. Simulation aside, HDLs are merely great shortcuts for implementing the hardware that the designer has visualised. For sure, I still 'see' schematics in my head when I write my VHDL.
Going back to your original post, I'd recommend to anyone starting out designing FPGAs to do more reading about FPGA clocking. If you can meet the timing, the rest is junior engineering!
Cheers, Syms.
I found something...
Firefox doesn't know what to do with an htm file, Internet Explorer wants to install language packs for the English document...but will at least show it to you in English! :)
Save it before its gone! :)
Grant
Hey Eric,
Yeah, yeah, yeah. Sod that.
We're still waiting for the FPGA 'Six Not So Easy Pieces'.
Syms. :-)
No pictures for me. :-(
Symon schrieb:
Hi Simon, You need to download the pictures separately and store them in the same directory as the html code:
Don't worry about the background and logo crap.
Have a nice synthesis Eilert
This seems to be problem with firefox. Internet explorer manages to view site correctly with the embedded images.
OK, got it! thanks guys.
Eric, Peter, Don't worry! it's not completely lost...
You can find the text here:
and... guess... the images are here:
Regards Sandro
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.