Ring oscillator design

digital design · Apr 23, 2013

Hi
I need to design a ring oscillator on FPGA. I create it by a chain of inverters and one AND gate for reset oscillation. There is no problem in simulation but when I program FPGA, it doesn't work and I don't know why!!
I use IGLOO Actel FPGA.
Please guide me.
thanks

mrflibble · Apr 23, 2013

It doesn't work because ring oscillators are touchy little creatures.

Chain of inverters. Okay, good start. How many inverters did you use? Suppose you used 5 or so, what you want to do is split up that chain in two parts and then put a placement constraint on them. You put 2 inverters over there, and the other 3 inverters waaaay over there on the die. That should give you 1) long routing delay 2) yesyes no doubt a lower frequency than the awesome 1 GHz you were trying to get and 3) a good chance of it actually working.

Oh and you want to put a FF as a divide by 2 stage after the output. Just because you can. And because it helps in getting a roughly 50% duty cycle.

Hope that helps.

barry · Apr 23, 2013

Also, make sure your synthesis tool is not eliminating those redundant inverters. Logically, 1 inverter is the same as 99 inverters and unless you force the tool to leave those inverters alone, it will optimize them away.

But (IMO) don't use a ring oscillator, you're just asking for trouble. If you can afford an FPGA in your design, you can afford an external oscillator.

digital design · Apr 23, 2013

Thanks for your guidance. I don't know how I should split up the chain and force it to implement two inverter chains far apart!
I don't know how I should set constraint!! I guest Synplify tool optimize circuit and remove all buffers and inverters! I should force it to keep all of them but I don't know how!
Can you help me in this way?

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I want to use Flash*Freeze mode of IGLOO kit. When an IGLOO device is in Flash*Freeze mode, none of the input signals (external clock) are seen by the internal logic. so I must create a ring oscillator in FPGA that is the internal clock source to the design.
I follow attached documentation step to step. About ring oscillator there is no problem in simulation but I have some problems on board. Ring oscillator didn't work on board. there are 4 step in section ring oscillator of this documentation. I can't set constraint of step 3.

barry · Apr 23, 2013

I am not familiar with Actel parts or that kit you're using, etc., but in Synplify you can use the "preserve" attribute to keep elements from being optimized out.

lucbra · Apr 26, 2013

I would assume you have read the application note on the MicroSemi website about that topic? It points you in the good direction.

Godup · Sep 15, 2013

mrflibble said:
It doesn't work because ring oscillators are touchy little creatures.

Chain of inverters. Okay, good start. How many inverters did you use? Suppose you used 5 or so, what you want to do is split up that chain in two parts and then put a placement constraint on them. You put 2 inverters over there, and the other 3 inverters waaaay over there on the die. That should give you 1) long routing delay 2) yesyes no doubt a lower frequency than the awesome 1 GHz you were trying to get and 3) a good chance of it actually working.
Oh and you want to put a FF as a divide by 2 stage after the output. Just because you can. And because it helps in getting a roughly 50% duty cycle.

Hope that helps.

Thanks for the break down but pls throw more light (enlighten) on using FF as a clock divider. How is it done? Kindly give a sketch of design and attach to your comment. Am also experiencing a similar challenge.

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barry said:
Also, make sure your synthesis tool is not eliminating those redundant inverters. Logically, 1 inverter is the same as 99 inverters and unless you force the tool to leave those inverters alone, it will optimize them away.

But (IMO) don't use a ring oscillator, you're just asking for trouble. If you can afford an FPGA in your design, you can afford an external oscillator.

Am also experiencing a similar challenge, but unfortunately i would have to use a ring oscillator because am designing the chip to respond to temperature changes. How do i get to know if the tool (QUARTUS II) had optimized the ring oscillator and where in the tool can i force it to stop optimizing the design? Attached is a snapshot of my code (VHDL) and the netlist viewer of the design. Thank you

mrflibble · Sep 15, 2013

Use the ring osc output as a clock, route clock to clock input of FF. Slap on an inverter and you're done.

Code:

reg divided_clk;

always @(posedge ring_osc_clk)
    divided_clk <= ~ divided_clk; // divided clock at half the frequency of ring_osc_clk

Godup · Sep 15, 2013

Thanks for the quick one... but i like to know if that code is VHDL, i would prefer VHDL. Also what kind of FF? +++ Slap on an inverter what does it mean???

mrflibble said:
Use the ring osc output as a clock, route clock to clock input of FF. Slap on an inverter and you're done.

Code:

reg divided_clk; always @(posedge ring_osc_clk) divided_clk <= ~ divided_clk; // divided clock at half the frequency of ring_osc_clk

mrflibble · Sep 15, 2013

The above code is for a FF, with the output connected to an inverter, and that output is going back to the FF input. As in toggle the flip-flop on each positive edge of the clock.

FvM · Sep 15, 2013

Am also experiencing a similar challenge, but unfortunately i would have to use a ring oscillator because am designing the chip to respond to temperature changes. How do i get to know if the tool (QUARTUS II) had optimized the ring oscillator and where in the tool can i force it to stop optimizing the design? Attached is a snapshot of my code (VHDL) and the netlist viewer of the design. Thank you

You are doing it basically right with syn_keep attribute. To view the delay_line details, you just ned to double click the block. You can also look at the resource usage to know if the ring oscillator has been implemented.

Godup · Sep 15, 2013

Thank you.. i got it now, i appreciate ur post. But do you know how to force a compiler from optimizing a design on the Quartus II software??? Or is there a Reserved word in VHDL that could do that?

mrflibble said:
Use the ring osc output as a clock, route clock to clock input of FF. Slap on an inverter and you're done.

Code:

reg divided_clk; always @(posedge ring_osc_clk) divided_clk <= ~ divided_clk; // divided clock at half the frequency of ring_osc_clk

mrflibble · Sep 15, 2013

I'm not that familiar with Quartus. You'd have to read the constraints user guide for that sort of thing.

Godup · Sep 15, 2013

Godup said:
Thank you.. i got it now, i appreciate ur post. But do you know how to force a compiler from optimizing a design on the Quartus II software??? Or is there a Reserved word in VHDL that could do that?

Thanks FVM, just answered that!

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FvM said:
You are doing it basically right with syn_keep attribute. To view the delay_line details, you just ned to double click the block. You can also look at the resource usage to know if the ring oscillator has been implemented.

Thanks man... got those block starring at me... didn't know i needed to double click. Attached is what it looks like. Pls assess.

FvM · Sep 16, 2013

Why are you now showing RTL instead of gate level view? RTL doesn't tell about logic removed during optimization.

You might be confused by the parallel 80 bit inverter, that's how things are compacted in RTL view.

The logic gate usage of the respective sub-entity is the most simple way to check if the inverter chain has been implemented.

Godup · Sep 17, 2013

FvM said:
Why are you now showing RTL instead of gate level view? RTL doesn't tell about logic removed during optimization.

You might be confused by the parallel 80 bit inverter, that's how things are compacted in RTL view.

The logic gate usage of the respective sub-entity is the most simple way to check if the inverter chain has been implemented.

Thanks, i got it now. Do u have any ideal as to forcing split on ring oscillator as mrflibble suggested? How is this constraint done in VHDL???

FvM · Sep 17, 2013

I don't see why we should split a ring oscillator.

Nevertheless you are free to prescribe an exact postion for each logic cell in assignment editor (presumed you understand the syntax).

mrflibble · Sep 17, 2013

What he meant to ask was "How do I constrain something to a particular location?" aka read the fine manual.

Godup · Sep 17, 2013

FvM said:
Nevertheless you are free to prescribe an exact postion for each logic cell in assignment editor (presumed you understand the syntax).

Not too sure about syntax now but my code is structural so created several objects of the ring oscillator attached together by SIGNAL lines. Then in assignment editor i found pins assigned... so where exactly can a do position fixing???

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mrflibble said:
aka read the fine manual.

. is it a material (for the tool or device)??? do i have to download that or its in the tool already (where can i get it)???

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I do hope u don't mean node assignment/location for logic placement.

Ring oscillator design

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