Crystal input for FPGA

I am posting this just as a suggestion for future FPGAs. It would be
nice if a crystal can be connected to FPGA to provide clock instead of
a oscillator. The cost difference between an oscillator and a crystal
is significant. A 50MHz oscillator (2.5v) fro S3E costs $3.27. A 100MHz
one is around $6. But all the crystals are less than $1. I have worked
with PIC microcontrollers in past and they have the option of
connecting both.

I may be missing something here or may be this is already solved in
some other way. I searched through Xilinx website but could not find
anything. Please let me know if there is an existing solutiuon.

Thanks
Sumit

shrutisumit@gmail.com wrote:

> I am posting this just as a suggestion for future FPGAs. It would be
> nice if a crystal can be connected to FPGA to provide clock instead of
> a oscillator. The cost difference between an oscillator and a crystal
> is significant. A 50MHz oscillator (2.5v) fro S3E costs $3.27. A 100MHz
> one is around $6. But all the crystals are less than $1. I have worked
> with PIC microcontrollers in past and they have the option of
> connecting both.
> 
> I may be missing something here or may be this is already solved in
> some other way. I searched through Xilinx website but could not find
> anything. Please let me know if there is an existing solutiuon.
> 
> Thanks
> Sumit
> 
If you're counting pennies like that, consider rolling your own crystal 
oscillator using one of the tiny logic inverters (M74VHC1GU04 or 
equivalent).  The inverter will cost pennies, the crystal will still be 
a buck, you'll still need the handful of parts you'd need with the PIC, 
and you'll still need the months of engineering time and hassle with 
purchasing and manufacturing to get and keep things at a rock-solid 
reliable level under all conditions and manufacturing variations.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

shrutisumit@gmail.com wrote:

> I am posting this just as a suggestion for future FPGAs. It would be
> nice if a crystal can be connected to FPGA to provide clock instead of
> a oscillator. The cost difference between an oscillator and a crystal
> is significant. A 50MHz oscillator (2.5v) fro S3E costs $3.27. A 100MHz
> one is around $6. But all the crystals are less than $1. I have worked
> with PIC microcontrollers in past and they have the option of
> connecting both.
> 
> I may be missing something here or may be this is already solved in
> some other way. I searched through Xilinx website but could not find
> anything. Please let me know if there is an existing solutiuon.

XTAL osc were there, but removed many generations ago.

These days, you can get cheap SOT23 Osc from Linear/maxim, that
can clock FPGAs in many applications.

-jg

Been there, done that.
XC3000 had (has) two pins that wrap around a single-stage buffer, and
are meant to be connected to a xtal. Add a biaing resistor plus the
obligatory caps to form a Colpitts oscillator.
It was a support nightmare. Even if 99% of applications had no problem,
the remaining 1% drove us crazy. Too little gain, too much gain, too
low a frequency (32 kHz), too high a frequency (>100 MHz), doesn't
start at cold, bad pc-board layout, etc
Canned oscillators are made by experts, using exactly the best chip for
the particular frequency, and use the smallest amount of power. And
they are surprisingly cheap, far less than the $6 quoted in the posting
here.
You can even get a 312.5 MHz oscillator for a few dollars...(Sits in
every cellphone)
Never again will we put that driver into an FPGA !
Peter Alfke, Xilinx
PS: when I was at AMD, we second-sourced the 8051. Most of Intel's mask
revisions were caused by their oscillator circuit...
===================================

Tim Wescott wrote:

> shrutisumit@gmail.com wrote:
> 
>> I am posting this just as a suggestion for future FPGAs. It would be
>> nice if a crystal can be connected to FPGA to provide clock instead of
>> a oscillator. The cost difference between an oscillator and a crystal
>> is significant. A 50MHz oscillator (2.5v) fro S3E costs $3.27. A 100MHz
>> one is around $6. But all the crystals are less than $1. I have worked
>> with PIC microcontrollers in past and they have the option of
>> connecting both.
>>
>> I may be missing something here or may be this is already solved in
>> some other way. I searched through Xilinx website but could not find
>> anything. Please let me know if there is an existing solutiuon.
>>
>> Thanks
>> Sumit
>>
> If you're counting pennies like that, consider rolling your own crystal 
> oscillator using one of the tiny logic inverters (M74VHC1GU04 or 
> equivalent).  The inverter will cost pennies, the crystal will still be 
> a buck, you'll still need the handful of parts you'd need with the PIC, 
> and you'll still need the months of engineering time and hassle with 
> purchasing and manufacturing to get and keep things at a rock-solid 
> reliable level under all conditions and manufacturing variations.

Good idea, but the FPGA will NOT like the sine drive - there are 1G 
variants, that ARE designed for Crystal Osc, [1GU04+1G14 in one package]
Philips have a couple, TI have one,
eg 74AUP1Z04

-jg

All,

Nothing like failure to cause one to abandon a feature forever.

A crystal oscillator that: always starts, is always the right frequency,
and is cheap -- is best left to those who have solved it for a few dozen
useful frequencies (and even they have their share of headaches).

It would be (and is) a horrible business risk to attempt to supply a
circuit that would always work for every possible crystal from near DC
to daylight.

Can anyone guess how many different parameters there are to specify for
a crystal?  It is not just the frequency....

Austin

Austin Lesea wrote:
> All,
> 
> Nothing like failure to cause one to abandon a feature forever.
> 
> A crystal oscillator that: always starts, is always the right frequency,
> and is cheap -- is best left to those who have solved it for a few dozen
> useful frequencies (and even they have their share of headaches).
> 
> It would be (and is) a horrible business risk to attempt to supply a
> circuit that would always work for every possible crystal from near DC
> to daylight.
> 
> Can anyone guess how many different parameters there are to specify for
> a crystal?  It is not just the frequency....

The companies that do this properly, typically have 4 gm's or transfer 
gains selectable by fuses, that cover 1-2 decades of frequency/option.

Most vanilla Osc circuits struggle with overtone crystals (above appx 
25-40MHz), and for FPGAs that is on the 'low' side of useful.
So above this, and you are pretty much into Osc-Modules (or the cheaper 
SOT23's if you can tolerate their precision )

Silicon MEMS 'crystals' have been talked about, but still seem niche 
devices. - and no, these will not appear inside your FPGAs :)

-jg

> Nothing like failure to cause one to abandon a feature forever.
>
> A crystal oscillator that: always starts, is always the right frequency,
> and is cheap -- is best left to those who have solved it for a few dozen
> useful frequencies (and even they have their share of headaches).
>
> It would be (and is) a horrible business risk to attempt to supply a
> circuit that would always work for every possible crystal from near DC
> to daylight.

Of course it is understandable that you want to minimize your risk. However, 
mosts user would benefit from this, as everyone needs an clock from 
somewhere. I cannot really believe that this is rocket-sience, as almost 
every uC out there has this. Most likely you can buy the IP for some bucks 
so that you have not to reinvent the wheel yourself?

An addition or alternative to the crystal-oscillator could be a calibrated 
internal oscillator with reasonable precision (e.g. +/-1 %).

Maybe X, A and L can look at Actel (Fusion), they have solved this almost 
perfectly, at least according to the first look at the datasheet...

Thomas

www.entner-electronics.com

> Most vanilla Osc circuits struggle with overtone crystals (above appx 
> 25-40MHz), and for FPGAs that is on the 'low' side of useful.

Why not using the PLLs for this?

Thomas 

Peter Alfke wrote:
> Been there, done that.
> XC3000 had (has) two pins that wrap around a single-stage buffer, and
> are meant to be connected to a xtal. Add a biaing resistor plus the
> obligatory caps to form a Colpitts oscillator.
> It was a support nightmare. Even if 99% of applications had no problem,
> the remaining 1% drove us crazy. Too little gain, too much gain, too
> low a frequency (32 kHz), too high a frequency (>100 MHz), doesn't
> start at cold, bad pc-board layout, etc
> Canned oscillators are made by experts, using exactly the best chip for
> the particular frequency, and use the smallest amount of power. And
> they are surprisingly cheap, far less than the $6 quoted in the posting
> here.

Where can I find those. I may be looking at the wrong place. I need a
2.5V one. Here is a link for digikey where the quoted price is $5.97

http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=375484&Row=706244&Site=US

Sumit

> You can even get a 312.5 MHz oscillator for a few dollars...(Sits in
> every cellphone)
> Never again will we put that driver into an FPGA !
> Peter Alfke, Xilinx
> PS: when I was at AMD, we second-sourced the 8051. Most of Intel's mask
> revisions were caused by their oscillator circuit...
> ===================================