Re: 2 leg crystal on FPGA: Lattice vs Xilinx

Austin

Might be worth making the suggestion to your sister grouping of GPD of
adding a dedicated oscillator crcuit to their range of products. Given
a lot of micros do that already there would be some logic in adding
such a circuit in the future to the low cost sector FPGA families.

John Adair
Enterpoint Ltd.

On 28 Sep, 21:21, austin <aus...@xxxxxxxxxx> wrote:
Antti,

The circuit works, only because the IO acts like an inverter with a very
small delay.

If the IO has more delay, than the circiut will not start up (always).

We do not recommend the use of a crystal like this, as we have
experience that it doesn't always start up.

Will it work once? Sure. Will it works always, nope.

I suppose Lattice isn't large enough to worry...

Austin



Antti wrote:
Hi

I know many wise men has said NO NO, but

1)
http://www.latticesemi.com/forums/forum/messageview.cfm?catid=42&thre...

Lattice engineer suggest that it works (assumable reliable) on machXO

the IO technology between machXO and Xilinx FPGAs isnt so big so I
wonder why cant it be done with Xilinx ?

for what I see is following

25MHz crystal
27p caps
560 series
1M parallel

when using LVCMOS33 SLEW=FAST

then there is some sort of overdrive, that makes oscillation to
periodically stop and restart
200 us work then 75 idle, then restarts again, the FPGA input sees
however nice 25MHz
from the crystal ALL time, (also when the output doesnt swing)

by simply changing slew=slow the circuit does start work reliable.

so any technical reasons why this circuit can not (should not) be
used??

crystal vs oscillator price difference is still some 0.30 USD, so why
waste the pennies

Antti- Hide quoted text -

- Show quoted text -


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