Re: 2 leg crystal on FPGA: Lattice vs Xilinx
- From: Peter Alfke <alfke@xxxxxxxxxxxxx>
- Date: Fri, 28 Sep 2007 19:49:47 -0700
On Sep 28, 5:05 pm, John Adair <g...@xxxxxxxxxxxxxxxx> wrote:
AustinJohn, "been there, done that".
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.
XC3000 used to have a single-stage dedicated inverter, exactly for
that purpose. It caused us a lot of support grief. Between 32 kHz and
100 MHz, there is a big variation in xtals, and there also was a
sensitivity to Vcc ramp-up rate. Nobody wants a circuit to work "most
of the time".
I also remember that many of the Intel mask revisions of the 8051 were
oscillator-related. (We second-sourced that at AMD)
My advice has always been: spend a few pennies on an oscillator
circuit made by specialists for a special purpose. And definitely do
not abuse a multi-stage I/O circuit to be the xtal inverter circuit.
Far too much gain and uncontrolled phase changes at very high
frequencies.
Peter Alfke
.
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