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Old 04-15-2006, 07:09 AM
Jim Granville
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Default Re: humble suggestion for Xilinx

John Larkin wrote:
> On Sat, 15 Apr 2006 10:45:06 +1200, Jim Granville
> <[email protected]> wrote:
>
>
>>John Larkin wrote:
>>
>>>Since the max serial-slave configuration rate on things like Spartan3
>>>chips is, what, 20 MHz or something, you might consider slowing down
>>>the CCLK input path, and/or adding some serious hysteresis on future
>>>parts. On a pcb, CCLK is often a shared SPI clock, with lots of loads
>>>and stubs and vias and such, so may not be as pristine as a system
>>>clock. CCLK seems to be every bit as touchy as main clock pins, and it
>>>really needn't be.

>>
>>Wouldn't one expect this to be 'normal design practise' ?
>>
>>I suppose Xilinx missed that obvious feature, becasue there are no
>>other Schmitt cells on the die, and even tho the CPLDs have this,
>>I'm sure their inter-department sharing is like most large companies
>>

>
>
> I have it secondhand (one of my guys tells me) that all S3 inputs have
> about 100 mV of hysteresis. But that's not enough to improve noise
> immunity in most practical situations.
>
> It's good that FPGAs keep getting faster, but not all applications
> need all that speed, and pickiness about clock edge quality can be a
> real liability in a lot of slower applications.


True - there is also a slight speed penalty for the full schmitt cells,
so that's a reason why the speed-at-all-costs FPGA sector ignores the
benefits.
Still, the news from Steve K is good

-jg


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