On 2/5/10 6:14 AM, "paul swed" <[email protected]> wrote:
On Fri, Feb 5, 2010 at 9:04 AM, Didier Juges <[email protected]> wrote:
>
> Part of the requirement is that the devices be immune (as much as
> practical)
> from SEU malfunction. I was told Atmel (or Actel?) makes flash-based small
> FPGAs that may fit the bill. Most SRAM devices are deemed to be excessively
> sensitive to SEU, even though I cannot imagine how a CPLD/FPGA could be
> made
> that does not use SRAM at all. Maybe it's a matter of quantity? A few
> working registers may be an acceptable risk, but the entire device
> operating
> from SRAM is not acceptable?
It is somewhat relevant to time-nuttery, with respect to the recent discussion
about CPLDs. The Actel parts we use for spaceflight are anti-fuse type, so the
logic configuration is radiation hard. However, the actual gates and latches
you instantiate are susceptible to SEU, so if you need an "upset proof"
implementation, you have to go to TMR type schemes (many of which are
automatically implemented by the design tools.. That is, they have TMR
registers, etc. as part of the libraries)
I think there are onchip charge pumps and such for generating internal bias
voltages, and I don't know what the noise implications of them might be.
But the SRAM devices aren't all that susceptible to upset on a "per gate"
basis. The problem is that if you have a 3-6 million gate part, the
probability of an upset "somewhere" is fairly high (in a world where we worry
about 1E-12 rates). Again, you can use TMR type techniques. You can google
"Xilinx Upset probability" or "Xilinx radiation effects" and get some typical
numbers. There are a variety of schemes for scrubbing the configuration memory
(basically always rewriting the configuration, so that a configuration upset
doesn't last for very long)
One needs to be careful in looking at upset rates in SRAM based parts, too...
You need to distinguish between an upset in the data and an upset in the
configuration memory, and they have different rates.
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
