[time-nuts] T.I. questions

Mike Monett timenuts at binsamp.e4ward.com
Sat Feb 7 21:40:35 EST 2015


>magnus at rubidium.dyndns.org said:

>> The typical noise generator chips uses a PRNG based on DFFs and XOR
>> gate(s). A typical weakness is that the chain of DFFs is to short,  causing
>> a relatively high rate of cycling, which hearable as a beating. However, for
>> some uses, that is OK.

>The buzzword for the typical PRNG is LFSR - Linear Feedback Shift Register.

>Many years ago, Xilinx published a good app-note on this topic.

Several, in fact. I spent some time tracking them down. Here's the previous
list:

  xapp210
  xapp211
  xapp217
  xapp220

But I found one more - XAPP052. It has a feedback table from N=3 to 168.
The table is repeated in later app notes. Also, Xilinx has apparently lost
XAPP217. It no longer appears on their web site, but I found it. Here's the
updated list:

  XAPP052 LFSR Generators N=3 to 168
  http://www.xilinx.com/support/documentation/application_notes/xapp052.pdf

  XAPP210 LFSR in Vertix Devices
  http://www.xilinx.com/support/documentation/application_notes/xapp210.pdf

  XAPP211 PDF PN Generators Using The SRL Macro
  http://www.xilinx.com/support/documentation/application_notes/xapp211.pdf

  XAPP217 Gold Code Generators
  77-42925.pdf http://application-notes.digchip.com/077/77-42925.pdf

  XAPP220 LFSRs As Functional Blocks in Wireless Applications
  http://uglyduck.ath.cx/PDF/Xilinx/Spartan3/appnotes/xapp220.pdf

Mike Monett



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