Actually, you're not far off. It's a serial shift chain which is shiftedEven state-of-the-art CPUs today commonly use scan-chains (via JTAG)Is there some blog somewhere that explains how scan-chains work (not
for debuggin.
how they're used, but how they're implemented inside the CPU)?
Intuitively they sound very costly to me, because of things like the
need to run extra wires all over the place. I'm obviously
missing something.
one-bit at a time to capture flop states. Each chain is a single wire;
a chip may have a few dozen individual shift chains.
https://www.design-reuse.com/articles/48331/scan-chains-pnr-outlook.html
Thanks. Wow. So it is really that bad, huh?
I also liked the note about speed limits and power consumption, how
shifting a state (in or out) causes (almost) all the flip-flops to
change state at each cycle, thus leading to very high power consumption.
What's the approximate cost of those scan chains. I.e. if we were to
take an existing working design and replace all the "flip-flop with
scan-chain" with "plain flip-flops", how much smaller would the
resulting chip be, how much faster could it run, and how much less power
could it consume?
I assume the cost in terms of power consumption is small because in
normal use, the scan-chain part stays completely stable so that barring
leakage it should not consume any power, save for the indirect costs
like the need to move the other bits over greater distances when
the extra wires of the scan chains get in the way.
Stefan
| Date | Sujet | # | Auteur | |
| 17 Apr 25 |  Re: register sets | 56 | Robert Finch | |
| 17 Apr 25 |  Re: register sets | 53 | Stephen Fuld | |
| 17 Apr 25 |   Re: register sets | 1 | Robert Finch | |
| 17 Apr 25 | Re: register sets | 46 | MitchAlsup1 | |
| 18 Apr 25 |  Re: register sets | 45 | Robert Finch | |
| 18 Apr 25 |  Re: register sets | 44 | MitchAlsup1 | |
| 20 Apr 25 | Re: register sets | 43 | Robert Finch | |
| 21 Apr 25 | Re: auto predicating branches | 42 | Robert Finch | |
| 21 Apr 25 | Re: auto predicating branches | 41 | Anton Ertl | |
| 21 Apr 25 | Is an instruction on the critical path? (was: auto predicating branches) | 1 | Anton Ertl | |
| 21 Apr 25 | Re: auto predicating branches | 39 | MitchAlsup1 | |
| 22 Apr 25 | Re: auto predicating branches | 38 | Anton Ertl | |
| 22 Apr 25 | Re: auto predicating branches | 1 | MitchAlsup1 | |
| 22 Apr 25 | Re: auto predicating branches | 36 | Anton Ertl | |
| 22 Apr 25 | Re: auto predicating branches | 35 | MitchAlsup1 | |
| 23 Apr 25 | Re: auto predicating branches | 3 | Stefan Monnier | |
| 23 Apr 25 | Re: auto predicating branches | 2 | Anton Ertl | |
| 25 Apr 25 | Re: auto predicating branches | 1 | MitchAlsup1 | |
| 23 Apr 25 | Re: auto predicating branches | 31 | Anton Ertl | |
| 23 Apr 25 | Re: auto predicating branches | 30 | MitchAlsup1 | |
| 24 Apr 25 | Re: asynch register rename | 29 | Robert Finch | |
| 27 Apr 25 | Re: fractional PCs | 28 | Robert Finch | |
| 27 Apr 25 | Re: fractional PCs | 27 | MitchAlsup1 | |
| 28 Apr 25 | Re: fractional PCs | 26 | Robert Finch | |
| 28 Apr 25 | Re: fractional PCs | 15 | MitchAlsup1 | |
| 29 Apr 25 | Re: fractional PCs | 14 | Robert Finch | |
| 5 May 25 | Re: control co-processor | 13 | Robert Finch | |
| 5 May 25 | Re: control co-processor | 12 | Al Kossow | |
| 5 May 25 | Re: control co-processor | 11 | Stefan Monnier | |
| 6 May 25 | Re: control co-processor | 3 | MitchAlsup1 | |
| 7 May 25 | Re: control co-processor | 1 | MitchAlsup1 | |
| 15 Jul 25 | Re: control co-processor | 1 | MitchAlsup1 | |
| 7 May 25 | Scan chains (was: control co-processor) | 7 | Stefan Monnier | |
| 7 May 25 | Re: Scan chains (was: control co-processor) | 2 | Al Kossow | |
| 7 May 25 | Re: Scan chains | 1 | Stefan Monnier | |
| 7 May 25 | Re: Scan chains | 3 | MitchAlsup1 | |
| 7 May 25 | Re: Scan chains | 2 | Stefan Monnier | |
| 8 May 25 | Re: Scan chains | 1 | MitchAlsup1 | |
| 15 Jul 25 | Re: Scan chains | 1 | MitchAlsup1 | |
| 29 Apr 25 | Re: fractional PCs | 10 | Robert Finch | |
| 29 Apr 25 | Re: fractional PCs | 9 | MitchAlsup1 | |
| 30 Apr 25 | Re: fractional PCs | 8 | Robert Finch | |
| 30 Apr 25 | Re: fractional PCs | 6 | Thomas Koenig | |
| 1 May 25 | Re: fractional PCs | 1 | Robert Finch | |
| 2 May 25 | Re: fractional PCs | 4 | moi | |
| 2 May 25 | Re: millicode, extracode, fractional PCs | 2 | John Levine | |
| 2 May 25 | Re: millicode, extracode, fractional PCs | 1 | moi | |
| 2 May 25 | Re: fractional PCs | 1 | moi | |
| 30 Apr 25 | Re: fractional PCs | 1 | MitchAlsup1 | |
| 15 Jul 25 | Re: register sets | 5 | John Savard | |
| 15 Jul 25 | Re: register sets | 4 | MitchAlsup1 | |
| 19 Jul 25 | Re: register sets | 3 | Robert Finch | |
| 19 Jul 25 | Re: register sets | 2 | Anton Ertl | |
| 19 Jul 25 | Re: register sets | 1 | MitchAlsup1 | |
| 15 Jul 25 | Re: register sets | 2 | John Savard | |
| 15 Jul 25 | Re: register sets | 1 | MitchAlsup1 |
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