Date: Sat, 13 Feb 2021 06:10:08 +0000
To: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Libre/Open blockchain / cryptographic ASICs
Precedence: list

Good morning Luke,

Another thing we can do with scan mode would be something like the below ma=
sking:

    input CLK, RESET_N;
    input TESTMODE;
    input SCANOUT_INTERNAL;
    output SCANOUT_PAD;

    reg gating;
    wire n_gating =3D gating && TESTMODE;
    always_ff @(posedge CLK, negedge RESET_N) begin
      if (!RESET_N)   gating <=3D 1'b1; /*RESET-HIGH*/
      else            gating <=3D n_gating; end

    assign SCANOUT_PAD =3D SCANOUT_INTERNAL && gating;

The `gating` means that after reset, if we are not in test mode, `gating` b=
ecomes 0 permanently and prevents any scan data from being extracted.
Assuming scan is not used in normal operation (it should not) then inadvert=
ent ESD noise on the `gating` flip-flop would not have an effect.

Output being combinational should be fine as the output is "just" an AND ga=
te, as long as `gating` does not transition from 0->1 (impossible in normal=
 operation, only at reset condition) then glitching is impossible, and when=
 scan is running then `TESTMODE` should not be exited which means `gating` =
should remain high as well, thus output is still glitch-free.

Since the flip-flop resets to 1, and in some technologies I have seen a res=
et-to-0 FF is slightly smaller than a reset-to-1 FF, it might do good to in=
vert the sense of `gating` instead, and use a NOR gate at the output (which=
 might also be smaller than an AND gate, look it up in the technology you a=
re targeting).
On the other hand the above is a tiny circuit already and it is unlikely yo=
u need more than one of it (well for large enough ICs you might want more t=
han one scan chain but still, even the largest ICs we handled never had mor=
e than 8 scan chains, usually just 4 to 6) so overoptimizing this is not ne=
cessary.


Regards,
ZmnSCPxj