On Sat, Feb 13, 2021 at 6:10 AM ZmnSCPxj <zmnsc...@protonmail.com> wrote: > > Good morning Luke,
morning - can i ask you a favour because moderated (off-topic) messages are being forwarded https://lists.ozlabs.org/pipermail/bitcoin-dev-moderation/ could you send these instead to libre-soc-...@lists.libre-soc.org? many thanks, l. > Another thing we can do with scan mode would be something like the below > masking: > > input CLK, RESET_N; > input TESTMODE; > input SCANOUT_INTERNAL; > output SCANOUT_PAD; > > reg gating; > wire n_gating = gating && TESTMODE; > always_ff @(posedge CLK, negedge RESET_N) begin > if (!RESET_N) gating <= 1'b1; /*RESET-HIGH*/ > else gating <= n_gating; end > > assign SCANOUT_PAD = SCANOUT_INTERNAL && gating; > > The `gating` means that after reset, if we are not in test mode, `gating` > becomes 0 permanently and prevents any scan data from being extracted. > Assuming scan is not used in normal operation (it should not) then > inadvertent 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 > gate, 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 > reset-to-0 FF is slightly smaller than a reset-to-1 FF, it might do good to > invert 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 are > targeting). > On the other hand the above is a tiny circuit already and it is unlikely you > need more than one of it (well for large enough ICs you might want more than > one scan chain but still, even the largest ICs we handled never had more than > 8 scan chains, usually just 4 to 6) so overoptimizing this is not necessary. > > > Regards, > ZmnSCPxj _______________________________________________ bitcoin-dev mailing list bitcoin-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
