[prim] Add and use a prim_const_buf and prim_const_sec_anchor_buf primitive cells

[vogelpi]

Description

The hardware revision constant in LC_CTRL is currently defined by a SystemVerilog parameter which is then used as is in the design. As a result, synthesis will optimize the parameter into the logic and it's not possible to change the hardware revision number when doing a metal spin. We should add a new primitive cell that is mapped to something like tie cells with synthesis constraints on it to prevent optimizations.

Similarly, we're using prim_sec_anchor_buf primitives for buffering constants inside OTP_CTRL (e.g. static scrambling keys) and KEYMGR (diversification constants). In both cases these are netlist secrets. So we want to ensure that we can track these signals during back end design.

So the task here is to:

• Add a prim_const_buf and a prim_const_sec_anchor_buf where the latter is a wrapper around the former one to ensure the name remains available in the netlist.
• Work out the synthesis constraints for these new const buffer cells / tie cells. We want to avoid that multiple such cells get combined into a single one.
• Use those cells in:
  • LC_CTRL for the hardware revision
  • KEYMGR[_DPE] for netlist constants going through prim_sec_anchor_buf stages
  • OTP_CTRL for netlist constants going through prim_sec_anchor_buf stages
  • Any other modules?

[gautschimi]

here is some data to make a assessment what makes sense to implement:

asap7 area comparison:

• smallest buffer area: 0.27 x 0.27 = 0.0729um² (BUFx2_ASAP7_75t_L)
• tie cell area: 0.162 x 0.27 = 0.04374 um² (TIEHIx1_ASAP7_75t_L)
• smallest inv area: 0.162 x 0.27 = 0.04374 um² (INVx1_ASAP7_75t_L)

=> TIE cells are in the same order as a buffer/inverter cell

number of cells:

• otp_ctrl: 7 x 128 + 4 x 64 = 1152
• lc_ctrl: 64
• keymgr: 2560
• keymgr_dpe: 1536

=> there are indeed almost 4k TIE cells. (top_earlgrey has 33k buffer, and 148k inverters)

Options:

• A: we can add a parameter to prim_const. with this parameter you can control what kind of cell you want to insert (special analog cell, buffer with const assignment, or tie cell)
• B: another prim that instantiates special cells
• C: we only use the prim_const for hardware revision and drop the changes in otp_ctrl and keymgr

[gautschimi]

Ok after a short call with @meisnere we decided to change the implementation a bit. We will go for option B (see above):

1. prim_const - instantiates TIE cells
2. prim_secure_anchor_const - instantiates prim_const
3. prim_const_id - instantiates special cell

The first 2 are already added, and the third is new.

prim_secure_anchor_const will be used in keymgr and otp_ctrl for seeds or keys. Basically anything that needs to be secret. prim_const_id will be instantiated in lc_ctrl for the hardware revision.

• The generic implementations will have a simple assign statement + comment.
• The asap7 implementation will instantiate TIE cells for both

@vogelpi @meisnere @andreaskurth do you agree?