toy-sram/custom

Custom Cell Design

Prerequisites

• Magic (VERSION TODO) - Layout
• Xschem (VERSION TODO) - Schematic capture
• Netgen-lvs (VERSION TODO) - Layout Vs. Schematic
• Ngspice (VERSION TODO) - SPICE simulation

10T RAM Cell

• domino 10T 2R1W
  • domino background
    • clocked logic family (each gate has a clock)
      • clock low, 'precharge' -> node high
      • clock high, 'evaluate' -> node based on input(s), can only go low now
      • clock signal ensures critical path only traverses through cells during 'evaluate'
    • since only switches in one direction, no pmos pullups needed
      • effective transistor load is lower for previous stage
        • smaller circuit for given capacitance, or
        • stronger drive for given capacitance
        • faster since no pmos/nmos contention during switching
    • noninverting logic
      • each domino cell is a single-stage dynamic cell followed by an inverter
      • glitchless

• array
  • in cell, read is bottom of domino (evaluate)
  • subarray is 16x12 cells
  • 'local eval' is top of domino (precharge) and inverter (NAND2 between (2) subarrays)
  • (8) subarrays create 64wx24b 'hard' array
  • surrounding logical array does strobe/addr predecode and addr/data latching; (3) subarrays for 64x72_2R1W
  • DDR adds double in/out latches, early/late select, and DDR-compatible strobe

Local Eval

• precharge
• 2:1 select

• nand2 with precharge pmos pullups on RBL_R, RBL_R
• insert the pullups between p/n

Precharge Pullup Only

• magic

  • load sky130_fd_sc_hd__inv_1 (open technology mgr)
  • gds in console
  • now have sky130_fd_sc__hd__inv_1.gds

• klayout sky130_fd_sc__hd__inv_1.gds

  • hack
  • check with magic -dnull -noconsole -rcfile magicdrc_pullup.tcl toysram_local_pullup.gds while hacking
  • drc clean with nfet side mostly removed; can't remove layer 81 (get drc); because i haven't removed bottom of cell yet (gnd, etc)?

• now stack these with 12x2 nands to make local_eval_stack

Tools

cd xschem
export PDK_ROOT=/data/projects/open_pdks/sky130
# yikes! nfet references through current dir
ln -s $PDK_ROOT/sky130B/libs.tech/xschem/sky130_fd_pr sky130_fd_pr
xschem --rcfile $PDK_ROOT/sky130B/libs.tech/xschem/xschemrc 10T_toy_xschem.sch

xschem --rcfile $PDK_ROOT/sky130B/libs.tech/xschem/xschemrc local_eval.sch

cd magic
export PDK_ROOT=/data/projects/open_pdks/sky130
magic -rcfile .magic_tech/.magicrc 10T_toy_magic.mag
console:
'gds' creates gds
'lef writeall' creates lef


magic -rcfile .magic_tech/.magicrc local_eval.mag
# use options->cell mgr to select cell; then can see layout (top is (UNNAMED) at start)
# windows->set editable
# box upper pmos section and 'a' (select all)
# 'move up 50'

Setup

A template .magicrc file can be found in magic/.magic_tech. This config file works with the SKY130 technology. Changes will need to be made to the .magicrc file so that PDK_ROOT points to this repo. All the relevant Magic tech files are also found in magic/.magic_tech.

The SKY130 model files are found in sky130A/ which are used when simulating a SPICE file in Ngspice.

A tcl script to generate SPICE from Magic layout can be found in magic/magic_spice_extract.tcl. A tcl script to run LVS can be found in lvs/runlvs_single.sh.