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19 Nov 2023

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sky130 analog simulation with xschem

Based on Stefan's Jan 2023 video "Analog simulation with xschem and the skywater 130nm Process Development Kit (PDK)"...

See also: Xschem tutorials: Lots of videos, etc.

I already have:

$ ngspice --version
ngspice compiled from ngspice revision 31

$ xschem --version
XSCHEM V3.4.5

$ magic --version
8.3.331

$ ls -l ~/.xschem/
-rw-rw-r-- 1 zerotoasic zerotoasic 22187 Nov 18 23:58 xschemrc

Steps:

  1. Install xterm and gvim: 
    sudo apt install vim-gtk3 xterm
    The default terminal and editor can be specified in ~/anton/sky130_example/xschemrc
  2. Per this: 
    mkdir ~/.xschem/simulations
cat >>~/.xschem/simulations/.spiceinit <<EOH
set ngbehavior=hsa
set ng_nomodcheck
EOH

    This file sets some default behavior for reading .lib files and speeds up loading pdk model files.

  3. Go into ~/anton/sky130_example and run xschem
  4. From the welcome page, select test_inv and press e on the keyboard to go into it.
  5. Click the 'Netlist' button in the top-right corner (or just press the n key). Result: 
    $ ls -l ~/.xschem/simulations/*.spice
-rw-rw-r-- 1 zerotoasic zerotoasic 3308 Nov 19 21:27 /home/zerotoasic/.xschem/simulations/test_inv.spice
  6. Go to Simulation => Configure simulators and tools:
    • For spice ensure Ngspice interactive is selected.
    • For spicewave ensure Gaw viewer is selected.
  7. Exit the config and click Simulate button in top-right corner. This should pop up a window running ngspice for a while.
  8. Data is shown in the terminal. Ctrl-click the button under the graph area in xschem to load and graph the results: ngspice simulation results for ring oscillator example

Next steps

  • Create hierarchical design:
    • Voltage divider that ranges from 0.0V to 0.7V from 1.8V source: High-side resistance is 61.111%, low-side is 38.889% divided into (say) 8 steps, with 0.0V being the lowest.
      • Example using 0.7um-wide resistors, and trying to cap to 70uA (i.e. 100uA per 1um width):
        • 1.8/7e-5 => 25.714k-ohm total resistance.
        • High side resistance therefore is: 15714k-ohm; at 48.2 ohms per square, we need 326 squares. At 0.7um width, this is 228.21um length.
        • Low-side total is: 10k-ohm
        • Divide the low side by 7 resistors (~1429-ohm each): 29.647 squares each, or 20.753um
    • Tap transistors with common output.
  • Watch other videos from 0173

Notes