Layout

The open source tools don’t have any automatic analog layout. To my knowledge, there is no general purpose analog automagic layout anywhere in the world. It’s an unsolved problem. Many have tried (including myself), but none have succeeded with a generic analog layout engine.

There are a few things, though, that could help you on the way.

Setup

I assume that you have the latest and greatest aicex\ip setup.

See SKY130NM Tutorial if aicex is unfamiliar.

Let’s assume we use jnw_gr05_sky130a to test out our layout

cd aicex/ip/
cd jnw_gr05_sky130a
git checkout a1e3dfc324194729e042f5e653777b052759863b
cd work

CICPY

The first thing we need to do is to place all transistors. I do have a script to help. Install cicpy.

cd aicex/ip/cicpy
git checkout master
git pull 
python3 -m pip install -e .
cd ..
cd cicspi 
git checkout main 
git pull
python3 -m pip install -e .

Placement

To generate an initial placement we can do the command below. If a layout exists it will be overridden

cd jnw_gr05_sky130a/work 
cicpy sch2mag JNW_GR05_SKY130A OTA_Manuel

_{Figure 1: Initial cicpy sch2mag placement of OTA_Manuel in Magic - all transistors in one row, 19 DRC errors}

The layout engine has no idea what components belong together, for example, the current mirror below should have been placed together

_{Figure 2: Xschem schematic of the OTA bias circuitry, where the current mirror pair xa07/xa20 should have been placed together}

We can instruct the layout engine by adding a “group” name to the instance name. The instance name always starts with x<something><number> where the something can be nothing, or a group name (a,b, not a number).

The rules for placement are:

Sort all instances by groups
Sort all groups by instance name
Place the first instance.
For all instances: If the next instance has the same group, then add on top. Otherwise increment the x location.

As such, if I rename my instances, as shown below,

_{Figure 3: The same OTA schematic after renaming instances with group prefixes (xa, xb, xd, xf, xg) to guide the placer}

Then the layout becomes a bit better

cicpy sch2mag JNW_GR05_SKY130A OTA_Manuel --gbreak 3 --xspace 34000 --yspace 30000

The gbreak command inserts a “group break” after the fourth group, such that a new Y coordinate is selected.

The X and Y space is for the distance between groups. The unit is “Ångstrøm”, so 1 um is 10 000 Å.

_{Figure 4: Placement after grouping, shown as instance boxes: devices of the same group stack vertically, and –gbreak 3 starts a new row}

_{Figure 5: The same grouped placement with all layers drawn, DRC clean}

Summary

The one-page version of this chapter:

Layout is the final translation: from schematic to the polygons the fab will etch
Matching is geometry: symmetry, proximity, dummies, and common centroid where it counts
The parasitics are part of the circuit - extract and re-simulate before believing any layout
DRC and LVS are not suggestions: clean both, every time

Would you like to know more?

Hastings, The Art of Analog Layout - the book on matching, and worth owning

The Magic and netgen documentation, which is the practical companion for the flow used here