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Add xschem write-up draft
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‎README.md

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# learn-sky130
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Learning to do things with the Skywater 130nm process
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Learning to do things with the Skywater 130nm process.
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* [Designing and simulating an inverter schematic using xschem and ngspice](schematic/xschem/getting-started.md)
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‎schematic/xschem/getting-started.md

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# Getting started with xschem with sky130
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Let's build an inverter using xscheme in the
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[Skywater 130nm process](https://skywater-pdk.readthedocs.io).
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## Background
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You are working for Inverter-Tech Inc. which specializes in taking
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something and doing the opposite. So far the company has made a living
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by producing batteries that have their polarities inverted. Sales have been
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going down, and they now want to break into the digital era. All the rage
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in the year 2020 is digital inverters. Taking a 0 and turning it into a 1 seems
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to be what all the kids are talking about.
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You have been tasked to use the newly open-source'd process called
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[sky130](https://skywater-pdk.readthedocs.io) to design the company's new
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crown jewel - the Digital(TM) Inverter(R).
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### What is Schematic Capture and why do we need it?
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It is the process where you *capture* an idea as a *schematic*. It is a term
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used to describe producing a schematic which can be used to describe some
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electronic circuit. When simulating the component or later designing the
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full chip you will use the output of the schematic to verify that the final
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result is what your schematic describes. This output is called a *netlist*.
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Alternativly you can also design your component in a
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Hardware Description Language (HDL) such as Verilog or VHDL. This can the
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be used to automatically produce the netlist. It is a very good way to make
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large digital designs, but for analog designs it quickly becomes complicated.
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In order to design our inverter, we will have to describe it as an electronic
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circuit in some form of schematic editor. Here we will use xschem for this
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purpose. We could have described it in HDL as explained earlier, but the goal
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of this tutorial is to learn how to manually design an inverter using analog
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components.
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### What is a process?
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A process commonly refers to a fabrication process for which chips are made.
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Skywater 130 nm is the name of the process we use here, which is usually
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shortened to `sky130`. There are hoards of other processes out there but what
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makes sky130 special is that it is open-source and does not require signing of
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an non-disclosure agreement (NDA) in order for it to be used.
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Every process has so called *primitives* that are the basic building blocks
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we can use to define a chip in that process. It is crucial that we use those
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blocks and when we simulate that we use *models* that describe how those blocks
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work in an environment that is as close to what we will have our chip running in
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as possible.
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The collection of files needed to use a process is called a Process Design Kit
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or *PDK*. In our case the PDK we will use is `sky130_fd_pr` which stands for
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Skywater 130nm Foundry Primitives.
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## Installation
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NOTE: The following assumes Ubuntu 20.04. For other Linux distributions the
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steps might differ somewhat. Versions might have been updated since the writing
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of this guide and you might need to adopt some steps accordingly. Fixes are
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very much welcomed!
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The tools we will install are:
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* xscheme - The schematic capture tool
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* ngspice - The circuit simulator
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* gaw - A waveform viewer to view the simulation results
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### Installation of ngspice
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A decently recent version of ngsice is present in the Ubuntu repository so
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installing it as easy as:
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```
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sudo apt install ngspice
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```
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### Installation of gaw
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Gaw is not packaged for Ubuntu so we will have to build it ourselves.
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**TODO**: Write this in a nicer way, this is just a dump right now
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```
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wget http://download.tuxfamily.org/gaw/download/gaw3-20200922.tar.gz
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sudo apt install libgtk-3-dev
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make -j$(nproc)
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sudo make install
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# Important!
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# Start gaw once and exit it
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gaw
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# Now change the rcfile
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$EDITOR ~/.gaw/gawrc
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# Find up_listenPort = 0 and set it to 2020.
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# This is needed for xschem to integrate well with gaw
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```
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Optional: `gaw examples/rlc_lpf_trans.dat` to figure out how the tool works.
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### Installation of xschem
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**TODO**: Write this in a nicer way, this is just a dump right now
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```
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sudo apt build-dep xschem # todo: expand to what is actually needed
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sudo apt install xterm graphicsmagick ghostscript
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# GraphicsMagick is needed for PNG export, ghostscript for pdf
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git clone https://github.com/StefanSchippers/xschem.git
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./configure
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make -j$(nproc)
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sudo make install
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```
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1) Go into Simulation -> Configure simulators and tools.
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1) Make sure "Ngpsice Batch"[1] and Gaw is selected.
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**Tip**: I like ngspice batch to be the following which allows you to see the status:
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`$terminal -e 'set -o pipefail; (ngspice -b -r "$n.raw" "$N" | tee "$n.out") || (echo -e "\n** ngspice exited with error code $? **\nPress enter to close"; read)'`
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1) Keep Fg checked and Status unchecked.
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1) Press "Accept and close"
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[1] Ngspice batch is needed for the raw-file to be created which Gaw uses to populate its table.
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### Installation of sky130 primitives
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**TODO**: Describe why we are doing this maybe.
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```
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cd /usr/local/share/
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sudo mkdir sky130_fd_pr
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chown $USER sky130_fd_pr
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git clone https://foss-eda-tools.googlesource.com/skywater-pdk/libs/sky130_fd_pr sky130_fd_pr
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```
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## Basics of xschem
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These two videos of using xschem are well worth the time. Watch them and follow
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along.
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* [Editing and Simulation in xschem](https://xschem.sourceforge.io/stefan/xschem_man/video_tutorials/editing_and_sim.mp4)
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* [Probing signals with gaw](https://xschem.sourceforge.io/stefan/xschem_man/video_tutorials/probe_to_gaw.mp4)
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Congrats! You simulated a schematic using open-source tools!
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## Designing an inverter
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TODO: Describe the process
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**NOTE**: To use the pmos4/nmos4 components, you should use these parameters:
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```
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model=sky130_fd_pr__pfet_01v8 w=1 l=1 m=1 spiceprefix=X
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```
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The `spiceprefix=X` is needed because the FET is defined as a so called
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subcircuit and will not be correctly located otherwise. If you get
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an error like `could not find a valid modelname` you might have forgotten to
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add this property.
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The final result is available in `inverter.sch`.
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If you installed the PDK somewhere else make sure to update the `.lib` statement
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in the SPICE component in the lower-right corner.

‎schematic/xschem/inverter.sch

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v {xschem version=2.9.8 file_version=1.2}
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G {}
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K {}
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V {}
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S {}
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E {}
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N 150 -600 560 -600 { lab=vcc}
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N 150 -340 420 -340 { lab=GND}
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N 560 -360 560 -340 { lab=GND}
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N 420 -340 560 -340 { lab=GND}
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N 560 -550 560 -540 { lab=vcc}
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N 560 -510 580 -510 { lab=vcc}
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N 560 -600 560 -550 { lab=vcc}
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N 560 -460 640 -460 { lab=inv_out}
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N 420 -360 420 -340 { lab=GND}
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N 420 -460 520 -460 { lab=in}
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N 420 -460 420 -420 { lab=in}
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N 560 -370 560 -360 { lab=GND}
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N 560 -480 560 -460 { lab=inv_out}
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N 520 -510 520 -460 { lab=in}
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N 520 -460 520 -410 { lab=in}
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N 560 -460 560 -440 { lab=inv_out}
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N 580 -560 580 -510 { lab=vcc}
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N 560 -560 580 -560 { lab=vcc}
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N 560 -410 580 -410 { lab=GND}
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N 580 -410 580 -360 { lab=GND}
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N 560 -360 580 -360 { lab=GND}
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N 560 -380 560 -370 { lab=GND}
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N 150 -600 150 -490 { lab=vcc}
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N 150 -430 150 -340 { lab=GND}
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C {vsource.sym} 420 -390 0 1 {name=V.100MHz value="pulse(0 1.8 1ns 1ns 1ns 5ns 10ns)"}
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C {code_shown.sym} 680 -170 0 0 {name=SPICE only_toplevel=false value=".lib /usr/local/share/sky130_fd_pr/models/sky130.lib.spice tt
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.tran 0.1n 1u
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.save all"}
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C {lab_pin.sym} 150 -570 0 0 {name=lsupply sig_type=std_logic lab=vcc}
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C {title.sym} 170 -50 0 0 {name=l5 author="Christian 'bluecmd' Svensson"}
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C {pmos4.sym} 540 -510 0 0 {name=M1 model=sky130_fd_pr__pfet_01v8 w=1 l=1 spiceprefix=X m=1}
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C {nmos4.sym} 540 -410 0 0 {name=M2 model=sky130_fd_pr__nfet_01v8 w=1 l=1 spiceprefix=X m=1}
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C {gnd.sym} 240 -340 0 0 {name=l1 lab=GND}
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C {vsource.sym} 150 -460 0 1 {name=V1.8 value="pwl 0 1.8"}
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C {lab_pin.sym} 420 -450 0 0 {name=l3 sig_type=std_logic lab=in}
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C {lab_pin.sym} 640 -460 2 0 {name=l4 sig_type=std_logic lab=inv_out}

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