This project ports an automatic analog layout generation tool called aloe to work with SkyWater 130 nm technology. Aloe uses a digital place and route tool together with a genetic algorithm to meet different analog layout specifications. Using this tool, we generated several layouts for a bandgap voltage reference (BGR) circuit on this chip, and we compared these with a manually laid out version from the previous offering of this course.

Table of Contents

• Part A - BGR Circuit Understanding
• Part B - Analog Standard Cell (Stem Cell) Generation and Examples

Part A - BGR Circuit Understanding

A1. Current vs Voltage relationship of diode

$I =I_S \cdot\left(e^{\frac{q \cdot V_f}{k \cdot T}}-1\right) \cong I_S \cdot e^{\frac{q \cdot V_f}{k \cdot T}} \mid V_f \gg \frac{k \cdot T}{q}$

$V_f = V_T \cdot \ln\left(\frac{I}{I_S}\right)$

A2. Get to know Bandgap Reference (BGR) Voltage Circuit

Abstraction:

• We want a voltage generation circuit, which is independent of temperature.
• Idea 1: target_voltage = PTAT_voltage + CTAT_voltage (PTAT: proportional to absolute temperature; CTAT: conversely proportional to absolute temperature)
  Idea 2: target_voltage = R * (PTAT_current + CTAT_current)
  Here we use Idea 2.

A3. Simplified BGR Circuit Schematic (comparator abstraction)

Conditions:
(1) Va = Vb
(2) Sizes of PMOS P1, P2, P3 are the same
(3) R1 = R2

Then:
(1) $\mathrm{I}1 = \mathrm{I}2 = \mathrm{I}3$ (current mirror banks)
(2) $\mathrm{I}1\mathrm{b} = \mathrm{I}2\mathrm{b} $
(3) $\mathrm{I}1\mathrm{a} = \mathrm{I}2\mathrm{a} $
(4) $-\mathrm{Vb = V_{f2} + dV_f = V_{f1} = -{Va}} $
(5) $\mathrm{d V_f=V_{f 1}-V_{f 2}=V_T \cdot \ln\left(\frac{I}{I_s}\right)-V_T \cdot \ln\left(\frac{I}{N \cdot I_s}\right)=V_T \cdot \ln (N)}$
(6) $\mathrm{I2a =dV_f/R3} $
(7) $\mathrm{I2b = V_{f1}/R2} $
(8) $\mathrm{I3 = I2 = I2a+ I2b}$
(9) $\mathrm{V_{ref}=R4 \cdot I3=R4 \cdot\left(\frac{V_{f1}}{R 2}+\frac{d V_f}{R 3}\right)=R4 \cdot(I2a+I2b)}$, where $\mathrm{I2a}$ is PTAT current and $\mathrm{I2b}$ is CTAT current.

Part B - Running codes

$ git clone https://this_repo_link.git 
$ cd Open-ALOE
$ source setup.csh  

Step 1 - Build Stem Cell Library

$ cd ./stem_cell_generator 
$ sh run_generator.sh  

Step 2 - Run PnR Optimization

Play around toy examples (skywater130-analog-flow using mflowgen)

inverter
ring oscillator

Reference

For the BGR part, we referred bandgapReferenceCircuit. In the layout part, some of the tcl codes referred open-pmic.