TailGator

Note

• If you are a student who has requested to continue this project as part of senior design for MIL, please fork this repository for your project and update the submodules link inside of the MIL Electrical Team SubjuGator 9 repository.
• Feel free to reach out to the project's hardware designer (Yovany Molina) for any questions you might have.
• Assembled boards and spare PCBs and parts may still be in the lab.
• Good Luck!

Introduction

The TailGator Interconnect System (TGIS) project provides a modular connection system and standard for easily connecting, replacing, and upgrading underwater robotics hardware in space-constrained robotics designs. Several other standards exist for modular electronics hardware, including:

• Adafruit Feather Specification
• Sparkfun MicroMod Specification
• Arduino Shield Specificaton
• PCI-e Specification

However, no single standard contains the pin density, physical dimensions, signal integrity, power delivery, and cost effectiveness we are looking for. The TailGator Interconnect System aims to solve this problem for the Machine Intelligence Laboratories' SubjuGator 9 Autonomous Underwater Vehicle platform while making the design open source to help the robotics community.

Our production release demonstration can be found here: https://youtu.be/vW_x0FsKn2Q

Electrical Specifications

The current design of TGIS can supply:

• 3.3V @ 4A (13.3W)
• 5V @ 4A (20W)

Reserved lines may be used to supplement the 3.3V or 5V lines. Board designs may also include a 5V to 3.3V LDO regulator to suppliment the 3.3V line on a specific board in the system.

Pinout

Hardware

Currently, the project contains Altium schematics and layouts for three boards used for power, communications, and alpha-build testing. These can be found in the PCB/TailGator Interconnect System folder:

• Power Input Board (TGIS Main) V1: Accepts power for the 3.3V and 5V rails used for TGIS boards while regulating it against reverse polarity and overvoltage. Also offers overcurrent protection.
• USB to CAN Board (TGIS USB to CAN) V1: Transcribes USB serial messages into CAN packets that are sent to all connected boards in the TGIS system. This board has inputs for JTAG, external CAN connections using RJ45, and SWD for debugging the board's RP2040 microcontroller.
• Breakout Board (TGIS Breakout) V1: Allows access to the TGIS connector lines using typical 0.1" (2.54mm) headers. This is going to be used for alpha-build testing of the different features currently implemented across all produced boards.

Other folders contain incomplete designs for other planned boards.

Documentation

For those without access to Altium Designer, Altium Designer Viewer, or similar compatible applications, documentation for each produced hardware design is available under the Project Outputs folder for each respective project. Links are also provided here for convienience:

• Power Distribution Board
• USB to CAN Board
• System Status Board
• Termination Board
• Breakout Board
• Handwritten Development Notes taken throughout the project are also available in PDF format.

Libraries

The Altium libraries used for the project have been compiled into an integrated library for use with your own projects involving the TailGator Interconnect System. You may find TGIS.intlib file in PCB/Library/Project Outputs for TGIS.

Simulations

There are LTSpice simulations for some parts of the circuits we designed. Libraries used are included.

• 3V3_Protection.asc is an LTSpice simulation of the 3.3V protection circuits in the Power Input Board. This circuit contains reverse polarity protection, overcurrent protection, and overvoltage protection.
• 5V_Protection.asc is an LTSpice simulation of the 5V protection circuits in the Power Input Board. This circuit contains reverse polarity protection, overcurrent protection, and overvoltage protection.

LTSpice Libraries

The libraries in the simulations folder are used for some of the components in the LTSpice circuits.

• fuse2.lib is a library of fuses compiled by user aurvii based on a fuse model by Helmut Sennewald. It can be found in the Official LTSpice Support Group.
• st_standard_sensitve_scr is a library of sensitive and standard SCRs/Thyristors by STMicroelectronics. It can be found here.
• Zener_DiodesInc.lib is a modified version of this zener diodes library from Diodes Inc with .ends statements moved to new lines to appease LTSpice.

All other models used come standard with LTSpiceXVII.

To install these libraries, you must copy and paste them into your LTSpice subcircuit folder. Depending on your version of LTSpice, on Windows machines, this could be:

• %localappdata%\LTspice\lib\sub
• %userprofile%\Documents\LTSpiceXVII\lib\sub
• %programfiles%\LTC\LTSpiceXVII\lib\sub

or other directories.

Simulation Results

5V_Protection.asc

3V3_Protection.asc

Hardware Test Results

V1

Hardware testing for the power input board was performed using a ODP3122 power supply, DL3021 electric load, and an MSO7014A mixed signal oscilloscope. Most of this equipment was interfaced using python code to send SCPI messages.

The fuse did work in the overvoltage protection case, but activated after a very long amount of time. We will try to use fast acting automotive fuses in future revisions.

Known Issues

• The mezzanine connector used has too small of a pitch to be reliably soldered using reflow. We recommend switching to conan connectors since they have a slightly larger (1.0mm) pitch. 60 pins should still be enough to have plenty of reserved pins for future expansion.
• The BOM has the wrong SWD connector.
• Some parts on the system status board (like the LEDs) require an active USB connection to function. They should instead be powered from the same source as the RP2040 (which is the selectable power source).
• The reset circuit on the system status board does not function correctly.
• The breakout board should be expanded to have plug and receptacle connections like other boards so it can be used in the front or the back.
• The headers to disable I2C on the termination board should be replaced with 0 ohm resistors that are not placed by default.
• The mounting holes should be M2.5 as MIL has switched to that screw size being the standard for all boards.
• The power distribution board has a higher voltage drop across the protection circuitry, meaning that there is still a problem if bypass mode is used without first checking the input voltage. We recommend keeping the crowbar circuit design from V1.
• CAN signals cannot pass through the USB to CAN board, but can pass through all other boards, including the System Status Board. Further investigation is needed.
• JTAG chaining was never implemented since no microcontrollers on designed boards supported JTAG. They may need buffers between boards for proper operation.
• Some JTAG signals (TRACECLK, TRD*, VREF, GNDDetect, JnSRST) may be omitted from the connector or otherwise tied to a constant value.

Software

Our work includes the design of firmware for the TGIS's system status board, which can display diagnostic information about the system.

For the production release, the firmware has been fully verified to support all major components on the system status board PCB:

• OLED display, prototyped on the Adafruit OLED Feather and communicating via I2C;
• LIS3DH IMU (tested previously in the Alpha Build);
• Micro SD card reader, using 4-pin SPI to communicate;
• Leak alarm, a piezo buzzer activated via GPIO; and
• Leak detector, prototyped via a push button connected to an input pin on the Feather RP2040.

All of the above components have been updated to work concurrently on embedded-hal version 1.0.

We are using the RTIC Framework to provide concurrent sensor access and display interfacing. This app can be seen in the RTIC_App directory of this repo.

Installation

Prerequisites:

• Working Rust toolchain for embedded development on the RP2040 (e.g., capable of running the Embedded Rust lab code)
• Adafruit Feather RP2040
• All of the above major components (LIS3DH, OLED, SD card reader) and connectivity to the feather (jumper cables and/or Feather boards)
• (Note: for testing the CAN demonstration, a pair of RP2040 devices each attached to a CAN transceiver via two GPIO pins for the Rx and Tx lines is required. One device must be flashed with the transmitter code--CAN_Demo/CAN_Transmsit--and the other with the receiver code--CAN_Demo/CAN_Receive. The transmitter should have a digital input component such as a button attached to the appropriate GPIO pin, and the receiver must be attached to a computer via USB where minicom shoud be run to inspect serial output at baud rate 115200.)

Clone this repository: https://github.com/yomole/TailGator.git, then open the app (RTIC_App) and inspect the code.

From within the app, run cargo run from within the RTIC_App directory with your Feather RP2040 connected to a debug probe over SWD. This will automatically flash the software to the Feather and run the program with rich debug output.

Alternatively, in .cargo/config, you can set the runner to be elf2uf2-rs instead of probe-rs by commenting and uncommenting the appropriate lines. cargo run will then work with a Feather RP2040 set to receive a UF2 file, but of course you will need a debug probe to see the output.

Known Issues

1. CAN bus communication has not yet been integrated with the system status board firmware due to a need for more up-to-date CAN drivers.
2. SWD port on the system status board is nonfunctional, and so debug output can only be demonstrated on the testbench via a Feather RP2040 and appropriately wired components. This may require reconfiguring GPIO pin selection for the devices under test.

Time Tracking

Google Sheet

Ackowledgements

This project is made in collaboration with:

• Machine Intelligence Laboratory

Thanks to Carsten for supporting our switch to the Rust RTIC framework and for providing a secondary Adafruit Feather RP2040 to use as an SWD debugger, as well as an OLED display and associated connector cable.