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Reference Design For A GPIO-based Parallel Bi-Directional Bus

Student: Pranav Kumar
Mentors: Andrew Henderson, Zubeen Tolani


Create a hardware/software design will incorporate shift registers (and potentially logic level converter solutions as appropriate) to allow hardware to communicate with hardware via a parallel, bi-directional bus.

Tasks Done

I have completed the task required as described on the ideas page, and created a pull request, as listed here

About Me

IRC: pranav_kumar
E-Linux Username: pranav083
School: UIET,Panjab University,Chandigarh
Country: India
Primary language: English,Hindi
Typical work hours: 9:30 - 23:00 IST GMT/EST/PST to Ist Adjusted Time
Previous GSoC participation: This is my first time participation in gsoc .I got intrested for it by seeing the vast amount of development for open source hardware around the world .And i also want to become a part of it.
Skills: C,Python,ROS,OpenCV,Embeddded C,Electronics Prototying,circuit designing

Tools(proficient) : Git, Linux

Experience : C, Python, Java, HTML, CSS, JavaScript, Arduino

Hardware Skills: Raspberry Pi, ICs, Circuit Designing, Atmel chip, I2C, circuit debugging

Reference Design For A GPIO-based Parallel Bi-Directional Bus

Project name: Reference Design For A GPIO-based Parallel Bi-Directional Bus


swift register 74hc299

In digital circuits, a shift register is a cascade of flip flops, sharing the same clock, in which the output of each flip-flop is connected to the 'data' input of the next flip-flop in the chain, resulting in a circuit that shifts by one position the 'bit array' stored in it, 'shifting in' the data present at its input and 'shifting out' the last bit in the array, at each transition of the clock input.

This is 74hc299 which is 8-bit universal shift register figure on the right side of the page .

As many projects require a large number of input/output pins to make the project interfacing other the other microcontroller it's having a huge requirement GPIO(e.g game controller etc.). As pocket beagle is small yet very power packed low-power board in which the GPIO are pinmux with the other peripherals and having many functionalities on the same pin like (SPI, USART), that led to the shortage of pins on the board.

As it can be seen that GPIO having simple input /output (without pinmux) are very less in number which makes it really hard to use pocket beagle to use in projects where their large requirement of I/O pins. Buts as it has two powerful PRU of with a 32‐bit RISC processor at 200MHz with single-­cycle pin access for hard realme. So it makes it very easy for implementing multiple inputs and outputs control at high speed through a few GPIO pins.

gpio without pinmux

Pins are shown above that have a boundary of yellow rectangle shows the no. of pin available in pocket beagle that is without pinmux with them.

Project Overview

the BeagleBone family has a large number of pins available on their P8/P9 headers, projects requiring a large amount of GPIO I/O may still be infeasible due to pinmux conflicts with other interfaces (SPI, UARTs, etc.). The newer PocketBeagle platform is even more restricted in the number of available GPIOs for interfacing. In addition, bi-directional bus communication using GPIOs is infeasible, as each GPIO is configured for either input or output. This project will combine your hardware and software skills to address these limitations by creating a shift register-based hardware design that will allow serial streams of data going in/out of the PocketBeagle or BeagleBone family GPIOs to be mapped into a bi-directional communications bus. Some practical uses of this are for GPIO-based game controller inputs (using *lots of buttons*), communication address/data buses for interfacing with older microcontrollers and microprocessors, and providing large numbers of logical bi-directional GPIOs.

Sometimes a project needs to have multiple inputs /outputs just for example interfacing LCD Display ,Interfacing a multiple button controller like game controller, there is need for interfacing external memory like (eg. 8kx8 SRAM older low memory version or 28f400 Flash Memory ) in which their needs to be need of multiple data line to be there in a controller to use it.

Detailed Description

Progress till now

  • First of all, I studied the internal mapping of the PRU in the pocket Beagle and find out the usable GPIO in the pocket beagle that will be suitable for this project.
GPIO used in this project
  • Then, I was able to interface swift register 74hc595 with the pocketBeagle and control the series of 8 LED with it by writing C code in the user space.
  ** Code for controlling the shift register.
  ** YouTube video of the working code uploaded.
  • As the 8-bit universal shift register stated was not available to me when I started a search for another swift register that was 74hc299 and in the meantime, I started to study this shift register and make the circuit for the interfacing of the different module that can be made by using pocket beagle, logic level converter,74hc299. Here is the link of the different file GitHub.
  • see the general configurable [circuit] that can be used in this project.
  • Added a GitHub repository for the project as there was no repository made for it.
  • studied about kernel module, device driver development and little about device tree execute small for example code for it.

[hendersa]: Mention your existing C code here and link to the GitHub project for it. For additional preliminary work, consider adding a few functions in your C code to set the read/write mode of the bus and to read/write data to/from the bus. This way, you can show that you've thought through the entire bus interfacing process and have a preliminary prototype that uses the (very slow) filesystem I/O interface. The logic in this prototype can then be used as a blueprint to build a more complex code. This will make your proposal much stronger.


Community Bonding Period

  • Studying and getting further in-depth knowledge of the PRU and knowing the further use cases of the project through the mentor and another community member.
  • Discussing the type of coding standard to be followed in the code with the mentors
  • If any changes are suggested making changes in according to that
  • Discussing the guideline followed for documentation
  • knowing more about the device tree and device driver development.
  • Studying the Datasheet so that during the implementation period it makes easier to apply things in a more smooth way.
Week 1 : Milestone #1
  • Continue my working on the swift register 74hc299 interfacing with the pocket beagle will work on further improving the code for different use cases
  • Testing it with different input and output modes on different GPIOs and able to interface different functionality in an efficient way
  • Check for signal accuracy and any delay if occurring which occurring while running different test cases using CRO and logic analyzer.
Week 2 : Milestone #2
  • working on the communication protocol for that can be done through the gpio understating the delay in the signal if arises due to cascading of the shift register.
  • In the meantime check for any loss which is occurring in this circuit
  • Testing and Debugging the circuit making circuits for the by implementing it with different input and output with different programs and making the design of the circuit on PCB as an addon for pocketbeagle.
Week 3 : Milestone #3
  • As I will get be known to most aspect of the hardware and PRU for now so it will be easier to exploit more from it, to move toward the development of kernel drivers
  • using the interrupt handler for the more efficient functioning of gpio of it.
  • Understanding and applying the device tree and kernel driver development.
  • developing Linux kernel module for the kernel userspace
Week 4 : Milestone #4
  • Continue working on the kernel driver development
  • testing and debugging if any problem is caused in between the development
  • expanding device driver capability to communicate taking input or output at different baud rates
  • Preparing slides of the completed work and check for any bugs in the driver.

by this time the time for Phase I evaluation: I will deliver most kernel driver, circuits, effects of the shift register on the different GPIOs of the pocket beagle

Week 5 : Milestone #5
  • Try to fix any bug if it is there in the kernel driver and if something proposed during evaluation.
  • Till now will start to developing library in c for the userspace of the Linux.
  • In the meantime checks the effect of the generated library with the beagle board and the shift registers 74hc299.
Week 6 : Milestone #6
  • continue the work on the development of userspace library.
  • Studying the response time from the controller and
  • testing and writing code for it as with the
  • Testing the communication and synchronization with other peripherals and logic analyzer for 8-bit, 16-bit and 32-bit for now.
Week 7 : Milestone #7
  • Testing delay in the signal using a logic analyzer and improving the quality of the signal from the made library.
  • Debugging the signal and working on the interface of any memory
  • trying to figure out communication with different baud rate that are use for communication
Week 8 : Milestone #8
  • making an easy to use library in c for different popular shift register available in the market that are of the same type.
  • Fixing bugs and adding any wanted material for the library if it left during the development period.
  • Documenting the work done for now and preparing slides of the completed work.

By this time the time for Phase II evaluation: I will be able to deliver library for the userspace.

Week 9 : Milestone #9
  • fixing bugs error and making support and making any changes as suggested.
  • making or implementing the library made a with a game controller pocket beagle.
  • creating and modifying the code so that its usability can increase.
Week 10 : Milestone #10
  • Making some ready to use library support for different general project applications
  • Documentation of the work for using the library and with different projects with the pocket beagle.
  • its time to test these on real hardware support.
Week 11 : Milestone #11
  • making the first project that will be a game controller using the pocket beagle.
  • Documentation of the work done for the project
  • Making an easy DIY guide for this on the beaglebone project page.
Week 12 : Milestone #12
  • Interfacing a simple small memory unit with the Shift register and able it use its memory for R/W
  • Documenting the work and making stuff ready for the project
Week 13 : Milestone #13
  • Making the final circuit diagram for the project and a small PCB layout for the Pocket Beagle and incorporating shift register on it
  • Ask the community member to review the project and give feedback on it.
Week 14 : Milestone #14
  • Making changes according to the Feedback on the project .
  • preparing final slide and video of the work for documentation.

Final Goals:

Successfully completing the project and creating a library for it and making a whole new open source code and DIY guides by which it help the community further to explore the Pocket Beagle.


As I am from Electronics background and always involved in creating in making projects related to micro-controller.I like to learn new things in my day to day life. I have made a 3D printer that works both as a 2D printer and 3D printer using the hardware modules of a 3d printer and Open Source Marlin Firmware. Worked on finite state machines using an embedded system. And I take sessions in our college to spread knowledge among ourself with common interests. I love to contribute to open source where people can help each and clearing one another problems.

Previous Contributions to Open-Source

  • Making a lighter and less complex multifunctional line Following algorithm using FSM that is easy to implement and build.
  • Lead the team of NGO making projects for school going children with full DIY guide and Documentation Github.
  • Making a short script for login into our university wifi Github Link.
  • Take the initiative to have an Open-Source Community section of India on OSRF official ROS-Discourse so that people can openly know to each other.

Contributions to this project

There was no upstream repository for this project. So, I make one and started contributing to it by starting from the interfacing shift register github. And for this project, I have created another repository on GitHub
All the future update for the project will get updated in the GitHub link listed above.


  • Participated in Hacktober fest 2018.
  • Suggesting an efficient prototype of Harvester at eYIC, IIT Bombay(Country level competition) Docs.
  • Won PocketBeagle from Mouser electronics event at IIT Roorkee, India.


As I will be on my break period from my college after semesters exam. So, I can devote my full time on this project and there will be enough time for me to learn, explore and implement new things and successfully able to complete this project on time.


It will largely impact the way people who want multiple inputs or output capability in their projects or those who want to interface different memory device with the pocket beagle. it also opens new capability by open an entire world of a new project that can get incorporated with the help of pocket beagle for ex. making a scientific calculator with the help of it, etc.

What community members speak

After making this project for the community, I am looking forward to working more with the hardware-software integrated project and want to help pocketbeagle to made easy to use in projects for newbies.

Sure, that would be very useful. You'll have to think about how you'd do that in the PRU firmware.
                                            -Andrew Henderson(hendersa)

If your mentor isn’t around?

First of all, before asking any question in with the mentor I would like to know the problem personally through google it and if then I am not able to find the answer then ask it the IRC channel and there is always likely to be someone in the community that can issue the problem in case if mentor is not around their.

Future Contributions

As always I will like to contribute further in the open source community and spread my word to my local community by spreading the easy of making project using beaglebone family and its advantages over other controllers, And the way people help each other especially.