Table of Contents

Background

BeagleBone PRUSS

The BeagleBone Black is a low-cost, high-performance embedded platform for hobbyists and other uses. It is based upon a TI AM335x 1GHz ARM® Cortex-A8. Features (from the product website) include:

Processor: AM335x 1GHz ARM® Cortex-A8

Connectivity

The board comes pre-loaded with and Ångström Linux distirubtion and many other options can be considered for development and operating the device. It is a very powerful device with a large number of peripherals built in making it well-suited for a number of soft real-time applications.

To support demanding hard-real time constraints or to off load processes from the main CPU, the processor chip includes two programmable real-time (PRU) cores. These are reasonably high speed, 32-bit RISC cores where every instruction takes a deterministic 5 ns to complete. The devices have dedicated peripherals, access to the main system's peripherals, and ability to communicate with the main processor through interrupts, shared memory or DMA.

These devices can be used to create programmable real-time peripherals and subsystems. The downside is that they are specialized and need low-level (assembly) programming (at least until someone develops a compiler) and configuration.

Goals

The goal is to experiment with the PRU devices and modify a small program to send a burst of bits at a given rate. The ARM processor should query the user for a 32-bit hexadecimal number and transmit it out serially on a GPIO pin LSB first. The period of transmission for each bit in microseconds should also be sent by the ARM to the PRU. A small program based on the TI examples has been provided to show two methods of sharing memory with the PRU and ARM as well as for using the PRU timer (you can also rely on the 5 ns instruction timing but then become more sensitive to code changes).

Not the TI examples are not production quality code but samples for demonstration.

Prelab

Familiarize yourself with the sample code and with the PRU assembly language. We will not be writing much code so you do not need to become expert but should be able to read the sample program and make modest changes.

Plan modifications to accomplish the goals above and prepare your code for the lab.

Procedure

  1. Log on to the laboratory workstation
  2. Using antistatic precautions, unpack the BBB. Plug in the USB connection (under the 4 LEDs by the power connector) to the BBB and the PC. This will provide power as well.
  3. The BBB should be visible as a flash drive. Double click on Start.html
  4. MORE to be added here: terminal, scp, Bonescript to shut off LEDs
  5. Copy (scp) the example files to the BBB (the zip file can either be expanded on the PC or BBB). These files should be placed in the myapp directory under the example_apps directory (~/Desktop/am335x_pru_package-master/pru_sw/app_loader/example_apps) Note to tech: should set up a proper user and so on for these boards
  6. cd to the example_apps directory
  7. Edit the DIRS file to add the source and paths to the files in myapp. This will allow make to find and move the files appropriately
  8. run make to make the sample apps and the new app
  9. cd bin
  10. ./pru_ex to run the program
  11. Repeat steps 5 - 10 for your version of the program to build and debug your program Note to tech: would be good to get a debugger
  12. When your program is working demonstrate it to the TA
  13. Determine the maximum frequency (minimum bit period) that you can reliably send the data and confirm with an oscilloscope.
  14. How could you improve the performance? How could the ARM/PRU programs be altered to send the data whenever a new signal was required (do not implement)?

Submission and Evaluation

As well as you demonstration you should submit a brief lab report. For the lab report submit your code, documenting the changes, and the answers to the questions above using the prism submit command.

Resources