ECE497 BeagleBone PRU

Revision as of 09:11, 25 October 2012 by Millerap (talk | contribs) (How it Works)
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thumb‎ Embedded Linux Class by Mark A. Yoder

Team members: Mark A. Yoder, Bryan Correll, Andrew Miller, Peter Ngo, James Popenhagen

Executive Summary

Give two sentence intro to the project.

Give two sentences telling what works.

Give two sentences telling what isn't working.

End with a two sentence conclusion.

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Installation Instructions

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User Instructions

Always run the following before doing anything with the PRU:

beagle$ modprobe uio_pruss

Building and Running the GPIO_PWM_PRU Example

This example is located in the GPIO_PWM_PRU directory in the AM335x_PRU_BeagleBone git repository, and can be pulled with the following:

git clone git://

This example uses the gpio and delay loops to approximate a PWM using the user LEDs on the BeagleBone. It is based on an example provided by Lyren Brown and documented by boxysean at

In GPIO_PWM_PRU all of the complicated Makefiles and directories used to make a multitude of examples at once have been stripped away to allow the user to compile one individual program that will run on the PRU.

The readme.txt file in the GPIO_PWM_PRU directory provides a walkthrough for compiling and running blinker on the BeagleBone.

The first step to compiling a program for the PRU is to make sure prussdrv.c is made and up to date. This is the file provided by TI that contains all of the C functions that allow for communication with the PRU. To do this, do the following:

cd <directory>/AM335x_PRU_BeagleBone/GPIO_PWM_PRU/interface

CROSS_COMPILE is specified as "" because this is running on the BeagleBone itself and the Makefile is setup to defaultly cross compile the code from another linux machine.

Once this is completed, the pasm_source must be set for the BeagleBone's linux operating system:

cd ../utils/pasm_source

Note: The above instructions need to be done for every time the BeagleBone boots up and these directories should be included with any code that you write for the PRU

Now, the BeagleBone is ready to compile the example code. Navigate to the example's root directory again:

cd ../../

This will compile the blinker.c file and output it to the bin folder. After this point, the assembly file needs to be compiled into a .bin file. This is done in the bin folder.

cd bin

Now, there should be a blinker.bin file in the folder. running the blinker executabile will put the blinker.bin file on the PRU and start it running. Use the following:


How it Works

//in the overview talk about the period being 5ns

Registers r5 and r6 are the duty_cycle and period respectively. The duty_cycle is a number smaller than the period that the accumulator r4 counts up to before setting the output to zero. When the r4 = period, r4 resets and the output is set to 1. This gives the following for for OnTime and OffTime.

SecondsPerCycle = 5*10^-9
OnCycles = 2 + (duty_cycle)*3 + 2
OffCycles = 2 + (period - duty_cycle)*3 - 1 + 2
TotalCycles = 7 + (period)*3

These equations can be used to create a very exact PWM output by setting duty_cycle and period to the values you wish to use. The code that was compiled and run above has a period of about a second and a duty cycle of about 50%.

There are a few macros defined at the beginning of the program. These macros are the location of GPIO1's memory space, the location of its set registers and the location of its clear registers. The BeagleBone's GPIO pins must be turned off and on using these two different memory locations. Setting the set register to 0 does not turn off its respective GPIO pin.

r2 stores the value that is going to be written to either set or clear gpio. r3 sores the address that r2 will be written to. within the first 3 lines of PWM_ON these values are set such that r2 will turn on the user LEDs. The instruction that actually turns it on is SBBO. This takes the value of r2 and writes it to memory location r3 with an offset of 0.

Here is a complete guide to the PRU's Assembly Instructions from TI


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Theory of Operation

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Work Breakdown


10/22: We should have all research done. Update documentation with every Milestone.

10/26: We should be able to show something, an example or simple implementation.

10/29: Ability to send different lengths to turn on an LEDs.

10/31: Ability to send different lengths to multiple LEDs.

11/2: We should be able to demo our overall work, possibly have some things to fix before presentation.

11/4: Finalize presentation

11/6: Presentation


Most of our research has come from internet resources listed below:

Future Work

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thumb‎ Embedded Linux Class by Mark A. Yoder