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BeagleBone cape eases access to the Sitara SoC’s PRU

Nov 6, 2014 — by Eric Brown — 3,893 views

TI has launched a $39, Linux-ready cape for the BeagleBone Black for evaluating the Sitara SoC’s programmable real-time unit for applications like robotics.

BeagleBone Black Rev C

The PRU-ICSS (Programmable Real-Time Unit and Industrial Communication Subsystem) coprocessor, typically shorthanded to PRU, is built into the Texas Instruments Sitara AM3559 system-on-chip that drives the Linux-ready Beaglebone Black SBC. The Sitara SoC’s PRU, which comprises dual, 200MHz, 32-bit RISC microcontrollers, is designed for customizing I/O that requires deterministic, real-time processing and ultra-low-latency. Yet, the PRU is so notoriously difficult to program, most BB Black hackers are hardly aware it’s there.

TI Sitara’s CPU/PRU architecture
(click image to enlarge)

Almost a year ago,’s Jason Kridner, who also works at TI, told LinuxGizmos that the community planned to make the PRU easier to use. Now, TI has followed up with a cape add-on board to help make that happen.

PRU Cape
(click image to enlarge)

The PRU Cape provides evaluation and program assistance for the dual-core PRU subsystem found in four of the six Sitara AM335x SoCs: the AM3356, AM3357, AM3358, and the BB Black’s AM3359. The Linux code that ships with the cape is also usable in the new quad-core PRU models found in all four of the newer, Cortex-A9-based Sitara AM437x SoCs, says TI.

PRU Cape hardware details (left) and block diagram
(click images to enlarge)

The PRU Cape, which also works on the original BeagleBone models, provides the following features:
  • BeagleBone/BeagleBone Black Expansion connector
  • Hardware UART to PRU subsystem
  • 2x GPIO push button switches
  • HDQ temperature sensor monitoring using 1-wire interface
  • Audio jack connection output
  • 7x PRU0 and PRU1 LEDs
  • LCD Connector for optional character display, compatible with Newhaven NHD-0208AZ-RN-YBW-33V

The PRU Cape is supported with Linux source code, as well as a number of demos. The demos cover LED, push buttons, audio, and UART topics. The code is available as part of the Sitara Linux SDK, which is based on mainline Linux. Other software resources include a new PRU-optimized C compiler in TI’s Code Composer Studio.

TI has pushed its PRU concept as a lower-cost, easier alternative to FPGAs and ASICs. Although it’s beyond the scope of the average SBC hacker, it offers some unique properties for developers working on real-time and/or motor control applications. The PRU offloads processing from the ARM processor to manage deterministic tasks such as time-critical I/O manipulation, as well as complex motor control functions involved with 3D printers, LED lighting control, thermal printing, and robotics, says TI.

PRU hardware (left) and software architecture diagrams
(click images to enlarge)

The PRU is said to offer single-cycle direct access to I/O pins within 5ns, and helps enable fast real-time responses, specialized data handling operations, custom peripheral interfaces, and offloading tasks from the other processor cores. You can use the PRU to create customized interfaces including advanced PWMs, motor control, RS485, sample rate conversion, and sensor monitoring, says TI. The PRU can also generate “bit-banging serial interfaces like I2C and SPI,” says the company.

Further information

The PRU Cape is available now for $39. More information, including direct sales, are available at the TI Pru Cape product page. The company has also posted a white paper on the PRU in PDF format.

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