Ti AM33XX PRUSSv2

The PRUSS (Programmable Real-time Unit Sub System) consists of two 32-bit 200MHz real-time cores, each with 8KB of program memory and direct access to general I/O. These cores are connected to various data memories, peripheral modules and an interrupt controller for access to the entire system-on-a-chip via a 32-bit interconnect bus.

PRUs are programmed in Assembly, with most commands executing in a single cycle with no caching or pipe-lining, allowing for 100% predictable timings. At 200MHz, a single cycle most operations will take 5ns (nanoseconds) with exception of accessing memory external to PRU to execute.

Available PRU Resources


Click here for a full list of register mappings.

Per PRU

 * 8KB program memory
 * Memory used to store instructions and static data AKA Instruction Memory (IRAM). This is the memory in which PRU programs are loaded.


 * Enhanced GPIO (EGPIO)
 * High-speed direct access to 16 general purpose output and 17 general purpose input pins for each PRU.
 * PRU0
 * pr1_pru_0_pru_r30[15:0] (PRU0 Register R30 Outputs)
 * pr1_pru_0_pru_r31[16:0] (PRU0 Register R31 Inputs)
 * PRU1
 * pr1_pru_1_pru_r30[15:0] (PRU1 Register R30 Outputs)
 * pr1_pru_1_pru_r31[16:0] (PRU1 Register R31 Inputs)
 * Hardware capture modes
 * Serial 28-bit shift in and out.
 * Parallel 16-bit capture on clock.
 * | MII standardised capture mode, used for implementing media independent Fast Ethernet (100Mbps - 25MHz 4-bit).


 * A 32-bit multiply and accumulate unit (MAC)
 * Enables single-cycle integer multiplications with a 64-bit overflow (useful for decimal results).


 * 8KB data memory
 * Memory used to store dynamic data. Is accessed over the 32-bit bus and so not single-cycle.
 * One PRU may access the memory of another for passing information but it is recommend to use scratch pad or shared memory, see below.


 * | Open Core Protocol (OCP) master port
 * Access to the data bus that interconnects all peripherals on the SoC, including the ARM Cortex-A8, used for data transfer directly to and from the PRU in Level 3 (L3) memory space.

Shared Between PRUs

 * Scratch pad
 * 3 banks of 30 32-bit registers (total 90 32-bit registers).
 * Single-cycle access, can be accessed from either PRU for data sharing and signalling or for individual use.


 * 12KB data memory
 * Accessed over the 32-but bus, not single-cycle.

Local Peripherals
Local peripherals are those present within the PRUSS and not those belonging to the entire SoC. Peripherals are accessed from PRUs over the Switched Central Resource (SCR) 32-bit bus within the PRUSS.

Attached to the SCR bus is also an OCP slave, enabling OCP masters from outside of the PRUSS to access these local peripherals in Level 4 (L4) memory space.


 * Enhanced Capture Model (eCAP)


 * Industrial Ethernet Peripheral (IEP)


 * Universal Asynchronous Receiver/Transmitter (UART0)
 * Used to perform serial data transmission to the TL16C550 industry standard.
 * 16-bit FIFO receive and transmit buffers + per byte error status.
 * Can generate Interrupt requests for the PRUSS Interrupt Controller.
 * Can generate DMA requests for the EDMA SoC DMA controller.
 * Maximum transmission speed of 192MHz (192Mbps - 24MB/s).

Communication
Communication between various elements of the PRUSS or the wider SoC may take place either directly, over a bus, via interrupts or via DMA.

The following lists will expose all possible communication approaches for each likely scenario.

For communication via interrupts, please first read the section on the PRUSSv2 Interrupt Controller.

Click here for a full list of PRUSS Interrupts.

The current example PRU loader uses UIO, but this ideally should be replaced with remoteproc rather than poking at the registers from userspace. In the mean time, according to this discussion: we can use the included script and load the uio_pruss userspace driver.

Interrupts
Each PRU has access to host interrupt channels Host-0 and Host-1 through register R31 bit 30 and bit 31 respectively. By probing these registers, a PRU can determine if an interrupt is currently present on each host channel.

To configure

Beaglebone PRU connections and modes

 * Note1: The PRU0 Registers{30,31} Bit 4 (GPIO3_18) is routed to P9_42-GPIO0_7 pin. You MUST set GPIO0_7 to input mode in pinmuxing.


 * Note2: The PRU0 Registers{30,31} Bit 6 (GPIO3_20) is routed to P9_41-GPIO0_20(CLKOUT2). You must set GPIO0_20 to input mode in pinmuxing.

Assembly
The complete list of PRU assembly instructions can be found at TI.

Four instruction classes

 * Arithmetic
 * Logical
 * Flow Control
 * Register Load/Store

Instruction Syntax

 * Mnemonic, followed by comma separated parameter list
 * Parameters can be a register, label, immediate value, or constant table entry
 * Example
 * SUB r3, r4, 10
 * Subtracts immediate value 10 (decimal) from the value in r4 and then places the result in r3 (or r3 = r4 - 10)

Resources
The documentation on the subsystem is [[media:spruh73c.pdf|here]]. TI does not support this subsystem and all questions/inquires/problems should be directed to the community.
 * AM335x PRU support package on Github
 * AM335x PRU-ICSS Reference Guide


 * PRU assembly syntax highlighting for TextMate, Sublime Text, etc.

Examples

 * BeagleBone_6502_RemoteProc_cape
 * BeagleBone Nixie Cape PRU App
 * PRU Soft UART Driver