As of Processor SDK Linux v6.1, AM335x supports up to 6 additional UARTs through PRU-ICSS GPIO pins. The UART function is implemented in PRU firmware. The firmware is self-contained in the same PRU, which means the firmware does not use shared resources within an ICSS including IEP timer and Scratchpad.

PRU Soft UART Features

Up to 3 independent UARTs are supported per PRU with the following features:

  • Common baud rates up to 115200 Mbps
  • 5, 6, 7, or 8-bit characters per transfer
  • 1, 1.5, or 2 stop bit generation
  • Even, odd, or none parity bit generation and detection
  • Auto-RTS/CTS based harware flow control
  • Full-duplex communication

PRU Soft UART Unsupported Features

The following features are not supported in PRU Soft UART:

  • Full modem control
  • Software flow control

PRU Soft UART Pin Configuration

The PRU GPIO pins are assigned for the functional pins of each UART port. The PRU GPIO pin assignment is specified in the “ti,pru-suart-pins” properties in the device tree bindings.

The device tree “ti,pru-suart-pins” property contains 2 or 4 byte values. The first two values are for TXD and TXD pins, and next two values are optional which are for CTS and RTX pins. The value is the index of the PRU GPIOs defined in PRU R30 and R31.

For details please refer to the kernel device tree binding doc:


The Processor SDK Linux kernel also provides an device tree example for Beaglebone Black:


This example defines the 3 UARTs on PRU0 without hardware flow control and 3 UARTs on PRU1 with hardware flow control. All the UART pins are available on the Beaglebone Black expansion headers.

PRU Soft UART driver Kernel Config

The PRU Soft UART Linux kernel driver depends on the PRU-ICSS kernel driver. So the following kernel Kconfig options should be enabled to use the PRU Soft UART module.

Device Drivers  --->
      SOC (System On Chip) specific Drivers  --->
              [*] TI SOC drivers support  --->
                      <M>   TI PRU-ICSS Subsystem Platform drivers

Device Drivers  --->
      Character devices  --->
              Serial drivers  --->
                      <M> TI PRU Software UART suppor