nRF RPC UART transport

The nRF RPC UART transport allows you to use the Remote procedure call library (nRF RPC) library to execute procedures on a remote processor that is connected with the local processor using the UART interface. This transport is used in the Protocol serialization samples.

Configuration

Use the CONFIG_NRF_RPC_UART_TRANSPORT Kconfig option to enable the nRF RPC UART transport.

Define the nordic,rpc-uart property in a devicetree overlay file. The property is used to select the UART peripheral that is used by the API serialization libraries within the nRF Connect SDK, such as Bluetooth Low Energy Remote Procedure Call or NFC Remote Procedure Call.

/ {
   chosen {
      nordic,rpc-uart = &uart1;
   };
};

Frame encoding

Note

The current frame format is experimental and is a subject to change.

An nRF RPC packet that is sent using the nRF RPC UART transport is encoded within a frame whose format resembles the one used by the HDLC protocol:

• Each frame shall start and end with the delimiter octet (0b01111110 = 0x7e).

• Each two subsequent frames may be separated by more than one delimiter octet.

• Each byte of the nRF RPC packet shall be encoded according to the following rules:

  • If the byte matches one of the special octets, it shall be encoded as the following two octets:

    • the escape octet (0x7d),

    • the input byte XORed with 0x20.

  • Otherwise, the byte shall be encoded into a frame’s octet without changes.

• The last two bytes of the frame contain the nRF RPC packet checksum, in little-endian byte order. The checksum is calculated using the CRC16_CCITT function with the initial value 0xffff.

Special octets

The following octets transmitted over the UART interface have a special meaning:

Value	Meaning
0x7d	escape
0x7e	delimiter

Encoding example

If the following nRF RPC packet is sent using the nRF RPC UART transport:

80 01 ff 00 00 61 7e f6

Then the following octets are transmitted over the UART interface:

7e 80 01 ff 00 00 61 7d 5e f6 6d 72 7e

Reliability

When the CONFIG_NRF_RPC_UART_RELIABLE Kconfig option is selected, the nRF RPC UART transport enables the reliability feature.

The reliability feature introduces the following changes to the transport protocol:

• The receiver of a valid frame acknowledges the frame by replying to the sender with the frame’s checksum field.

• If a sender has not received an acknowledgment within a certain time, it retransmits the frame. The time (in milliseconds) is defined using the CONFIG_NRF_RPC_UART_ACK_WAITING_TIME Kconfig option.

• If the sender has not received an acknowledgment after a certain number of attempts, it gives up and reports the transmission error. The number of attempts is defined using the CONFIG_NRF_RPC_UART_TX_ATTEMPTS Kconfig option.

• The frame’s checksum field is composed of two values:

  • the most significant bit is the sequence bit that is flipped by the sender for each new transmission.

  • the remaining bits are 15 least significant bits of the nRF RPC packet checksum.

• If the received frame has the same checksum field as the previous one, it is rejected as a duplicate.

API documentation

Header file: include/nrf_rpc/nrf_rpc_uart.h

Source file: subsys/nrf_rpc/nrf_rpc_uart.c

nRF RPC UART transport