UART RAW protocol

The UART RAW protocol implements the Serialization PHY API using the UART interface.

UART RAW for serialization uses four standard UART lines: RX, TX, /CTS, and /RTS. Hardware flow control is enabled. The protocol supports full duplex communication. Every packet consists of a two-byte header followed by payload. Packet header contains the number of bytes of the payload. Header is transmitted with the least significant byte first.

UART RAW packet

Packets are transmitted in the following format:

1. First, on the RX line, length is received as the header: [RX_HEADER]=[0x0004] .
2. It is then followed by the payload [RX_PAYLOAD]=[0x00 0x78 0x41 0x03] .
3. After the reception of a packet, transmission starts on the TX line. Packet format is the same as during reception: [TX_HEADER]=[0x0006] , [TX_PAYLOAD]=[0x01 0x78 0x00 0x00 0x00 0x00] .

UART driver

The UART RAW protocol for serialization is implemented in the ser_phy_nrf51_uart.c file. It uses app_uart.c as a the low-level UART driver.

The implementation is event-driven. Events from the low-level driver are handled in a static ser_phy_uart_evt_callback() function. Three types of app_uart_evt_t events are processed: APP_UART_COMMUNICATION_ERROR , APP_UART_TX_EMPTY , and APP_UART_DATA .

• In case of an APP_UART_COMMUNICATION_ERROR event, error source (taken from app_uart_evt_t structure) is checked. If the source is parity or overrun error, then the upper layer is notified with SER_PHY_EVT_HW_ERROR . Error source is passed in error_code member of the ser_phy_evt_hw_error_params_t structure.
• APP_UART_TX_EMPTY indicates a TXDRDY interrupt. In such case, a static function for transmission is called: ser_phy_uart_tx() .

  Note

  When transmission of a packet begins, the ser_phy_uart_tx() function is called for the first time in the ser_phy_tx_pkt_send() function. Later, it is only called in the app_uart_evt_t event callback.

• APP_UART_DATA indicates an RXDRDY interrupt. In such case, a static function for reception ser_phy_uart_rx() is called, with the received byte as a parameter. The received byte is taken from the app_uart_evt_t structure.

The following ser_phy_evt_t events are sent to the upper layer:

• SER_PHY_EVT_HW_ERROR , when a hardware error is reported by the low-level driver.
• SER_PHY_EVT_RX_BUF_REQUEST , when a header is received and therefore the payload length is known. From this point, reception is paused until the ser_phy_rx_buf_set() function is called by the upper layer, indicating whether a receive buffer has been allocated or not. Subsequently the packet payload is received or dropped.
• SER_PHY_EVT_RX_PKT_RECEIVED , when all bytes have been received. This event indicates how many bytes have been received to which memory address.
• SER_PHY_EVT_RX_PKT_DROPPED , when all bytes are clocked in, but the packet is discarded because no receive buffer was allocated by upper layers.
• SER_PHY_EVT_TX_PKT_SENT , when all bytes are transmitted.

Function ser_phy_open() initializes UART using the APP_UART_INIT macro with configuration structure of type app_uart_comm_params_t as input. It also registers the SER_PHY event callback.

Function ser_phy_close() closes UART using the app_uart_close() function. It also deregisters the SER_PHY event callback.

Note

During initialization of UART, the pin connected to the RX line is configured as pull-down until the first byte is received. This is done to avoid false byte detection due to glitches on the line when the other side is turned off or starting up.