The UART HCI protocol implements the Serialization PHY API using the UART interface.
UART HCI for serialization uses four standard UART lines: RX, TX, /CTS, and /RTS. Hardware flow control is enabled. The protocol supports full duplex communication. The serialization version of the HCI protocol has a similar feature set as the stand-alone version provided in HCI transport library. However, implementation details are different due to API, signaling, and memory management required by the Serialization PHY API.
The packet format complies with the standard. However, because of memory shortage in the connectivity chip, the current implementation limits the packet size to 384 bytes. Every payload packet consists of a four-byte header followed by payload and CRC field.
The packet header contains SEQ, ACK numbers, data integrity check (DIP), reliable packet (RP) flags, LEN - numbers of bytes in the payload, and HCS - header checksum. Payload packets use TYPE=0xE
- a vendor-specific type. Acknowledgment packets use TYPE=0x0, SEQ=0, DIP=0, RP=0, LEN=0
.
The basic communication flow implemented in the HCI layer is shown in the figure below:
The driver is split into two layers - HCI and SLIP.
The HCI layer is implemented as two event-driven state machines with separated micro schedulers for events coming from SER_PHY, SLIP layers, and timer.
The operation of receiver and transmitter state machines is shown in the following UML state diagram:
UART HCI SLIP packets are transmitted in the following format, where both TX_PACKET_START
and TX_PACKET_END
are a 0xC0
byte:
[RX_PACKET]=[0xC0 0xE8 0x3E 0x00 0xDA 0x41 0x42 0xDB 0xDC 0x78 0x8D 0xC0]
At the beginning and the end of the packet, there are 0xC0
bytes that indicate packet start and packet end.0xC0
symbols, the content of the packet is received. It includes higher level (HCI) header, CRC, and payload. There is also a SLIP escape sequence present: [0xDB 0xDC]
, which is an encoded 0xC0 character.The UART HCI SLIP protocol for serialization is implemented in the ser_phy_hci_slip.c
file. It uses the UART driver as a low-level driver.
The implementation is event-driven. Events from the low-level driver are handled in a static uart_event_handler()
function. Three types of nrf_drv_uart_event_t events are processed: NRF_DRV_UART_EVT_ERROR, NRF_DRV_UART_EVT_TX_DONE, and NRF_DRV_UART_EVT_RX_DONE.
Following ser_phy_hci_slip_evt_type_t, the events are sent to the upper layer:
Function ser_phy_open() initializes UART using the nrf_drv_uart_init() function and prepares for the reception of incoming packets. It also registers the ser_phy_hci_slip_evt_type_t event callback.
Function ser_phy_close() closes UART using the nrf_drv_uart_uninit() function. It also deregisters the ser_phy_hci_slip_evt_type_t event callback.