Data Sheet

AP6255

Datasheet

8.4

UART Interface Description

The UART is a standard 4-wire interface (RX, TX, RTS, and CTS) with adjustable baud rates from

9600 bps to 4.0 Mbps. The interface features an automatic baud rate detection capability that

returns a baud rate selection. Alternatively, the baud rate may be selected through a

vendor-specific UART HCI command.

UART has a 1040-byte receive FIFO and a 1040-byte transmit FIFO to support EDR. Access to the

FIFOs is conducted through the AHB interface through either DMA or the CPU. The UART supports

the Bluetooth 5.0 UART HCI specification: H4, a custom Extended H4, and H5. The default baud rate

is 115.2 Kbaud.

The UART supports the 3-wire H5 UART transport, as described in the Bluetooth specification

(Three-wire UART Transport Layer). Compared to H4, the H5 UART transport reduces the number

of signal lines required by eliminating the CTS and RTS signals.

The BCM43456 UART can perform XON/XOFF flow control and includes hardware support for the

Serial Line Input Protocol (SLIP). It can also perform wake-on activity. For example, activity on the

RX or CTS inputs can wake the chip from a sleep state.

Normally, the UART baud rate is set by a configuration record downloaded after device reset, or by

automatic baud rate detection, and the host does not need to adjust the baud rate. Support for

changing the baud rate during normal HCI UART operation is included through a vendor-specific

command that allows the host to adjust the contents of the baud rate registers. The BCM43456

UARTs operate correctly with the host UART as long as the combined baud rate error of the two

devices is within ±2%.