Serial Interface

Bluetooth Xpress BLE provides a flexible serial interface.

The serial protocol for issuing commands and receiving responses can be used locally via the UART interface or remotely via the BLE interface. The protocol is easy to use for both humans and machines alike.

Bluetooth Xpress modules also allow data to be streamed directly between the UART and BLE interfaces.

The serial bus mode determines whether data is streamed, or which interface can send commands to Bluetooth Xpress modules.

For a demonstration of serial bus modes, bus mode selection, and remote control of a Bluetooth Xpress module, see the Bus Mode Selection and Remote Control application note.


UART Protocol

UART baud rate is determined by the ua b variable. The default is 115200.

UART flow control is determined by the ua f variable. The default is 0, no flow control.

The serial port settings, corresponding to the Bluetooth Xpress module defaults, are 115200 8N1 with NO hardware flow control:

See Getting Started, Evaluating BGX with a Terminal Program.

Hardware Flow Control

Hardware flow control is used to regulate the flow of serial data between a Bluetooth Xpress device and a connected serial device such as an MCU. The following diagram shows typical connections to implement hardware flow control.

Flow Control Connections

Enabling Flow Control

Hardware flow control is enabled or disabled by the ua f variable. When enabled, the BGX device will utilize the hardware flow control signals. When disabled the hardware flow control signals RTS and CTS are not used.

Flow Control Signals

The Bluetooth Xpress device provides active low serial hardware flow control signals RTS and CTS with the following functions:

SignalDirectionFunction
RTSoutputdriven by BGX to indicate to MCU that BGX can receive data
CTSinputdriven by MCU to indicate that BGX can send data to MCU

Flow Control Operation


BGX-to-MCU

NOTE: The following diagrams are meant to help with understanding the overall data flow. They show a simplified representation of internal buffers of the BGX and other devices and should not be taken literally.

As BGX receives data from a remote Bluetooth device, it transmits the data to the MCU via the serial TX line. The MCU signals that it can receive data by asserting its RTS signal. In this condition the BGX will continuously transmit any data it receives to the MCU.

Data Flow to MCU

When the MCU needs to halt data flow for internal buffer management or some other reason, the MCU deasserts RTS. In this condition the BGX will stop transmitting data to the MCU and will begin to accumulate data in its internal buffers.

MCU Stops Incoming Data

Eventually the internal buffer will fill and BGX will stop acknowledging data from the remote Bluetooth device.

BGX Buffer Fills

This will eventually cause the remote Bluetooth device to stop sending data.

Remote Stops Sending

When the MCU asserts RTS again, then the BGX will resume transmitting data to the MCU, and its internal buffer will drain and begin to acknowledge data from the remote Bluetooth device.

Remote Configuration

If the remote Bluetooth device is another BGX acting as central, then it should be configured to require acknowledgement on write. This is done using the ce w m variable, by setting the value to ack.

If the remote Bluetooth device is a mobile device using the Xpress Mobile Framework, then it will stop transmitting data to the BGX when the BGX buffer is full due to CTS being deasserted.


MCU-to-BGX

As the MCU sends serial data to the BGX, the BGX internal buffer level will vary as it transmits the data to the remote Bluetooth device.

Data Flow From MCU

If the remote device stops receiving data (because of buffer fullness or other reason), the BGX will not transmit data to the remote device and the BGX internal buffer will begin to fill.

Remote Device Stop Data Flow

If the BGX internal buffer approaches full, it will deassert RTS to indicate it cannot receive any more data. The MCU should detect this condition on its CTS line and should stop sending data to the BGX.

BGX Buffer Full

Once the remote begins accepting data again, then the BGX internal buffer level will drop and it will re-assert its RTS signal. The MCU observes this on its CTS signal and can then resume sending serial data.

Flow Control Time Limit

The maximum amount of time that data flow can be stopped by hardware flow control is 30 seconds. If data flow does not resume within 30 seconds, then the connection will be dropped. For more information about the 30 second timeout, refer to the following sections of the Bluetooth Specification:


Serial Bus Modes

A Bluetooth Xpress module can operate in any one of three distinct bus modes:

The diagram below shows a schematic representation of the three bus modes.

There are various ways to make Bluetooth Xpress BLE transition between bus modes including:

Manual and Automatic Bus Mode Selection

When a Bluetooth Xpress module boots up, it may be desirable to control it via the UART interface, or by a remote BLE device.

To allow for either option to work without further configuration, Bluetooth Xpress modules support a choice of manual and automatic bus mode selection.

Bus mode selection depends on whether the str_select GPIO is configured. See the gfu (GPIO function) command.

When str_select IS configured:

When str_select IS NOT configured:

str_sel GPIOBus mode selectionDescription
configuredmanual- bus mode initializes to the value set by bu i
- mode_sel GPIO determines if the bus is in STREAM mode or local COMMAND mode
- if the bus is in STREAM mode, the remote device can use the mode characteristic to switch to remote COMMAND mode, if enabled. See sy r e
not configuredautomatic- if a remote device is connected: the bus is in STREAM mode
- if NO remote device is connected: the bus is in local COMMAND mode
- if the bus is in STREAM mode, the remote device can use the mode characteristic to switch to remote COMMAND mode, if enabled. See sy r e

The state diagrams below show the possible transitions.

Manual Bus Mode Selection

Manual mode state diagram

Automatic Bus Mode Selection

Automatic mode state diagram

local COMMAND mode

To enter local COMMAND mode:

See Command Protocol for details of sending commands and handling responses.

STREAM mode

For details of this service, see BLE Services, Xpress Streaming Service

To enter STREAM mode:

In Stream mode, bytes or characters sent from the host to a serial interface are transparently pushed by a Bluetooth Xpress module to the remote device via the wireless interface.

Conversely, bytes or characters received from a remote device are transparently pushed by Bluetooth Xpress BLE to a serial interface connected to the host.

The Getting Started example is a typical application that uses stream mode.


Command Protocol

Both local COMMAND mode and remote COMMAND mode provide an asynchronous command interface that a host may use to send and receive control and setup information. Command mode is typically used by a host to configure Bluetooth Xpress modules.

Command mode is the primary mode used to configure Bluetooth Xpress modules. Grasping how command mode works, and the options provided, is the key to mastering the use of Bluetooth Xpress modules. The serial interface is designed to cater for humans and machines alike, and the interface is configurable to suit the needs of both.

In Command mode, control, data and debug logs (if enabled) are interleaved.

Configuration

In command mode, Bluetooth Xpress modules effectively provide a slave interface to the host. The host is the master and a Bluetooth Xpress module is the slave. The host initiates all transactions which follow the format of "host issues a command, the Bluetooth Xpress module provides a response". Bluetooth Xpress module commands are used by the host to read and write Bluetooth Xpress module variables, send control information, and send and receive data across network connections.

The system command mode variable (sy c m) is provided for convenience to make it easy to switch between human mode and machine mode.

Setting sy c m to human or machine conveniently changes the value of each of the following variables to configure the desired command mode.

Human-Friendly Command Mode

By default, the serial interface is configured to be human friendly with the following settings. See System Variables.

set sy c m human

is equivalent to:

set sy p all  -> Turn on all debug & informational prints
set sy c h 0  -> Disable response header
set sy c p 1  -> Turn on the user prompt
set sy c e 1  -> Turn on echo to see what you're typing

Machine-Friendly Command Mode

To configure the serial interface for machine-friendly operation, use the following settings. See System Variables.

set sy c m machine

is equivalent to:

set sy p 0    -> Turn off all debug & informational prints
set sy c h 1  -> Enable response header
set sy c p 0  -> Turn off the user prompt
set sy c e 0  -> Turn off echo

Command Format

<command name> [variable name <argument>]\r\n

Commands sent to Bluetooth Xpress modules are formatted as shown in the following table. The <command name> is a Bluetooth Xpress command issued by the host, and [variable name <argument>]\r\n is an (optional) Xpress variable name and accompanying argument terminated by a carriage return and newline character.

Response Format

Responses from Bluetooth Xpress module follow the format shown in the text below.

RXYYYYY\r\n
<response data>

where:

Response Error Codes

CodeDescriptionFrom Commands
0Successall
1Command failedall
2Parse errorall
3Unknown commandall
4Too few argumentsall
5Too many argumentsall
6Unknown variable or optionall
7Invalid argumentall
8Timeoutcon
9Security mismatchcon
10GATT Procedure Failedcon