BM71 MCU Interface

Last modified by Microchip on 2023/11/10 11:11

Overview

The BM71 provides a flexible MCU interface, which can be tailored to your specific needs. The most basic interface requires only two Universal Asynchronous Receiver Transmitter (UART) interface signals. Two additional control signals are also recommended: one to reset the module, and the other to enable BM71 firmware/configuration programming.

The BM71 is configured and controlled via binary packet communications over a UART interface. The "BLEDK3 Command Set" user's guide contains a complete command/event/response listing and details of the interface. Additionally, the module provides status indication/control via several programmable General Purpose Input/Output (GPIO) pins, simplifying MCU firmware and state-machine development.

Further documentation on the MCU interface and available GPIO pin functions may be found in the BM70/71 device datasheet.

BLEDK3 refers to the name of the application firmware running in the BM71 module.

All BM71 functionality referred to on this page requires v1.06 firmware. Instructions for updating module firmware may be found on the "BM71 Firmware Update" page.

Binary Packet Communications

The MCU host sends specially formatted Command packets to the module. Depending on the command, the module will issue Event response packets and eventually a Command Complete response packet as shown:

Event response packets and a Command Complete response packet

Commands cover the following operational categories:

  • Common (Reading basic device information)
  • Generic Access Profile (GAP) Commands (Bluetooth® Low Energy (BLE) Connection Management)
  • Generic Attribute Profile (GATT) Client (BLE Data Exchange)
  • GATT Server (BLE Data Exchange)
  • GATT Transparent
  • Pairing

Please refer to the BLEDK3 Command Set document for complete details.

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Command Packet

Command Packet

​A checksum is used to test data integrity on command packets.
To calculate:

  • Sign-extend to 16 bits, then add all required bytes.
  • Calculate the 2's complement on the lower 8 bits of the result.

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Response Packet ("Command Complete")

Response Packet "Command Complete"

​A checksum may be used to test data integrity on response packets.
To verify:

  • Sign-extend to 16 bits, then add all required bytes (including the checksum byte).
  • If there are no errors, the result will be 0xXX00.

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Example

The following depicts a command/response sequence for the Read Local Information command:

Command/Response sequence for the Read Local Information command

Command Packet

Command Complete Packet

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Basic Interface (Manual Pattern Mode)

In the simplest configuration, the module requires two UART interface signals (TX and RX). Two additional control signals are also recommended: one to reset the module, and the other to enable BM70 firmware/configuration programming.

Basic Interface diagram requiring two UART interface signals (TX and RX)

RST_N (Reset)

The Reset input pin (RST_N) is used to Reset the BM70/71 with an active-low pulse with a minimum pulse width of 63 ns.

An internal pull-up is enabled on this input, however, an external pull-up resistor of 4.7 kΩ is recommended.

P2_0 (System Configuration)

This pin is used to enable module firmware/configuration updates.

P2_0 (System Configuration) Operational Modes

P2_0 has an internal pull-up, enabling Application mode by default.

HCI_TXD / HCI_RXD

UART data output/input signals.

The default port configuration for the UART is described in the table below:

ParameterValue
Baud Rate115200
Data Bits8
ParityNone
Stop Bits1
Flow ControlNone

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Basic Interface (Auto Pattern Mode)

The basic signal interface for Auto Pattern mode (Transparent UART) adds two active-low UART flow control signals, P3_6/UART_RTS and P0_0/UART_CTS:

Basic Interface (Auto Pattern Mode)

BM71 UART RTS/CTS signals are reserved for Auto Pattern mode operation only.

UART RTS/CTS flow control timing diagram

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Special Function Pins

Several additional programmable GPIO signals are available to be interfaced with the host MCU. Some of these are used for control, while others may be used to monitor module state as an alternative to UART event packets.

Special Function Pins Connection Diagram

Some of these pin functions are dedicated, however, most can be re-programmed with the User Interface (UI) Configuration Tool.

P1_6 (UART_RX_IND Pin - Programmable)

When enabled, this active-low input pin function can be used to reduce the active current consumption of the module by disabling the UART HCI_RXD pin function. When enabled, the host MCU needs to insert a ~3 mS delay before sending a command packet as shown:

Host MCU inserts a ~3 mS delay before sending a command packet

UART_RX_IND pin function is enabled and assigned to a pin using the UI Configuration Tool.

P1_7 (PAIRING_KEY Pin - Programmable)

During an active BLE connection, this active-low input pin function can be used to force the module to terminate the link and enter into Standby mode. This signal needs to be applied for at least 160 mS as shown:

Standby mode. Signal needs to be applied for at least 160 mS

PAIRING_KEY pin function is assigned to a pin using the UI Configuration Tool.

During an active BLE connection, this active-low input pin function can be used to force the module to terminate the link and enter into Shutdown mode. This signal needs to be applied for at least 10 mS as shown:

Shutdown mode. This signal needs to be applied for at least 10 mS

The LINK_DROP pin function is not assigned to any pin by default. Assign to a pin using the UI Configuration Tool.

P0_2 (LED0 Pin - Dedicated)

This active-low output pin function can be used to signal various BLE connection states. Options are also provided to flash this output at various rates.

LED0 pin toggle functionality is configured using the UI Configuration Tool.

P2_7 (UART_TX_IND Pin - Dedicated)

This active-low output pin function can be used to signal the host MCU that data will be transmitted out of the HCI_TXD pin after a specific interval:

Active-low output pin function timing diagram

TWakeup_External_MCU_Wait_Time is configured using the UI Configuration Tool.

P1_3/P1_2 (STATUS2/STATUS1 Pins - Programmable)

When enabled, these output pin functions indicate a variety of module connection states, as shown in the following table:

STATUS1STATUS2StatusDescription
HHShutdown modeBM70 is shutdown
HLStandby modeBM70 sends advertising packets and is discoverable and connectable
LLConnected modeBLE Link is established (BLE GATT Operating mode)
LHConnected modeBLE Link is established (Transparent UART Service Operating mode)

STATUS2/STATUS1 pin functions are assigned to GPIO pins using the UI Configuration Tool.

Px_x (LOW_BATTERY_IND Pin - Programmable)

This active-low output pin function can be used to signal a low battery condition on VBAT. Additionally, the module can be configured to enter shutdown if the battery voltage remains low for a certain time interval.

​LOW_BATTERY_IND pin functionality is not assigned to any pin by default. Assign it using the UI Configuration Tool.
Related UI Config Tool Parameters:

  • Battery Detection Interval
  • Low Battery Level
  • Normal Battery Level
  • Low-Battery-to-Shutdown Time

Px_x (RSSI_IND Pin - Programmable)

If enabled, this output pin function can be used to signal the BLE connection link quality. The module checks two link quality levels which can be configured using the UI Configuration Tool. If link quality is lower than the Received Signal Strength Indicator (RSSI) Weak Threshold, then RSSI_IND output is low. If link quality is higher than RSSI Normal Threshold, then RSSI_IND output is high, as shown here:

Link Quality Indication diagram

​RSSI_IND pin functionality is not assigned to any pin by default. Assign it using the UI Configuration Tool.
Related UI Configuration Tool Parameters:

  • Link Quality Detection
  • RSSI Normal Threshold
  • RSSI Weak Threshold

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