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MSP430 USB Hardware Design

Because of potential use in the field, the BSL is password-protected to prevent unwanted access to

proprietary application software.

The BSL must be invoked, meaning that CPU execution must be transferred to it. On the USB BSL, this

can happen in one of three ways:

• The application software in main flash can jump into it

• A BOR reset while the reset vector is blank

• A BOR reset while the PUR pin is held high externally – perhaps by way of a pushbutton switch

Field applications of the USB BSL are likely to use the first method, because a main application is already

in control of the device. In this way, the developer can design a simple end user experience for the

update. However, if main software becomes corrupted, this method might not work.

The second method is used during production of a device based on the MSP430 MCU, because the reset

vector is blank when it leaves TI's factory; you can simply assemble the board and attach the device to a

USB host, and it will enumerate under BSL control. This method also plays an important role in recovering

from interrupted BSL sessions, because the reset vector is left blank throughout the update process.

The third method is the most reliable but often requires the additional cost of a pushbutton.

The BSL is designed to automatically recognize four frequencies applied to XT2:

• 4 MHz

• 8 MHz

• 12 MHz

• 24 MHz

If using BSL for production programming, you must use one of these four frequencies. If using BSL after a

JTAG session has been performed, you can use that JTAG session to program a modified version of the

BSL that can recognize your frequency.

All of this is described in the application note Field Firmware Updates on MSP430 MCUs (SLAA452).

Supporting this application note, the MSP430 USB Developers Package includes the Python-based

Firmware Updater application. This application is built on the open source package python-msp430-tools.

It can be used from a command line, or a GUI can be built on top of it.

4 MSP430 USB Hardware Design

4.1 TI Reference Design for USB Interface

Figure 8 shows a detailed reference design, and commentary follows it.

SLAA457A–September 2013–Revised May 2014 Starting a USB Design Using MSP430™ MCUs

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