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issue: September 2009 APPLIANCE Magazine

APPLIANCE Engineer - Electronics Report
Low-Power USB Connectivity


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A new MCU family features embedded USB and ultralow power consumption, providing easy USB solutions to battery-powered, portable applications.

Wider adoption of USB connectivity has led Texas Instruments Inc. (Dallas, TX, U.S.; www.ti.com) to integrate embedded full-speed USB (12 Mbps) into its MSP430 microcontrollers (MCUs). Helping to cut the power cord, the new MSP430F55xx family of MCUs allows designers to quickly implement USB functionality on battery-powered devices while minimizing power consumption and saving cost and board space.

According to Bhargavi Nisarga and Keith Quiring, MSP430 applications engineers, one major strength of the F5xx is its ultralow power consumption, which extends battery life for devices. “The F5xx’s CPU consumes up to half that of previous MSP430 designs, which already have long been recognized as the industry’s lowest. Our 12-bit analog-to-digital converter (ADC) with its integrated 1.5/2.5-V reference consumes half that of previous families,” says Nisarga.

Several power management techniques enable the F5xx to achieve high levels of integration at low power, the Comparator_B module being one of them. The comparator lets designers select the right tradeoff of performance versus clock availability with five low-power modes (LPM). LPM0 keeps the on-chip digitally controlled oscillator (DCO) running, which can provide a clock across the entire 25-MHz spectrum. “At 1 MHz, which is enough for many or even most applications, the device only consumes 80 µA. Together with MSP430’s intelligent peripherals, this enables a wide variety of operations to be conducted without CPU involvement,” explains Quiring. “Since the CPU is the largest power consumer, this translates to massive power savings.”

LPM3 powers down the DCO while keeping a low-frequency clock active, typically 10–40 kHz. Depending on the source of the clock, it consumes 1–2 µA. “This low frequency can still be used to enable a wide variety of functions, in particular, a timer that can wake the CPU periodically to perform other functions,” Quiring tells APPLIANCE. In LPM4, no clock is activated. “This achieves very low power consumption, but without an internal clock, the CPU is dependent on an interrupt being driven from the outside, like a push button, in order to wake,” says Quiring. “The final step is LPM5, which powers down the device completely and can be woken up by power cycle or interrupt on I/O.”

Quiring notes that OEMs often overlook a critical factor in evaluating an MCU’s low-power capability—the ability to quickly switch between these modes. Nisarga agrees. “Like all MSP430s, the F5xx can switch between LPM3, LPM0, and active mode almost instantly—always less than 5 microseconds,” says Nisarga. “This provides the ability to actually use the low-power modes because low current values are useless if you can’t wake from the mode fast enough to respond to events.”

The new MCUs contain an integrated low-dropout regulator (LDO), which helps designers save cost and board space. “While most modern USB-equipped MCUs operate from a nominal 3.3-V rail, the power provided by the USB host is 5 V. Somehow this 5 V must be reduced to something the MCU can use,” Quiring explains. “Unless the LDO is integrated, one must be added to the design. LDOs cost money, and space-constrained designs will feel the effect of fitting it on the board.”

Quiring says the F55xx devices are especially suited for portable medical instruments or fitness applications that measure data, such as heart rates, blood glucose levels, or blood oxygen levels. “The portable device can be used to gather data over the course of operation, and then periodically attach to a host via USB to upload the data, maybe charge a rechargeable battery, or maybe to update firmware on the device from the vendor’s website.”

For companies new to USB technology, TI offers a support plan that assist with the design process. For example, if a firm wants to allow its users to update device firmware via USB, “the on-chip bootstrap loader program has been adapted to use USB, and we’re providing a Visual Studio Express project as ‘starter’ for custom development,” Quiring says. “In most cases, the only step required of the OEM will be to insert their own firmware image for download, and customize the graphical user interface to their own liking, which for many folks should be possible within 15-30 minutes.”

The MSP430 USB Descriptor Tool’s help pane is another example, Quiring adds. “It methodically associates each GUI control back to a specific location in the USB specification, and helps the designer understand the tradeoff of the decision being made.”

 


 

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