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MSP430 USB Hardware Design

If using a crystal, the load capacitors should be properly chosen, according to the crystal's specification.

This is especially true if the oscillator will be shut down during USB suspend to save power consumption,

in which case improper capacitor selection could result in a slow response to USB resume. Improper

capacitor values could cause crystal startup to take longer than it should, and the device has a total of 10

ms to become USB-ready when the host performs a USB resume on the device. If the oscillator had been

disabled during suspend for power savings, then its re-enabling is part of this 10-ms budget. (A properly-

tuned oscillator will easily meet the requirements.)

Note that XT2 derives its power from the DVCC pin, rather from the internal USB LDO. Also note that XT2

consumes approximately 200-400 µA, depending on the frequency. (See the device data sheet for actual

values.)

4.3.2 Choosing a Frequency

A wide range of frequencies is possible, because the USB PLL has a very flexible programmable input

divider. Each MSP430 header file contains predefined constants for 43 possible frequencies, and many

more are possible. (The pre-defined ones are of the format USBPLL_SETCLK_xx_yy, where xx_yy is

frequency-specific.)

As shown in Section 4.3.1, each clock source is subject to a frequency range.

There are two considerations in frequency selection:

• Lower-frequency crystals or resonators consume less power than higher ones. (See the device data

sheet power consumption values for XT2.)

• If using the USB BSL for factory programming of the MSP430 device, only 4, 8, 12, and 24 MHz are

auto-recognized by the BSL. See Section 3.6.

USB-equipped MSP430 boards from TI usually use a 4-MHz ceramic resonator that meets USB precision

requirements. Being the lowest-frequency crystal possible, these have the advantage of minimizing power

consumption. 4 MHz is also auto-detected by the on-chip BSL.

4.4 Other Reference Design Commentary

The circuitry within the dashed rectangle in Figure 8 is required only if the bootstrap loader (BSL) is to be

invoked using a pushbutton switch, for performing firmware updates through USB. PUR is normally an

output, but in the moments following a BOR reset, it is an input that determines whether or not the BSL

will be invoked. This pushbutton is only one way to invoke the BSL; see Section 3.6 for more information.

The weak pulldown resistor on PUR ensures that the PUR pin stays low if the pushbutton is not pressed.

(It is weak enough to not disrupt D+ during USB operation.) This pulldown should never be eliminated, as

doing so might result in unintended invocation of the BSL.

The pullup resistor is specified as 1.4k. Those with USB experience are aware that the pullup value

specified in the USB specification (parameter R

) is 1.5k. The reason for this difference is that the

MSP430 device's PUR pin itself contributes approximately 100 Ω (shown as parameter RPUR in the

device data sheet). The sum of this amount and the external 1.4k resistor produces the complete value

. To be absolutely compliant, a 1% resistor is needed. This is because the sum of the errors on RPUR

and a 5% resistor slightly exceed the 5% allowed by the USB specification on R

. Practically speaking,

however, there is no problem using 1.4k ± 5%.

At 12 MHz, full-speed USB usually is not highly sensitive to transmission line characteristics, but it is

recommended to keep D+ and D- reasonably short.

A USB port can represent an ESD vulnerability, because it extends a conductive path to the outside of the

enclosure, which the user frequently touches. Under these conditions, the protection integrated into most

ICs is not enough to withstand the levels of discharge it might experience. Therefore, a dedicated ESD

suppressor is recommended, such as the TPD2E001 shown. It is important to follow all the design

recommendations in the TPD2E001's data sheet.

The diode on VBUS is placed there to ensure compliance with the USB 2.0 specification's requirement

that the device never source current toward the host over VBUS.

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

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