SLAA457B Application note

SLAA457B September 2013 – October 2018 MSP430F5500 , MSP430F5501 , MSP430F5502 , MSP430F5503 , MSP430F5504 , MSP430F5505 , MSP430F5506 , MSP430F5507 , MSP430F5508 , MSP430F5509 , MSP430F5510 , MSP430F5513 , MSP430F5514 , MSP430F5515 , MSP430F5517 , MSP430F5519 , MSP430F5521 , MSP430F5522 , MSP430F5524 , MSP430F5525 , MSP430F5526 , MSP430F5527 , MSP430F5528 , MSP430F5529 , MSP430F5630 , MSP430F5631 , MSP430F5632 , MSP430F5633 , MSP430F5634 , MSP430F5635 , MSP430F5636 , MSP430F5637 , MSP430F5638 , MSP430F5658 , MSP430F5659 , MSP430F6630 , MSP430F6631 , MSP430F6632 , MSP430F6633 , MSP430F6634 , MSP430F6635 , MSP430F6636 , MSP430F6637 , MSP430F6638 , MSP430F6658 , MSP430F6659 , MSP430FG6425 , MSP430FG6426 , MSP430FG6625 , MSP430FG6626

5.1 How to Choose a USB Device Class

Tradeoffs between the three device classes are shown in Table 5. (Plain text indicates advantages or neutral comments, and italic text indicates possible disadvantages.)

Table 5. Device Class Tradeoffs

Characteristic	CDC (Communications Device Class)	HID (Human Interface Device Class)	MSC (Mass Storage Class)
The interface generated on the host	Virtual COM port	A human interface device	Storage volume
Industry expertise for this interface	COM ports are common in the industry; widely-supported and well-understood	Unlike COM ports or storage volumes, HID interfaces are somewhat USB-specific, and less known in the industry	Storage volumes are common in the industry; widely-supported and well-understood
Installation on the host	Windows PCs must undergo a device installation process that requires end user interaction.⁽¹⁾ Admin rights on this Windows PC are required Despite the actual binary files already existing in Windows, the user must supply an INF file	Loads silently in most operating systems – simply begins working. No driver files required	Loads silently in most operating systems – simply begins working. No driver files required
How the end user interacts with it	An application on the host that interfaces with COM ports The application might be custom, or any existing application that uses COM ports	A custom application on the host that interfaces with HID devices	The device mounts a storage volume onto the system; applications read and write files on the volume. The application might be custom, or any application that reads or writes files
Driver certification needs	Unless the INF file is WHQL certified (signed), Windows will report that the driver is 'uncertified'. ⁽²⁾	No 'uncertified' message is generated	No 'uncertified' message is generated
Code footprint and complexity	Small code footprint (4K to 6K) Simple architecture	Small code footprint (4K to 6K) Simple architecture	Larger code footprint (8K to 15K) Requires a file system, increasing cost, size, and complexity.
Throughput	Fast (hundreds of KB/sec) Uses bulk USB transfers.	Slower (64 KB/sec) Uses interrupt USB transfers.	Fast (hundreds of KB/sec) Uses bulk USB transfers.
Good for point-to-point communication between host and device	Yes	Yes	No
Good for bulk data transfer	Yes	No	Yes

(1) This is the process that occurs the first time the device is attached to the host. In Windows, this process may begin with a "Found New Hardware" dialog box. The user must locate the INF file provided by the OEM.

(2) WHQL certification refers to Microsoft's Windows Hardware Quality Labs. See http://www.microsoft.com/whdc/winlogo.

Sometimes the choice of device class is clear. In other cases, the application can be considered general purpose, giving the developer options. Although there are many ways to approach this decision, one way is shown in Figure 11.

Figure 11. Example Process for Deciding on a USB Device Class

For general-purpose use, HID-Datapipe should be considered, because of its combination of silent loading on the host (easy for the end user) and its small footprint and simplicity on the MSP430 MCUs, which speeds development. Coding of a corresponding host application is made easier with the MSP430 Java HID Demo App.