3.2 USB API Stacks: Features

The API is the foundation of the MSP430 USB Developers Package. It supports three of the most common USB device classes:

• Communications Device Class (CDC): (ACM class) Results in a virtual COM port on the host
• Human Interface Device class (HID): The MSP430 USB Developers Package defines four subtypes:
  • Datapipe (an unformatted general-purpose interface resembling CDC)
  • Mouse
  • Keyboard
  • Custom
• Mass Storage Class (MSC): When the host sees an MSC interface, it begins mounting a storage volume from it.

These classes provide a good selection for general-purpose use. See Section 5.1 for a discussion of how to choose your interface.

Features of the API include:

• Small peripheral footprint (only the USB module and XT2 oscillator)
• Small memory footprint
• Can use either the DMA or CPU to move data
• Provided as source code
• BSD-licensed, providing maximum flexibility

CDC and HID-Datapipe provide the developer a simple data exchange interface. As a brief example of this interface, a simple application follows.

VOID main(VOID) { // Init clocks, power, ports WDT_A_hold(WDT_A_BASE); PMM_setVCore(PMM_BASE, PMM_CORE_LEVEL_2); initPorts(); initClocks(8000000); // CPU frequency, in Hz initTimer(); // Set up one-second intervals to wake from LPM0 USB_setup(TRUE,TRUE); // USB API call; initializes USB and connects to the host __enable_interrupt(); while (1) { __bis_SR_register(LPM0_bits + GIE); // Enter LPM0 sleep if (cdcSendDataInBackground(helloWorldStr, 12, CDC0_INTFNUM, 1000)) { handleFailedSend(); // Might fail if cable is disconnected, bus is // busy, or host has become unresponsive } } }

This application wakes up once per second (using a timer interrupt, not shown) and sends a string over USB to a virtual COM port on the host.

To send data over a CDC interface (to a virtual COM port), you can simply build the interface with the Descriptor Tool, prepare your data, and call the API construct function cdcSendDataInBackground(), which accepts these parameters:

• Data buffer
• The buffer's size
• Which CDC interface
• The number of retries before the attempt is considered a failure.

This is only one simple example; much more is possible with the USB API. See the MSP430 USB API Programmer's Guide and examples in the MSP430 USB Developers Package for more information.