SDAA280 March   2026 MSPM0G5187

 

  1.   1
  2.   Abstract
  3. 1USB Makes a Complex System Appear Simple
    1. 1.1 Why Has USB Been so Successful?
    2. 1.2 Why Does USB Look Simple?
  4. 2MSPM0 USB Silicon
    1. 2.1 How MSPM0 Devices are Documented
    2. 2.2 MSPM0 USB Module
  5. 3MSPM0 USB Hardware Design
    1. 3.1 Block Diagram
    2. 3.2 USB Mode of Operation
      1. 3.2.1 USB Device Mode: Bus Powered
      2. 3.2.2 USB Device Mode: Self-Powered
      3. 3.2.3 USB Host Mode Power Considerations
      4. 3.2.4 ESD Considerations
      5. 3.2.5 Layout Considerations
    3. 3.3 USB Clock Implementation
      1. 3.3.1 Selecting a Clock Source
      2. 3.3.2 Selecting a Clock Frequency
    4. 3.4 Example Implementation
  6. 4Software Overview
    1. 4.1 USB Stacks: Features
    2. 4.2 SysConfig Descriptor Tool
    3. 4.3 Selecting a Device Class
      1. 4.3.1 Example Process for Deciding on a USB Device Class
    4. 4.4 How to Select a Vendor ID (VID) and Product ID (PID)
      1. 4.4.1 Choosing and Obtaining VID and PID
      2. 4.4.2 Using VIDs and PIDs During Development
    5. 4.5 TinyUSB API Programmer's Guide and Examples
  7. 5Getting Started: Evaluating MSPM0 USB
  8. 6Summary
  9. 7References
  10. 8USB Glossary

3.3.1 Selecting a Clock Source

The biggest consideration when selecting a clock source is precision. The USB specification requires the clock have a tolerance of 2500ppm. Sources outside of this specification can cause decreases in consistent performance and fail to verify USB compliance.

There are three choices to source the reference clock:

Table 3-1 Clock Source Differences
Source Frequency Range When to Use?
External Clock Source 4 to 48MHz crystal oscillator, PLL to higher frequencies Crystal oscillators have the best precision and USB compliance, allowing the user to meet USB specification compliance.
Internal Clock Source 4 to 32MHz, PLL to higher frequencies Not typically recommended as the internal clock is often outside of 2500ppm, usable for prototyping.
USBFLL 48 to 60MHz No space for an external crystal, as the device leverages SOF packets for synchronization. Space and cost-constrained designs, however, require an active USB connection for clock synchronization.

If using an external crystal, make sure that the Crystal Oscillator guide is followed for maximum performance.