Everywhere you look there is another USB enabled device. It all started years ago with the personal computer. But now they are in every device we own, our phones, cars, picture frames, power outlets, televisions, the list goes on and on, but how does it even work? Well, we won’t get into the gritty details, but we will talk a little about crystals.
First some background. In full-speed mode, a USB peripheral operates at a clock frequency of 48 MHz and requires an extremely accurate timing element, which generally means an external crystal, but could also be a ceramic resonator or an integrated RC oscillator. Although each option has trade-offs, the important points are that the frequency of the timing element is a multiple of 48 MHz and has a very tight tolerance, within 0.25%.
But what if you don’t want to buy a crystal? They cost money and take up important real estate on your board? Well, here’s an excellent option: Crystal-less USB devices.
How we make it work. The integration of a 48 MHz internal reference clock (IRC), along with a supporting USB clock recovery circuit. Although the IRC is factory trimmed, the clock frequency can still vary as much as +/- 1.5%, obviously too high for guaranteed USB operation. The problem is solved through the use of the USB clock recovery circuit, bringing the accuracy of the timing element to within the requirements of USB operation. For additional technical support, please take a look at our Application Note: AN4905.
Striving to innovate. Kinetis is our broad portfolio of ARM based microcontrollers, developed to suit your various needs. We have general purpose products that balance Performance and Integration, as well as those that focus on Ultra-Low Power to Motor Control, among many others.
Within the Kinetis portfolio, we have a number of USB enabled devices, some with USB certification, but all ranging from Full Speed (FS) to High Speed (HS), including a wide range of supporting functionality.
We try to continually innovate and incorporate features that provide our customers with additional value, and from our perspective, crystal-less USB is definitely one of these differentiators. Helping to reduce power, lower cost and save valuable space, check out the crystal-less USB feature found on Kinetis K22, K24, K63, and K64!
Justin Mortimer is a Kinetis product owner.