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.
Crystal-less USB devices eliminate the need and additional cost incurred for
an external crystal! Check out Kinetis
K64, all of which support crystal-less, full-speed USB.
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:
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
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
Justin Mortimer is a Kinetis product owner.