Min. clock speed requirement

[vreg]

Hi,
I wanted to know why a certain component might require some minimum clock frequency in order to operate.
For example, I read somewhere that
when the USB interface is used in an application, the clock input must be at least 18 MHz in
order to support internal operations of the USB block...

 

[KlausST]

Hi,

this are only fragments...

Are you talking about a certain USB interface chip or a microcontroller?

If microcontroller:
The processing speed of microcontrollers depend on clock frequency.
The microcontroller needs to answer the incoming USB request within a limited time.
If it´s clock speed is too low it can´t react within the time and communication fails.



Klaus

 

[vreg]

I get your point..

But the USB block here is an on-chip peripheral of the microcontroller (LPC23xx), so shouldn't the clock to the USB block be same as the CPU clock and therefore the said situation shouldn't arise.

Did I miss something here?

 

[KlausST]

Hi,

yes and no.
Even the on chip periferal needs a number of clock cycles to react on the incoming data.

As long as the USB periferal has no PLL to synchronize to the USB data rate it needs a minimum clock frequency.

***
Additionally the USB periferal usually needs the microcontroller to communicate, within a maximum delay time.


Klaus

 

[Ishaan Karnik]

6 MHz - Low Speed USB
12 MHz - Full Speed USB
16 MHz - Full Speed USB (some implementation)
24 MHz (x20 PLL) - USB 2.0

However this speeds may vary with respect to hardware implementation of USB Phy