Objective Development Forums

Hi
I have some general questions about the USB. I saw the example such as easylogger. The questions are
why do you choose the frequency of 16.5M?
How do you calculate this frequency? Is this based on the USB speed of 12Mbps?
If I want to use external crystal, which frequency should I choose?

I tried to make the example of esaylogger. But it does not work well. I want to find the reason, and understand the basic principle.

I forgot to say
Thanks in advance for the help!!!

A low speed USB device sends with a bit clock of 1.5 MHz. The driver is easier to implement if the CPU clock is a multiple of that.

For crystal based designs, I would recommend 12 MHz as this is the oldest and therefore best tested implementation. But you can also use 15 or 16 MHz crystals, as long as you configure the driver for this frequency.

If you want to use the internal RC oscillator, the driver must also implement a synchronization mechanism (Phase Locked Loop). We have done that for 16.5 MHz only (this is a multiple of 1.5 MHz).

Thank you for the reply!!

By the way, do you plan to add 18 and 20 MHz routines? (Actually those could be probably made of the 12/15Mhz and 16MHz by adding a couple of nop's, if I am right...?)

We may add a 20 MHz module, but I don't see a demand for 18 MHz.

It may be good for overclocking mega8's and similar...