MCF52259 USB Module Clock

[eGuy]

Hi

If I want to use USB module (as HOST ) in 52259, the USB clock MUST be 48MHz, right? I saw this in the reference manual, but could not find the reason.

If I disable PLL, in order to generate all the component clocks I need (UART, USB, Ethnet etc), could I use external 16MHz oscillator, or I must use 48Mhz oscillator.

Basically, my question/problem is:

In my coldfire 52259 board, it has 16Mhz oscillator. If I want to disable PLL, could I generate all component clocks I need?  

Regards,

Message Edited by eGuy on 2009-07-20 11:40 AM

[RichTestardi]

Yes, the USB clock must be 48MHz to meet USB timing requirements.

See:   http://forums.freescale.com/freescale/board/message?board.id=CFCOMM&message.id=7107&query.id=25162#M...

So I think you are left with either having a 48MHz timing source and feeding USB directly from it (and the CPU potentially from the PLL, faster or slower), or having some other timing source and PLL'ing it to 48MHz for both the USB and CPU.

Why do you want to disable the PLL?

BTW, figure 7-1 in the 52259 Reference Manual is really helpful in understanding its clock options -- I am glad it was included again!

-- Rich

[eGuy]

Rich, 

In Reference Manual, 21.1.3,  it says that mininum internal bus clock for the ethernet FEC is 50MHz.

So if I use external 48Mhz oscillator, without PLL, I can't get needed clock for FEC. Right?

Regards,

J.

[GaryOlmstead]

eGuy-

Well, without the PLL, that's rignt.  With the PLL, it's easy.  Why don't you like the PLL?

Gary

Message Edited by Gary Olmstead on 2009-07-23 09:00 PM

[eGuy]

I knew that using PLL can give the >50Mhz clock.

As in my previous post, we want to disable PLL in our application. (to avoid any EMI issues. The product I am working on is very strict on EMI)

Without enable PLL, how can I get all needed clock (USB+Ethernet+UART....)? ----This is my original question.

Or, FEC ethernet module will work correctly in 48Mhz? 

Regards,

J.S

Message Edited by eGuy on 2009-07-23 04:13 PM

[RichTestardi]

Sorry, I don't know about the FEC clock requirements...

If you're disabling the PLL for EMI, I assume you've already done things like shut off the external clock pins?  That had a lot of effect for me (though I only verified it on the '21, not on the '59).  I run the external clocks only when I'm connected to the debugger, and not when I boot standalone...  (I use a run-time check on a register that is set by the cfg file to detect if the debugger is attached -- you can see how I do it in the skeleton.zip archive at the bottom of:  

http://www.cpustick.com/downloads.htm -- search in startup.c for "debugger_attached", and also look in the debugger cfg files.)

-- Rich

[eGuy]

Rich,

Thanks for your clarification.

Regards,

J.