Lpc2000

Anyone have a response to this?

If I use the LPC2148 am I essentially stuck with a 12MHz external 
crystal if I want to use USB and have the uC running at full speed?


--- In lpc2000@yahoogroups.com, "seangra" <sgraham@o...> wrote:
>
> --- In lpc2000@yahoogroups.com, Tom Walsh <tom@o...> wrote:
> 
> > >>I am not an expert in this, but I think the higher the 
multiplier
> > >>(= a lower external crystal freq) the more jitter you get on 
the
> > >>CCLK. So if you want a very stable CCLK you choose the highest 
> > >>supported external crystal frequency you can find.
> 
> > No, not really.  You have to be aware of PPM spec of the crystal.
> > Parts Per Million is a statement of the accuracy of the crystal 
at
> > a given frequency (Million being Million Cyles or MHz, Parts 
being
> > the cycles of drift).  But higher frequency 5PPM is no more / 
less
> > accurate than 5PPM at a lower frequency...  It does compute into 
a
> > percentage of error though.
> 
> That explains the vast difference in price between different 
> crystals all at the same frequency.
>  
> > For me, the primary choice of frequency was that of the
> > bootloader.  14.7456MHz crystal was one of the recommended
> > frequencies and it would give me a 58MHz clockrate for the
> > CPU.  I also have some MAX3100 SPI UARTs on my board, they 
> > needed a 3.686MHz clock, by using an external TTL oscillator 
> > to drive both the X1 input of the processor and drive the  
> > input to a divide by 4 counter (74HC93), I was able to  
> > satisfy the CPU and the UARTs with one clock source.
> 
> Doesn't the 74HC93 cost more than a 2nd crystal?
> 
> Do the serial busses' operational frequency depend on the "input 
> clock" from the external oscillator?  The spec says (for SPI) that 
> it can operate at maximum 1/8th the "input clock" (which I'm 
> assuming is Fosc)?
> 
> I just re-read the user manual (not the spec manual) and if I want 
> to use USB, then pretty much my only choice for external clock is 
> going to be 12MHz if I want full speed, right?  If I want to play, 
> 16MHz for either 48MHz or O/C to 64MHz, or 24MHz for 48Mhz or O/C 
to 
> 72MHz (probably a bad idea!). 
> 
> So if I want max performance out of my serial bus, then I want a 
> higher Fosc, which essentially limits me to a lower CCLK?
> 
> Sorry for the newb questions.
> 
> 
> > >Original question:
> > >
> > >>Assuming that I want to be able run at maximum speed, what 
> > >>oscillator/crystal should I choose?
> > >>
> > >>Normally the choice is dictated by the divider for the UART, 
> > >>but in this case (from what I understand) with the prescaler 
you
> > >>can get just about any baud rate you want from whatever 
> > >>source frequency you choose.
> > >>
> > >>So if 10*6 or 20*3 or 15*4 all yield 60MHz, what factors are 
> > >>there that influence which frequency should be used?
> > >>    
> > >> (For an LPC2148)
>