Lpc2000

--- 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)