Lpc2000

> Message: 25        
>    Date: Thu, 17 Nov 2005 09:37:47 -0800
>    From: "Joel Winarske" <joelw@...>
> Subject: RE: LPC214x crystal frequencies
> 
> > To get the 48 MHz necessary for the LPC214x USB, you must 
> use a crystal
> > of 12.0 MHz, 16.0 MHz, or 24.0 MHz.  None of these frequencies are
> > mentioned in the manual table 291 that lists the possible ISP baud
> > rates with various crystal frequencies.  Can you use serial ISP with
> > 12/16/24 MHz, and with which baud rates?
> 
> At 12MHz the highest the Phillips Flash utility will connect 
> is 38.4k baud.
> I cannot connect running a higher baud. 
> 
> Joel

I've been doing serial stuff from some time and this matches my experience.
With an integer-MHz rate, the divisors are such that most all uart chipsets
will work fine up to 38.4k, after that the round-off error is too much for
the chipset to synch up with, which is why you see alot of 7.3828MHz,
14.xxx, and 3.8xx crystals on demo boards.  I do alot of analog->digital
stuff and off-freq's tend to make that nasty (more divisor round-off), so I
just put up with running my uarts a bit slower.  I've noticed alot of
off-shelf devices with serial ports running at 38.4k lately, and I suspect
the same argument applies.  If you grab any of the Atmel AVR datasheets
(i.e. AtMega128) they've done the math for you in the "UART" section to give
exact numbers for %error given a crystal freq and a baudrate/divisor combo.
They state that %err's above 10% are not healthy, but in practice, I've seen
cheap serial ports on Wintel boxes give me garbage with errors as low as 3%.

If you are into that sort of thing, Silicon Lab's CP2102 UART<->USB bridge
will do integer baud rates (i.e. 128.0k instead of 115.2k, up to 1.0M, I
recall) and that can get you up to higher rates if you need to, and give you
usb in a 7mm sq. package.  I think I'm going to go direct to implementing
the usb driver thread that's been going the last few days for new designs
myself...

Steve

Steve Franks, KE7BTE
Electrical & Firmware Engineer
Tucson Embedded Systems
(520) 575-7283 x171
(Opinions expressed are sole-ly that of Mr. Franks, and not Tucson Embedded
System)

I've always thought the American eagle needed a left wing and a right wing.
The right wing would see to it that economic interests had their legitimate
concerns addressed. The left wing would see to it that ordinary people were
included in the bargain. Both would keep the great bird on course. But with
two right wings or two left wings, it's no longer an eagle and it's going to
crash.
    -Bill Moyers