>I'm using datasheet DS39632B.  Table 28-5 on page 372 ("USB voltage
>regulator specifications") lists C sub USB as parameter D324. The
>minimum value is 220 nF. No typical or max value is listed. The
>comment on this parameter is "Must hold sufficient charge for peak
>load with minimal voltage drop"
>
>Regards, Bob

Since the USB voltage regulator is a LDO (Low Dropout Regulator),
normally the capacitance and the ESR of the capacitor will
affect the stability of the LDO.

The following article may be a good reference.
http://www.national.com/nationaledge/jul02/article.html

I will suspect that the output capacitor for Vusb 
should be okay if it is higher than 220nF as long as the ESR 
is not so big. Therefore most ceramic capacitor (probably less 
than 4u7 to lower than cost) or Tantalum capacitor (lower value
is generally cheaper, e.g. less than 6u8) with capacitance bigger
than 220nF will be okay. Apparently low ESR is okay in this case 
even though some older LDOs have problems with ceramic capacitors. 
Take note that Vusb is not meant to drive high current.

The sentence in DS39632B "An external 220nF (+/-20%) capacitor
is required for stability" is misleading. I guess that the
author mentioned +/-20% to mean that the tolerance of the
capacitor is not so important.

According to the Microchip Forum posts, some of the most
of encountered problem with USB enumeration problem are
the following:
1) wrong configuration words
2) missing Vusb capacitor
3) lousy Vdd power supply filtering
4) wrong firmware

Regards,
Xiaofan
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist