From: "Michael F. Coyle"
Subject: Re: RS232 newbie - in need of assistance!
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Fri, 03 Jan 2003 23:56:08 GMT
NNTP-Posting-Date: Fri, 03 Jan 2003 18:56:08 EST
Organization: Optimum Online
"Matt Foster" wrote in message
> Jusging by around page 20 of
> Do i need 5.068?
More like page 73 (Sec 12.1)
I wouldn't recommend 5.0688. (Actual value, Microchip rouns off a bit.)
Without going into detail about Baud Rate Generator (BRG) design, each BRG
is optimized to use a certain frequency oscillator. The PC uses 1.8432 MHz.
Other designs use 5.0688. Other values are used, too.
Unfortunately, 5.0688 doesn't work well with the PIC BRG. You can't get the
exact baud rates you need -- they are off by about 3%. While many systems
will work with this amount of error, why risk it when you can generate the
exact values you need?
The PIC BRG works well with 2.4576 MHz, 3.6864 MHz and 4.915 MHz. These
frequencies will allow the BRG to be programmed to give the standard baud
rates *exactly*. Why settle for less?
My preference from the above values is 3.6864 because it alows use of 4 MHz
PICs at almost their maximum rated speed.
IMO, 5.0688 MHz should not be on the data sheet, except perhaps as an
example of what not to pick.
Painful amounts of detail available upon request.