Path: utzoo!attcan!uunet!isc-br!jimc
From: jimc@isc-br.ISCS.COM (Jim Cathey)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Serial port question
Message-ID: <2661@isc-br.ISCS.COM>
Date: 22 Nov 89 03:45:16 GMT
References: <1989Nov16.190032.13720@aucs.uucp>
Distribution: na
Organization: ISC Systems Corporation, Spokane WA
Lines: 28

In article <1989Nov16.190032.13720@aucs.uucp> peter@aucs.uucp (Peter Steele) writes:
>Does anyone know how to initialize the Mac serial driver to send at
>38400 baud? The contants are available to initalize it to 57600 and
>19200 baud but not 38400. Any reason why? I think that Microphone can
>send at 38400 so its probably possible.

According to IM volumes II & III, and Zilog's SCC manual, a constant of
1 will send at 38,250 baud.  The formula is:

sclk = 1/(3,672,000 / 16)
baud = 1/(2 * (CONSTANT + 2) * sclk)

(0 is 57375, 1 is 38250, 2 is 28688, 3 is 22950, 4 is 19125, ... 380 is 300).

The constant is dropped directly into the SCC's time constant register.

Just looking at the numbers, it looks like they used one of those
drop-a-half-cycle-every-eight-full-cycles (multiply by 15/16) counter
stages to get 3.672 MHz out of a 15.6672 MHz crystal.

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