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From: liber@reed.UUCP (Peter Liberman)
Newsgroups: net.micro.mac
Subject: External Clocking
Message-ID: <1427@reed.UUCP>
Date: Tue, 30-Apr-85 23:42:15 EDT
Article-I.D.: reed.1427
Posted: Tue Apr 30 23:42:15 1985
Date-Received: Thu, 2-May-85 02:20:56 EDT
References: <6240@ucbvax.ARPA>
Reply-To: liber@reed.UUCP (Peter Liberman)
Distribution: net
Organization: Reed College, Portland, Oregon
Lines: 21

Requesting advice on how to clock the SCC externally...

I need to get an unusual baud rate for async. serial communication
(31.25K for MIDI). Since I need at least the 16x clock to receive
reliably, I figure that my only chance is an external clock (500KHz/16
=31.25K).

I have read the SCC data sheet, but it is not the most self-explanatory
document ever written.  As Kevin Karplus noted on net.info-mac a while back,
register 11 seems to be the crucial one--to set the transmit and receive clock
to the TRxC pin the value is $28.

But it seems that no matter how R11 is set (the SCC otherwise configured
conventionally by Apple's async RAM driver), the external clock (at pin 7 of
port A) disrupts my program whenever the disk drive starts up (by preventing
it from stopping), and by freezing the mouse.

I assume that some other register needs to be set to disable the interrupts or
handshaking which is responsible.  Any ideas?

thanks, Peter Liberman, %Reed College box 1087, Portland Snoregon 97202