liber@reed.UUCP (Peter Liberman) (05/01/85)
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