>> If so, it is not a VIA bug but a PET software bug. >Then why don't the VIC-20 and 1540 use the serial register for transferring >data? Because the two issues are unrelated. The VIC20 + 1540 shift register issue is caused by a bug in shift in mode using an external clock, when the clock coincided with the o2 clock pulse. There was an official publication from MOS that included a hardware modification that could have been used on the VIC20. I can't find that but http://forum.6502.org/viewtopic.php?f=4&t=2175 has a similar description and circuit. The problem is also mentioned on page 14 of http://archive.6502.org/datasheets/synertek_sy6522.pdf The PET sound + tape loading is because sounds puts the shift register into shift out using a timer mode & tape uses the same timer for tape loading. There would always be a conflict as the tape load routines need to have the timer set to a specific value (which will mean the sound would always be the same), but the timer also works differently when it's in that mode. Jim Butterfield knew there was a problem on the PET and because the VIC problem involved the magic phrase "shift register" it was obvious to him that it was the same problem. > We early PET/CBM freaks knew, from playing music, that there was > something >wrong with the 6522's shift register: it interfered with other functions. >The rule was: turn off the music before you start the tape! (The shift >register was a popular sound generator). But the Commodore engineers, >who only made the chip, didn't know this. Until they got into final >checkout of the VIC-20. It was only obvious to him because he didn't understand the cause of either problem, only that it involved the shift register. Message was sent through the cbm-hackers mailing listReceived on 2014-07-09 09:00:02
Archive generated by hypermail 2.2.0.