David Wilson wrote:
Alex Freed wrote:Here is a question to anyone who programmed a Mockingboard: why were 6522 (VIA) chips used in the design?Until I lay my hands on a copy of the AY-8910 datasheet this is pure speculation - perhaps the setup/hold times for the chip are not met by the /DEVSEL signal and it was easier to interface to it using a VIA to generate chip select, read (and write?) signals with latched data on the other VIA port?
Actually, because of the way that the Apple generates the clock phases, many 65xx series support chips will have setup/hold time problems unless the ph1 clock signal is delayed somewhat. In the Mockingboard, this was accomplished with an inverter chain and an RC network. The 6522 was probably used because it has timers, one of which is used to generate interrupts to "pace" the reloads of AY registers. It has the happy consequence that the 6522's provide a lot of capability that can be used for other things. ;-) -michael New, faster SUDOKU v2.0 solver for Apple II's! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it's seriously underused."