Weird Clocks And A Two Chip Apple I

The Apple I, [Woz]’s original, had about sixty chips on a single board. Most of these chips were logic glue or hilariously ancient DRAMs. The real work was done by the 6502, the¬†6821 PIA, and the Signetics video chip. It’s a simple computer, really, and following the now popular tradition of two-chip computers, [Dave] built a replica of the Apple I using a 6502 and an ATMega.

The ATMega in this project takes care of everything – the 4k of RAM, the few bytes of ROM, the IO, and even the clock. With the 6502 you can have a little bit of fun with the clock; because the 6502 reads data off the bus a few nanoseconds off the falling edge of the clock and writes on the rising edge, [Dave] played around with the duty cycle of the clock to give the ATMega a bit more time to do its thing. With a 50% duty cycle, the 16Mhz ‘Mega has about eight cycles to decode an address and read or write some data. By making the low part of a clock cycle longer, he has about 45 cycles on the ‘Mega to do all the work. All of this was inspired by a fantastic tutorial on the 6502 clock.

Right now [Dave] has some hex values displaying on a small LCD, while the real I/O is handled by a serial connection to a computer. It’s retro enough, and a future update will include a faux cassette interface, possibly using an SD card for storage.