> If it detects Sonic the Hedgehog 2, a signal is generated on the ROM's D0 line, setting the Q bit of the 74HC74.
This is missing some important information. One input to the flip flop is D0 on the ROM, yes, but it only sets when there is a clock signal. Otherwise there would be some very interesting coding constraints.
The clock of the flip flop is connected to cartridge pin B31. Here's the best explanation I can find of that pin:
'/TIME' is the schematic label given to pin B31 on the cartridge slot. It is essentially a chip enable for the I/O port range $A130Fx, and is used for programming on-cart memory mappers. The only games I know of that use it are Beyond Oasis, Phantasy Star 4 (both for switching between ROM and SRAM at $200000-3FFFFF) and Super Street Fighter 2 (for a full-blown ROM banking chip). I don't know why it's called '/TIME'; perhaps they expected to connect an RTC chip to it.
So if you write to certain addresses, the genesis will flicker that pin, which is great for controlling an extra chip like this. (I mean, it's meant to control an extra chip, but it seems like the plan is a significantly smarter one than a flip flop.)
Hi, I'm the original author of that stack exchange answer. I'd be more than happy to add your information with full attribution.
Thanks for your explanation. I'm a programmer but don't have a strong electronics background. Clock signals, those chip enable signals and how they interact were difficult for me, especially at the time I wrote that.
Edit: I just added a full citation for your comment. Thanks again!
This is missing some important information. One input to the flip flop is D0 on the ROM, yes, but it only sets when there is a clock signal. Otherwise there would be some very interesting coding constraints.
The clock of the flip flop is connected to cartridge pin B31. Here's the best explanation I can find of that pin:
'/TIME' is the schematic label given to pin B31 on the cartridge slot. It is essentially a chip enable for the I/O port range $A130Fx, and is used for programming on-cart memory mappers. The only games I know of that use it are Beyond Oasis, Phantasy Star 4 (both for switching between ROM and SRAM at $200000-3FFFFF) and Super Street Fighter 2 (for a full-blown ROM banking chip). I don't know why it's called '/TIME'; perhaps they expected to connect an RTC chip to it.
So if you write to certain addresses, the genesis will flicker that pin, which is great for controlling an extra chip like this. (I mean, it's meant to control an extra chip, but it seems like the plan is a significantly smarter one than a flip flop.)