Can I use PPUGenLatch's value again?

Are you new to 6502, NES, or even programming in general? Post any of your questions here. Remember - the only dumb question is the question that remains unasked.

Moderator: Moderators

Post Reply
thomson
Posts: 12
Joined: Tue Apr 05, 2016 2:28 pm

Can I use PPUGenLatch's value again?

Post by thomson » Tue Apr 05, 2016 2:48 pm

I'm trying to develop nes emulator and I have a question about PPUGenLatch.
I read Wiki (PPU registers),and understood this latch came from capacitance of very long traces.
But I think if we read this value, it should be unset because remaining electric charges will disappear.

Am I wrong ? Can anyone help me?

Thank you.

lidnariq
Posts: 10238
Joined: Sun Apr 13, 2008 11:12 am
Location: Seattle

Re: Can I use PPUGenLatch's value again?

Post by lidnariq » Tue Apr 05, 2016 2:57 pm

They're just capacitors, not core memory. It's not the process of reading that causes the value to be lost: the output is buffered, not connected using an analog mulitplexer.

tepples
Posts: 22278
Joined: Sun Sep 19, 2004 11:12 pm
Location: NE Indiana, USA (NTSC)
Contact:

Re: Can I use PPUGenLatch's value again?

Post by tepples » Tue Apr 05, 2016 3:00 pm

I did some experiments with the phenomenon that FCEUX calls PPUGenLatch about a year ago when I was working on a controller detection routine. I found that reading it 64 times in a row did not unset it. See Riding the open bus.

thomson
Posts: 12
Joined: Tue Apr 05, 2016 2:28 pm

Re: Can I use PPUGenLatch's value again?

Post by thomson » Wed Apr 06, 2016 8:38 am

Thank you for your reply.

I suppose "capacitance of very long traces" means parasitic capacitance.
It can read many times because MOS has high input impedance and it will not decay in PPU cycle.

Is my understanding correct?

lidnariq
Posts: 10238
Joined: Sun Apr 13, 2008 11:12 am
Location: Seattle

Re: Can I use PPUGenLatch's value again?

Post by lidnariq » Wed Apr 06, 2016 10:14 am

Yes.

You can see the relevant bits of metal in the die by looking at Visual2C02.
(Go to the empty box next to the "Find" button and search for _io_db0 through _io_db_7)

thomson
Posts: 12
Joined: Tue Apr 05, 2016 2:28 pm

Re: Can I use PPUGenLatch's value again?

Post by thomson » Wed Apr 06, 2016 10:47 am

Thank you so much.

Post Reply