puzzling Nintendo 64 question

timmeh87 · Post by **timmeh87** » Fri Jan 09, 2009 12:37 am

First person to satisfactorily explain this wins a prize

Hint: middle trace is ground?

Ben Cebhrem · Post by **Ben Cebhrem** » Fri Jan 09, 2009 12:53 am

What do you want o know about it? Why the ground is middle, and it's split in two? Why are the two traces so large?

As far as the size and curious placement, I would have to assume it's meant to be a low impedance trace. Any ideas what the pin is for?

EDIT: It goes through R24 to VTERM. Value of R24 is 25 ohms.

timmeh87 · Post by **timmeh87** » Fri Jan 09, 2009 12:55 am

Yes, all of those things and whatever else.

I want to build my own PCB so Its sort of important. The traces goes to some passive component on the other side but im not sure what.

But yeah look... it splits and goes back on itself. why is that? why put the middle trace there?...

Its beside the ram so I bet it has to do with that...

Ben Cebhrem · Post by **Ben Cebhrem** » Fri Jan 09, 2009 1:03 am

Aye, they split it with a ground in between (I am assuming) in order to give it a low impedance signal. Something like those torrodial coils you see on power supply plugs, or if you prefer, those neat rings with wire lazily wound around them on your motherboard (I think they still do that at any rate...). This would make the trace relatively "clean" of noise and interference, so it might have something to do with the RAM, the video, audio... or it might even be a clock reference or voltage reference line.

Sorry I'm not of much help here in that reference- but until I traced it myself and understood electronics a good deal, I can only offer what little I have.

EDIT: After tracing it a bit, the clock goes into an IC, through some passive components (resistors, capacitors) and then onto VTERM. Let me check again here, and aggravate the hell out of people with constantly checking, and editing.

EDIT2: Jerk.

It goes through pin 3 of U15, then through R32- so I'm guessing it's a clock reference line. Somebody who knows electronics better than my poor self ought to slap me for making suggestions, or applaud me for guessing well out of my arse.

timmeh87 · Post by **timmeh87** » Fri Jan 09, 2009 1:05 am

Ill scope it one of these days

bacteria · Post by **bacteria** » Fri Jan 09, 2009 2:58 am

Perhaps an error in the software they used to make the board design??

Kyo · Post by **Kyo** » Fri Jan 09, 2009 4:07 am

I doubt that

timmeh87 · Post by **timmeh87** » Fri Jan 09, 2009 1:38 pm

Agree, after staring at a design like this for a year of your life you can see it with your eyes closed. Heck, even Im starting to memorize parts of it.

Its very deliberate, they used two vias in making this feature.

Whats Vterm? I think its Vref thats been voltage-divided and low-pass filtered?

Ben Cebhrem · Post by **Ben Cebhrem** » Fri Jan 09, 2009 3:59 pm

No idea what VTERM stands for

Some google searching and what not- U15 is an MX8330 chip, which means it is a RAMBUS clock generator. A datasheet is located here; I have no reasons to think this is a 'correct' or 'incorrect' datasheet, as I don't even have the knowledge to explain how a transistor works (much less build a circuit). Based on what little knowledge we do have etc, it definitely seems quite likely that's a clock reference line for one of the CPUs in regards to the RAMBUS (so it knows when to send/receive data). Having a clean source of that clock is then crucial for proper operation of the N64 (less garbage, won't crash after 10 minutes of mistimed information packet exchanges, and so on).

I can then infer that the RAMBUS is non-ECC type, though it'd be best to find out by looking at the chips themselves.

TL;DR- It's a clock reference line for RAMBUS. It's basically shielded from interference for better operation.

Post by **marshallh** » Fri Jan 09, 2009 11:00 pm

I believe the Rambus chips were ECC-capable, however the 9th bit was mainly used for pixel coverage data when rendering anti-aliased triangles to the framebuffer. Neat trick.