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PostPosted: Wed Jun 29, 2016 11:57 am 
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The problem with trying to implement the time delay in analog is that the SNES /RESET already relies on a large (1k) resistor in series, and any other parts you add could adversely affect that.

Also it's tricky to get consistent RC time constants that slow. But I guess you could try R·C = 2sec, R≟1MΩ → C=2µF. I'd also be worried about designing something before you could test it. And at this small of a current you'll definitely have to use 74HC/AC/&c instead of 74LS/&c.


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PostPosted: Fri Jul 08, 2016 8:32 am 
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lidnariq wrote:
The problem with trying to implement the time delay in analog is that the SNES /RESET already relies on a large (1k) resistor in series, and any other parts you add could adversely affect that.


So what would the alternative to analog be? A microcontroller? I'm assuming a microcontroller could probably replace the 74'74 and add the delay, but programming for one is out of my league for now. The hold-reset-to-switch thing would be a nice touch, but I think I'll just keep it simple. The two games that I'm going to be switching between aren't actually games, they're utilities. So aside from it just being neat, there's no real point to go out of my way to try and implement the delay.

lidnariq wrote:
I'd also be worried about designing something before you could test it.


I'm definitely going to do a test build before I order the boards. I knew OSH Park was $5/sq-in, but I guess I wasn't thinking that even these small boards would be ~$30 each. And you have to buy three, so, ~$90!


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PostPosted: Fri Jul 08, 2016 11:11 am 
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Ziggy587 wrote:
So what would the alternative to analog be?
Just electrically isolating the two signals using an inverter or buffer, so that the time delay part isn't drawing any current from the SNES/RESET line.

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I'm definitely going to do a test build before I order the boards. I knew OSH Park was $5/sq-in, but I guess I wasn't thinking that even these small boards would be ~$30 each. And you have to buy three, so, ~$90!
Ouch. That's like early-2000s board house prices.

Have you considered one of the cheap chinese vendors like Seeed? 10 boards, ~$20 shipped, takes a month to get to you. ( http://pcbshopper.com/ )


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PostPosted: Fri Jul 08, 2016 12:05 pm 
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lidnariq wrote:
Ouch. That's like early-2000s board house prices.


I was thinking about shopping around a little, especially since this will be my first board order, but I like what I'm seeing on OSH Park so far. Their prices include shipping, they ship in 12 days, and they're USA based (as am I) so I'll get them very quickly. They also have a $1/sq-in service, but it's a 150 square inch minimum.

edit: Hmm, I just put in the dimensions on PCB Shopper and it's showing that OSH Park is $30 for three boards ($10/ea) so maybe I misread something. Either that or PCB Shopper is wrong. OSH Park states "$5/sq-in for a set of three boards" which I interpreted as:

6 square inch × $5 × 3 = $90.

Seeed and others on PCB Shopper are showing some pretty hefty shipping charges and lead times, although I do realize those are just estimates.


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PostPosted: Fri Jul 08, 2016 1:11 pm 
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Ziggy587 wrote:
OSH Park is $30 for three boards ($10/ea) so maybe I misread something. Either that or PCB Shopper is wrong. OSH Park states "$5/sq-in for a set of three boards" which I interpreted as:

6 square inch × $5 × 3 = $90.
I'm pretty certain they mean "5$/in² of design, three instances"

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Seeed and others on PCB Shopper are showing some pretty hefty shipping charges and lead times,
Yeah, the shipping costs are noticeable but the PCBs are inexpensive, so it works out.

Although I'm a little surprised you're seeing higher prices; the lowest I'm seeing Maker Studio with $9.5 for 10× 10cm×5cm, $5.64 shipping, ETA 20 days.

Maybe the problem you're running in to is that the cheap board houses have a price point right at 5cm/10cm on each axis?


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PostPosted: Fri Jul 08, 2016 1:45 pm 
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lidnariq wrote:
I'm pretty certain they mean "5$/in² of design, three instances"


Looks like you're right. Elsewhere on the site they say, "For example, a 2 square inch board would cost $10 and you’d get three copies of your board." I guess I'm too used to ULINE, where you multiply the price each by the minimum quantity.

lidnariq wrote:
Although I'm a little surprised you're seeing higher prices; the lowest I'm seeing Maker Studio with $9.5 for 10× 10cm×5cm, $5.64 shipping, ETA 20 days.

Maybe the problem you're running in to is that the cheap board houses have a price point right at 5cm/10cm on each axis?


I was searching 4×1.5 inches, so ~10.16×3.8cm (rough estimate of one of my boards). I'm also searching with and without ENIG finish and double sided silk screen options as OSH Park includes both of those in their price.

I'd be funny if the threshold is 10cm, and I'm going over by a lousy 1.6mm. I don't think I can shrink the width any without messing up the key shape.


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PostPosted: Wed Jul 20, 2016 8:00 am 
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I have a question regarding the 74'161. I made a test build with IC sockets so I could easily swap things around. I wired a 74'161 like this:

Attachment:
File comment: http://www.sega-16.com/forum/showthread.php?28939-Guide-Basic-Genesis-Multicart
74161.JPG
74161.JPG [ 46.17 KiB | Viewed 1040 times ]


Everything is working fine, but I'm wondering if it's OK to leave the unused pins floating in the final design. Should I tie any of them high or low?


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PostPosted: Wed Jul 20, 2016 10:28 am 
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Unused outputs should float, but don't leave inputs floating unless they're specifically designed for that. They can pick up noise that causes the circuit to misbehave or use excessive power. An input designed to float will have an internal pull-up or pull-down resistor; check the datasheet.


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PostPosted: Wed Jul 20, 2016 11:17 am 
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In this specific case, the only inputs you've left floating are clocked inputs, which are mostly safe to leave floating.

Pedantically: this is not true of latched inputs. It is only true of ones that only do a thing in response to a rising or falling edge on another pin.


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PostPosted: Wed Jul 20, 2016 11:57 am 
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I was under the impression that even a latched input could lead to metastabilty in the latch if it's right at the crossover point when it's latched. Am I wrong?

You don't want Buridan's ass to kick yours.


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PostPosted: Thu Jul 21, 2016 8:10 am 
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The way my board is laid out the inputs on the '161 are right next to a ground plane, so I suppose I have no reason not to tie them all low. From what I've read, some people say to use a resistor to ground while others say either a resistor or directly to ground is OK. So my question is, is there an upside to using a resistor when I'm connecting unused inputs to ground?


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PostPosted: Thu Jul 21, 2016 8:31 am 
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Use a resistor when the pin is bidirectional (both input and output), like the data pins of a RAM. You can use a resistor for an input-only pin, but it'll save money not to.


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PostPosted: Fri Jul 22, 2016 7:00 am 
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It'll save some room on the board as well.

Thanks, everyone, for your help!


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PostPosted: Fri Jul 22, 2016 2:35 pm 
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Bregalad wrote:
You can even switch games during gameplay and get... somewhat unexpected effects. With the SNES I have no idea how this'll turn out, but on the NES I was able to get a game boot with different palette for instance.


With a Pac-Man PCB set up to dual boot Ms. Pac-Man, switching in the middle of the game is absolutely fine. Ms. Pac-Man is a binary hack of Pac-Man, so you get dropped into a fully working gamestate with slightly different logic.


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PostPosted: Wed May 17, 2017 9:49 am 
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Hey guys. I know this topic was from about a year ago, but I wanted to share my success and say thanks!

Image

Image

I was planning on making something special for the Racketboy Charity Auction, but since the forum participation has been dwindling the yearly auction didn't occur as it normally would have in past years. That being said, I stalled my work on this project for some time. But I would like to finally finish it so I can share it.

I have it shared on OSH Park right now ( https://oshpark.com/shared_projects/Ptrw5hb8 ). I still want to tweak it a little more before I share it properly, but this current design is 100% functional from my testing. I was also thinking about making a through hole version, as it seems like a lot of people complain about surface mount stuff.

I know the PCB thickness from OSH Park is off spec, but it was good enough for me to do a test build. I wouldn't recommend ordering the PCB from there because of this, but you can download the .brd from there.

I would appreciate any feedback, especially negative, in regards to the board layout as this was my first. The OSH Park link has nice high res pics of the front and back of the board, but I thought they would be too large to post on the forum. I'll resize them to fit properly when I get the chance.

edit: Resized the pics:

Image

Image

I also wanted to say thank you to everyone that helped me! This forum has been a great resource for me. Being that I only do this as a hobby, I don't know a whole lot about this kind of stuff. It's always a little intimidating asking for help when you barely know the questions you need to be asking, but everyone here has always been very kind and patient with my ignorance. So... Thanks, NESdev!


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