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PostPosted: Tue Apr 24, 2018 4:13 pm 
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Hi.
After discovering that using resistor networks to do level convertion can be reasonably safe and not wanting to "kidnap" that thread, I'm writting this to ask for advice on this.
Since I have a couple of games with 3v memories (and (un)fortunatelly am receiving some more as a gift), I think this can be an easier and quicker mod I could do on these games to make them safer.

What resistor values are fine?
Should they be pull up/down or the data will pass across them?
Thanks in advance.


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PostPosted: Tue Apr 24, 2018 6:09 pm 
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You should be able to get by with voltage dividers on the address & control lines since they're unidirectional. I don't really have a suggestion on bidirectional data lines though. Series resistors will limit the current which is better than unlimited, but the ideal value will depend on drive strength of the console and cart.

You may want to come up with a game genie type device that actually includes proper level shifters, or play on a clone console that runs close to 3v to get around the issue.

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PostPosted: Fri May 04, 2018 4:08 am 
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I've heard that using an HC part like an 74HC244 can make the conversion.
Is it true?


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PostPosted: Fri May 04, 2018 6:53 am 
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LVC


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PostPosted: Fri May 04, 2018 9:01 am 
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Fisher wrote:
I've heard that using an HC part like an 74HC244 can make the conversion.
Is it true?

All the info you need to determine this is spelled out in the datasheet if you know what you're looking for.

Standard HC family devices are not 5v tolerant when supplied by 3.3v. If you look at the Vi (input pin max voltage) it's likely Vcc or Vcc+0.5v. So if you supply Vcc with 3.3v, you'll be exceeding the max input voltage if you apply 5v.

LVC families are generally good for level shifting but are slightly less versatile gate because they can't be supplied by 5v. They will have a Vcc range typically of 1.6-3.6v and Vi max spec of 5.5v. So these can be supplied by 3.3v and have 5v signals applied to their inputs. Their outputs will be relative to the Vcc supply voltage, looking at Vo max it's likely Vcc. So supplied with 3.3v the output pins will be Vcc or less, so you can safely use their outputs to drive 3.3v flash/logic/etc.

AHC is a pretty convenient series because it can be supplied by 3-5v, and is 5v tolerant when supplied by 3v. Vcc is allowed between 2-5.5v, and Vi max is 5.5v. Where HC max Vi is depended on Vcc, AHC Vi max is independent of Vcc. Personally I really like to use these gates because I only have to buy one part in bulk and can utilize it in various designs. It can be supplied by 5v (unlike LVC) so I can use them for a design that's entirely 5v like NES UNROM mapper perhaps. But they can also be used as level shifters when supplied with 3.3v, the Vi max is still 5.5v. Not all devices are offered in AHC series though, but my most commonly used 4x OR gate is availabe with 74AHC32.

There are other families which are good for level shifting, but it comes down to what options are available for your desired logic function.

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PostPosted: Sat May 05, 2018 1:45 am 
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Wouldn't be my first choice, but you probably could use certain 74HC parts, always check the datasheet for the specific chip in question. This quote is from NXP's datasheet for their 74HC245:
Code:
Inputs include clamp diodes. This enables the use of current limiting resistors to interface inputs to voltages in excess of VCC


But it's usually going to be easier to just use LVC, AHC, VHC, LCX, LVT, etc.

As an aside about LVC, when you look into Picogate parts like LVC1G, 2G, etc. you'll find ones that can be powered at 5V. I'm using one like that in my controller port USB peripheral, but I selected one for its partial power-down support.


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