NC pins - connect to GND or VCC

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Kayumba
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Joined: Sun Mar 01, 2020 2:36 am

NC pins - connect to GND or VCC

Post by Kayumba »

Hi,

I looking for to make universal PCB that the size of flash Eprom like 29lv800 / 29lv160 / 29lv320 can be switched by jumpers.
The difference between those chips is that some of adress pins are not used (NC).
Can you tell me if NC pins should be better to connect them to GND or VCC ?

Thanks
lidnariq
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Re: NC pins - connect to GND or VCC

Post by lidnariq »

In general, it doesn't matter. However, you should look at the datasheets for every part you're planning on using and make sure that none of them are "Do Not Connect"
Kayumba
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Re: NC pins - connect to GND or VCC

Post by Kayumba »

lidnariq wrote: Sun Jun 06, 2021 3:22 pm In general, it doesn't matter. However, you should look at the datasheets for every part you're planning on using and make sure that none of them are "Do Not Connect"
So it better to connect them to GND or VCC vs leave it « not connected »
lidnariq
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Re: NC pins - connect to GND or VCC

Post by lidnariq »

In general, it won't hurt to tie a pin to fixed high or low. But you should check all the relevant parts you want to use and make sure those pins don't have more specific requirements.
Kayumba
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Re: NC pins - connect to GND or VCC

Post by Kayumba »

lidnariq wrote: Sun Jun 06, 2021 3:37 pm In general, it won't hurt to tie a pin to fixed high or low. But you should check all the relevant parts you want to use and make sure those pins don't have more specific requirements.
Thanks 🙏.

One more question,
For example :
For LoROM, Sram decoder 74’139 , pin 13 are connected to A20 (SNES cartridge edge 46).
If i will use 8mbit rom with flash Eprom mx29lv800, that dont have adresse pin A20,
Will decoder work’s ?
lidnariq
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Re: NC pins - connect to GND or VCC

Post by lidnariq »

Despite the simple names ("mode 20" or "lorom") there are multiple different layouts by that name. Per nocash's documentation:
https://problemkaputt.de/fullsnes.htm#s ... d1500games
some "LoROM" boards only support 1MB of ROM, some 2MB, and some 4MB.
Whether any given game will work is complicated.
This is why BSNES uses a manifest file instead of just relying on the internal headers.
But most of the time, they're interchangeable—hence why SNES9x and ZSNES only rely on the internal headers.
Kayumba
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Re: NC pins - connect to GND or VCC

Post by Kayumba »

lidnariq wrote: Sun Jun 06, 2021 3:37 pm In general, it won't hurt to tie a pin to fixed high or low. But you should check all the relevant parts you want to use and make sure those pins don't have more specific requirements.
How those pins can have more specific requirements if they are NC by datasheet ?
What I mean is, those pins can be used even if in datasheet its NC ?

I have one Chinese LoROM PCB with 16Mbit Flash eprom.
So, normally, when I look at in datasheet, pins 10 and 13 are NC .
But when I beep it, I see that it connected SNES Cartridge edge .
Do that mean that they should be connected even if its NC in datasheet cause those pins are used by other parts (like SRAM decoder etc) ?
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Quietust
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Re: NC pins - connect to GND or VCC

Post by Quietust »

More specifically, a pin marked as "NC" could be one of several things:

1. A pin that isn't connected to the internals of the chip at all, but exists solely because of the chip's physical form factor (e.g. a DIP chip needs to have an even number of pins, and a QFP chip is probably going to have a multiple of 4). In this case, it doesn't matter what you do - connecting it to VCC/GND or leaving it floating would have the same result, so it's best to just leave it unconnected.
2. An input pin that is unused in a specific configuration (e.g. based on how other input pins are connected). As above, you could connect it to VCC or GND, but there would be no benefit so you're better off leaving it unconnected.
3. An output pin that is unused in a specific configuration (e.g. an address line for a larger ROM size than you're using). In this case, connecting to VCC or GND could be dangerous as the chip might decide to randomly output the opposite value and cause it to source or sink significant amounts of current, potentially causing the chip to overheat and be damaged. In this case, you definitely want to leave it unconnected.
4. A bidirectional pin that's unused, basically a combination of #2 and #3.

No matter which one it happens to be, leaving it not connected is always the best choice.
Quietust, QMT Productions
P.S. If you don't get this note, let me know and I'll write you another.
Markfrizb
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Re: NC pins - connect to GND or VCC

Post by Markfrizb »

Many times the parts datasheet have a recommendation of what to do with pins if they are unused. For example, logic chips with unused gates should not float. The inputs that are unused should be tied high or low. If you use a chip that has 4 gates but you only use 2 of them, the other 2 should be managed and not left floating.
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krzysiobal
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Re: NC pins - connect to GND or VCC

Post by krzysiobal »

Additionally, if you're projecting a PCB that will be manufactured in many copies, have in mind that for example A21 for 29LV320 (4MB) = NC, but for 29LV640 (8 MB) it is normal input pin.

At some point of your manufacturing you might consider switching from 320 to 640 (because of price or lack in stocks) so even if you're not planning to use more than 4 MB, you still need to connect A21 to something in 640.
29LV memories can be T (top boot block) or B (bottom boot block), so if you plan backward compatibility with 320, the position of boot block should remain the same, which means that for T and B, A21 need to be wired differently. You should allow either programatically or at the moment of soldering chose where to connet A21. As a result, those upper lines that it some memories are NC should be connected to jumpers for more flexible design.
Kayumba
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Re: NC pins - connect to GND or VCC

Post by Kayumba »

krzysiobal wrote: Tue Jun 08, 2021 6:13 pm Additionally, if you're projecting a PCB that will be manufactured in many copies, have in mind that for example A21 for 29LV320 (4MB) = NC, but for 29LV640 (8 MB) it is normal input pin.

At some point of your manufacturing you might consider switching from 320 to 640 (because of price or lack in stocks) so even if you're not planning to use more than 4 MB, you still need to connect A21 to something in 640.
29LV memories can be T (top boot block) or B (bottom boot block), so if you plan backward compatibility with 320, the position of boot block should remain the same, which means that for T and B, A21 need to be wired differently. You should allow either programatically or at the moment of soldering chose where to connet A21. As a result, those upper lines that it some memories are NC should be connected to jumpers for more flexible design.
Thanks again.
That’s what i’am actually using on my PCB : the jumpers.
And it’s best solution for me.
Jumpers allow me to chose between LV320 (A21=GND) and for LV640 (A21= BA7).

I made some experience with LV640 (programmed with 4MB rom) and A21 not connected at all, to simulate LV320.
In this condition my Game ( with 4MB rom) was not loading at all.
After what i connected A21 to GND (to not leave it « floating » ) then game loaded with success!
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