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ATF1508 VCC requirements
http://forum.6502.org/viewtopic.php?f=10&t=5019		 Page 1 of 1
Author:  cbmeeks [ Fri Dec 22, 2017 5:12 pm ]
Post subject:  ATF1508 VCC requirements
I'm looking at the datasheet for the ATF1508.

Keep in mind, I know very little about CPLD's but I'm learning.

The datasheet specify there are two sources of power inputs. VCCINT and VCCIO. From what I understand, VCCINT is the standard 5V power input from which we need proper decoupling on each one. And there are only TWO on the chip.

For VCCIO, there are six. Now, if I wanted to work with 3v3 chips, I would put a 3v3 power input on them. Is that correct? Also, there seems to be two on the left, two on bottom, one on the right and one on top. Does that mean I can split them out and use 3v3 on the top, 5v on the left, etc.? Or, do all six VCCIO have to be the same voltage.

Finally, do I have to put decoupling caps on each VCCIO or can I just tie them all to VCC (5V) if I only want to work with 5v?

Thanks!
Author:  Dr Jefyll [ Fri Dec 22, 2017 5:25 pm ]
Post subject:  Re: ATF1508 VCC requirements
Ideally, all eight of those power pins would be given its own decoupling cap. However, if for some reason you want to avoid doing that then you could probably get away with 2 or more pins on a single cap if it's for pins that are physically close together. But capacitors are cheap, so it's hard to justify such a compromise.

I'm guessing (not having seen the datasheet) VCCINT powers the internal logic, which is probably 5-volt only. VCCIO is for I/O and could be less than 5V if that's what you want, but you said you want to work with 5v only so connect all the VCCIO's to 5V, too.
Author:  cbmeeks [ Fri Dec 22, 2017 5:29 pm ]
Post subject:  Re: ATF1508 VCC requirements
Dr Jefyll wrote:
you said you only want to work with 5v only so all the VCCIO's go to 5V, too.


That's correct but I am curious. If I wanted to work with 5V and 3v3 parts, would I put some of the VCCIO to 3v3 and some to 5v? Would the I/O pins on the left side, for example, use the voltages of the LEFT VCCIO pins?

Hope that makes sense.
Author:  Dr Jefyll [ Fri Dec 22, 2017 5:34 pm ]
Post subject:  Re: ATF1508 VCC requirements
cbmeeks wrote:
If I wanted to work with 5V and 3v3 parts, would I put some of the VCCIO to 3v3 and some to 5v?
That's right. Each VCCIO pin is associated with a group of IO pins, so if you want 3V operation on that group of pins then supply 3V to that VCCIO pin.

Best to read the datasheet, though, and double-check to see if there's anything else they have to say.
Author:  cbscpe [ Tue Dec 26, 2017 3:47 pm ]
Post subject:  Re: ATF1508 VCC requirements
I did not find any information regarding groups of pins having dedicated VCCIO. I think it is either 5 or 3V for all pins.
Author:  Dr Jefyll [ Tue Dec 26, 2017 6:00 pm ]
Post subject:  Re: ATF1508 VCC requirements
I made my comment without checking the datasheet -- I confess I got a little lazy. :oops: But now that I do check it seems I was on the right track regarding mixed 5V and 3V operation, but not about groups of pins. IOW it appears there is only one "group," as cbscpe said.
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And just a reminder, cbmeeks -- as the OP it would be helpful for you to post or link to the datasheet, simply as a convenience for those who might read your post and wish to respond. :)

cheers,
Jeff
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