NC pins on Mega8515 vs supply pins on Mega162

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I'm making a board which I want to be compatible with both ATMega8515 and ATMega162. Those chips are mostly compatible in terms of IO pinout, but there's one difference. The 8515 has 4 pins which are marked NC, whereas those are used for power supply on the 162. I'd like to design with belt and braces and connect the extra supply pins for use with the 162. Is this safe when using an 8515 on the same footprint?

The reason I'm asking this seemingly silly question is that I've heard a rumor that Atmel is sometimes using NC pins for internal test functionality. It would be a shame if an 8515 would go into test mode because one of the NC pins is connected to gnd or Vcc.

The datasheet just gives the standard warning: "NC = Do not connect (May be used in future devices)"

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I would follow the warning: "NC = Do not connect (May be used in future devices)"

Of course unless you ABSOLUTELY HAVE TO you should ditch the M8515 pinout completely and go with the M8535 pinout which is then compatible with modern AVRs like the Mega164p-Mega1284p series.

Otherwise you could fit 0R links on the NC pins and only populate them on the M162 boards.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Thanks for the advice. To start off, I don't actually have an indication that those extra supply pins are needed for my project. I'm already using the 162 on a board with only two supply pins and it's working, but again, by the belt and braces philosophy, routing through one of the ground/NC pins would allow a slightly shorter ground path on my otherwise cramped board. Nothing critical but would be nice.

As for switching parts, and going into the considerations of my projects, I've come to the conclusion that the 162 is the perfect fit. Some of the considerations are:
Pin compatibility with 8515 as well as 35 GPIO. 8535/164 only has 32 GPIO, and in that case I could just as well start from scratch and use a smaller AVR and some external latches. (I don't need the ADC anyway so Aref/AVcc are just wasted pins for me.)
16 MHz / 3.3-5V operation. The 8535/164 line requires a 5-ish V supply for 16 MHz operation, well at least on paper. I know AVRs can take a little overclocking, but I'd rather stay within spec.
But all of that is of course up to me in the end.

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Quote:
"NC = Do not connect (May be used in future devices)"

Typically here is what I have noticed:
Quote:
NC == not internally connected
DNC == do not connect

NC means "this design does not use this pin and you can do whatever you want with it"
DNC means "there is something connected inside, leave it unconnected".

Quote:
I'm already using the 162 on a board with only two supply pins and it's working

Do not do that. The chip relies on the even voltages on the supply pins.
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as well as 35 GPIO

As well as "works great with JTAGICEMk1".
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16 MHz / 3.3-5V operation.

The spec says 4.5V to 5.5V at 16MHz.
Quote:
but I'd rather stay within spec.

+1
ncccr

No RSTDISBL, no fun!

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You could check the NC pins with a DVM, to make sure that they are not connected to anything.

Leon Heller G1HSM

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Quote:

The 8535/164 line requires a 5-ish V supply for 16 MHz operation,

Using the ramp in the '164PA datasheet, 16MHz operation would require 3.8V supply. (3.78V)

We've used the '162 in a few apps over the years. A couple had memory-mapped interfaces and the '162 has external memory provisions. And the '162 has dual USARTS--kind of a rarity in that size AVR when I was your age.

If power consumption is at all a concern, than an e.g. '164 will run rings around the '162.

If cost is at all a concern, a '162 is $3.72/qty. 100; a '164A is $2.59.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.