Pin compatibility chart ?

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#1
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Is there a complete compatibility chart anywhere ? I've poked around, and found a few "sort-of"s, but I would really like a complete chart of all the compatible AVRs out there.

Like you can pop a mega8, mega48, mega88, mega168 into the same system, and hardware-wise, it will work. Mostly.

How about a grand overview chart of all the upgrade paths ?

Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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I'd like to know if I can put a 40 pin mega644 in the socket where my mega32 sits.

Imagecraft compiler user

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Broadly speaking, there are two "classes" of 40-pin DIPs / 44-pin TQFP/MLF.

There's the "analog" class derived from the AT90S8535. They feature an ADC.
This class includes the ATmega163, ATmega16, ATmega8535, ATmega32. If you start with "lower" devices in the chain, then moving upwards you will find compatible pin functionality. Extra functionality might be found only in the "higher" devices (such as extra PWM channels, etc.)

And there's the "digital" class derived from the AT90S8515. They feature an External Memory Interface.
This class includes the ATmega161, ATmega162, ATmega8515.

In the world of 64-pin TQFP/MLF, there's four broad categories: ATmega64/128, ATmega1281/2561, ATmega169/329/649, and ATmega165/325/645. Devices within each of these four families are horizontally pin-compatible. There's also upward mobility from the ATmega64/128 into the ATmega1281/2561.

In the world of 100-pin TQFP/MLF, there's three broad categories: ATmega640/1280/2560, ATmega3290/6490, and ATmega3250/6450.

The ATmega644 (and ATmega164P/324P if they ever roll out) are sort of a class by themselves, but they most closely resemble the "Analog" class, and a migration note exists for moving from the ATmega16/32 into the ATmega164/324/644.

In the 28-pin DIP/32-pin TQFP/MLF, pin compatibility inherits from the AT90S4414. As you note, there is 100% upward mobility from the ATmega8 into the ATmega48/88/168 (The pins are all there, and each special-function pin on the ATmega8 is still present on the 48/88/168. But some code changes are necessary). However, there are extra features in the 48/88/168 are missing in the ATmega8 which prevent backporting some applications.

I haven't spent enought time investigating the heritage of the ATtiny series to say much about them.