ATmega640 power limits

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I am considering a 640 for use in a system that will use most if not all of it's I/O ports. Does anyone have any thoughts on the the number of 74LS loads that can be run from a 640 without over heating the device. Most of the pins will be toggled at a fairly slow rate, around 1khz. After testing we will bond the part to the board.

Thanks

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Look in the datasheet under electrical specs or something, all the maximum ratings are listed there.

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There is is an individual pin limit AND a package limit.

True TTL is different because MOST of the input current happens when the gate input is low. The driver to the input has to sink current. Those specs are harder to fathom (or at least figure out how it correlates with TTL input behavior). As I recall for the Mega parts in general, it does not give you enough information to tell what the sinking limits are for the whole package. I would apply the same limit as stated for the general package, without regard to whether it is source or sink.

Odds are, you will probably run into inability of a port pin to pull the output low enough to make a TTL low before you run into other problems.

Jim

 

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I'm sure, you can drive a baking tray of LS-TTLs from a single AVR.
If I remember right, the LS-TTL fan-in was 0.32mA, so you can calculate the exact amount of inputs.

But I goes always the opposite way. Whenever an AVR can replace two TTLs, I do it.

Peter

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The ATmega640/1280/1281/2560/2561 data sheet version 2549L–AVR–08/07 on page 371, note 3 is all about combinations of pins current sink limits (IOL) for the entire chip. Note 4 covers the current source limits (IOH).

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Thank you.