GaryStofer 
Can someone shed some light on ATMELS nomenclature in naming their devices. For example what is the difference between an ATMEGA324P and an ATMEGA328A or -PA.
So far I can only figure out that the 32 in it's name means 32K of flash.
I have been looking for some sort of a key, an explanation of what the extra digits and letters in the name mean, but could not find anything like that on ATMELS website.
Gary
clawson 
Device numbers that end with a P are "PicoPower" devices and have been optimized for power sensitive applications and contain a number of specialised functions to aid in power usage reduction. Those that end with A are just a recent respin of existing silicon with identical functionality to the previous device but produced on a smaller fabrication process to reduce the die size, cost and power consumption further. Some P devices have now had the A respin so are PA devices.
Picopower (P) is explained here:
The digit after the flash size appears to have no consistent meaning. It seems to generally designate a "series" of devices with the same or substantially same internal architecture but varying with flash size.
Example: ATmega48, ATmega88, and ATmega168 can be thought of as members of the "8" series with 4K, 8K, and 16K flash, respectively.
But, then you have Mega325 and Mega3250 which are same architecture and memory size but significantly different packages, pin counts and I/O capacity.
Seems like a lack of foresight in choosing a numbering scheme. Unfortunately, they have gone though several numbering shifts, including a very recent one on numbering of USB-enabled devices. So, that also adds to the confusion.
Jim
glitch 
There have been several shifts over the years on how Atmel numbers the chips so there is no hard and fast rule. Though for modern "mega" devices the following tends to hold.
The flash size is always a power of two, this may be followed by one or two additional digits.
A 0 indicates a 100pin version of the non-zero counterpart, oterwise feature wise identical. [though with extra pins, comes extra GPIO ports]
Digits 1-9 [not all allocated yet] seem to indicate a family. With these dveices one can migrate amongst various flash sizes, while remaining feature & package compatible.
The digits are then possbly followed by one or two letter suffixes. As stated above the P indicates a pico-power device [some additional functionality exists to its non-P counterpart but remains mostly compatible], and the trailing A indicates a respin of an older device using the newer smaller manufacturing process, oterwise functionally identical.
