user11802: I have been working on creating a stand alone atmega2560 arduino. When trying to program it with a serial adapter using the rx and tx pins nothing was happening. After a lot of debugging I found the problem was with the chip. 
I found that the ones that worked said atmega2560 16AU 0721, the ones that did not work said Atmega2560 16AU 1241. When looking on eBay, i noted that the Atmega2560 16AU comes with many different versions of those last four numerals, does anyone know what the last four digits on top of the atmega2560 16AU mean?

user612: The data sheet (or supplemental product-line documentation) will specify the formal package marking codes, but for an ATmega, the 16AU is part of the variant specification but the following would appear to be date or lot codes.
These are probably not your problem.
More likely, the chip you are trying to interact with via the serial port

does not have a bootloader flashed (normally they are sold blank)
has a bootloader which requires a different baud rate
is fused for a clock source you are not providing
has the brownout detector set lower than the supply you are providing
is improperly or incompletely soldered
is damaged

While there was the infamous case of Sparkfun being sold a reel of power supply chips falsely marked as ATmega328's, this is still a relatively unlikely possibility compared to the more likely ones above, especially the simple lack of a bootloader.  And a date code is no proof of authenticity - though if you ended up co-mingly inventory from various sources/purchases, it might help you with preliminary sorting them.

11802: <p>I have been working on creating a stand alone atmega2560 arduino. When trying to program it with a serial adapter using the rx and tx pins nothing was happening. After a lot of debugging I found the problem was with the chip. </p>

<p>I found that the ones that worked said atmega2560 16AU 0721, the ones that did not work said Atmega2560 16AU 1241. When looking on eBay, i noted that the Atmega2560 16AU comes with many different versions of those last four numerals, does anyone know what the last four digits on top of the atmega2560 16AU mean?</p>
 612: <p>The data sheet (or supplemental product-line documentation) will specify the formal package marking codes, but for an ATmega, the <code>16AU</code> is part of the variant specification but the following would appear to be date or lot codes.</p>

<p>These are probably not your problem.</p>

<p>More likely, the chip you are trying to interact with via the serial port</p>

<ul>
<li><strong>does not have a bootloader flashed (normally they are sold blank)</strong></li>
<li>has a bootloader which requires a different baud rate</li>
<li>is fused for a clock source you are not providing</li>
<li>has the brownout detector set lower than the supply you are providing</li>
<li>is improperly or incompletely soldered</li>
<li>is damaged</li>
</ul>

<p>While there was the infamous case of Sparkfun being sold a reel of power supply chips falsely marked as ATmega328's, this is still a relatively unlikely possibility compared to the more likely ones above, especially the simple lack of a bootloader.  And a date code is no proof of authenticity - though if you ended up co-mingly inventory from various sources/purchases, it might help you with preliminary sorting them.</p>
