Search this site
Main‎ > ‎electronics‎ > ‎

ET_AVR_1281_transplant

I can't take credit for the idea, in fact, if one particular kind person at avrfreaks wasn't persistent with me, i would have ended up trying to solder the 1281 onto a basic tqfp adapter.. But, i decided to take the advice and buy an ET AVR Stamp module from futurlec and cut off the original atmega128, then solder the 1281 I bought from au.farnel.com onto it. Futurlec do not sell the 1281 unbfortunately because no doubt it would have been a bit cheaper.. All up about $50 for both, but i'm not regretting it! Well worth the money and effort.

The only real difference between the 128 and the 1281 is PIN1. on the 128 it's something called "PEN", which i'm not even sure what that is but on the ET-AVR Stamp schematic you can see it is externally pulled up by a resistor to +5v, on the board, which is a problem. On the 1281 PIN1 is PG5, and more importantly oc0b which is the output compare pin for timer zero, which I actually really wanted to use!  Timer  0 is the 8 bit timer, timer 1 is the 16 bit timer so it's a waste using it to generate a 8MHz clock, and then the atmega1281 does not have a oc2b! the 100 pin atmega1280 does, but not the 1281! it has oc3b which is another 16bit clock, but no 0c2b.
 
So to reclaim oc0b, i traced the pin labelled PEN back to a tiny smd resistor "R4" on the board and unsoldered it, problem solved, that's all i had to do. The last photo on the bottom is a close up of where I removed the resistor.

Here are some pics, this is the original stamp:



I tried to remove the atmega128 by desoldering it, but it was too small and too hard with a regular soldering iron, so i ended up carefully cutting it off:



This is after the cleanup job, ready to take the donor chip. This part was surprisingly easy, the little cut off leg segments actually wanted to stick to the iron rather then the board, I was worried it would be the other way around and i'd end up smearing crap around trying to get it off. So i just wiped each track with the iron once or twice, and after 2 or 3 tracks would wipe the iron off on my sponge.


Finished product, I did have a problem with a bridge that happened which seemed to be UNDER the pins! This was a pain to get out, i ended up actually cutting it with a thin blade! But apart from that i'm very happy with the result. Some of the solder joints on the left look pretty dodgy but i think that's the light, i've triple checked each joint under magnification and they all look pretty good.