Ok, finally google docs supports any type of file so I can post all the project files in one place: code, gerbers, gEDA design files and spreadsheet with the calculations, requirements. It is all here. I decided to try out pastebin, so I'm updating this post with the code for everyone to take a peek... In this version I'm still working on the "when to turn on" routine, I think it still doesn't work as I initially intended... pastbin doesn't look so good to paste long assembly lines, but here it goes anyway..
The software as it is now is working, it has some problems and "room for improvement" but it is working! You know the motto: Make it work, make it better!, make it faster!
One of the improvements would be to limit the time the light is ON to about 30min-45minutes, this would allow the battery charge to progressively increase during the good sunlight days and progressively decrease on a sequence of bad sunlight days (as in this year for example). Other improvement could be a full darkness detection and adjust a 12h no light clock from there. It has happened that during the day when a big cloud (and/or heavy rain/heavy snow) the measured light level goes down to the "night" level, then if there's enough battery the light turns on... it shouldn't and I'm sure this winter is exceptional, but the only way to prevent this is to detect the lowest darkness point and count 12 hours from there, and only from then on the light would turn on when it is dark.
In the code I added a debug routine that is included when the two variables are uncommented (DEBUG and DB_CNT), when I wanted to output a value I load it in the DEBUG variable and call dbug. The byte is output in the status LED serially MSB first, there is a long (2 cycles) high pulse followed by clock and MSB data, clock and bit 6 data... clock and LSB data.
Finally my biggest problem was the branch instructions, the AVR has the following: BRLO - Branch if Lower (UNSIGNED) BRLT - Branch if Less than (SIGNED) BRGE - Branch if Greater or Equal (SIGNED) BRSH - Branch if Same or Higher (UNSIGNED) I was not used to have these complex branch instructions, I'm used to Zero Flag set, Zero Flag Clear, Carry set, Carry clear... I'm specially not used to having a different branch for signed or unsigned... So I chose wrong.. result the jumps were not taken when they should. The ADC values are unsigned when read (0..255) but if you start calculating with them you can get signed (negative) values and then you have to use signed instructions. AVR assembly programmer beware!
A final word about the Code, it was assembled with Atmel AVR Studio Assembler (Win), it doesn't output the number of cycles per instruction... a bit of a let down... :-) And with this post I finish this project.... Woo Hoo!! Project is dead, long live the new Project...