Very strange that you get a negative voltage...Quote
glub0x
So after some testing, the pin n°19 is always at -0,7volts which is not nice...
Do i have to replace the 40 pins chip?
Have you measured this with only one power connected (either 12V PS or USB ) ? Do you then still get negative voltage?
Because, as I understand it, there should not be negative voltages anywhere on the board. So if there is, the only possibility I can think of is if you have some ground loop, i.e. if the 0V from the USB is not the same as the ground from the 12V PS. You might want to measure if there is any voltage difference between PS and USB ground lines. If it is not 0V, try to solve this first, e.g. using the same outlet for PC and PS.
Have you tried changing HEATER_0_PIN to 12 and, if so, can you then control the hotend via the heatbed connector? If yes, then you have at least a temporary solution as long as you don't need the heatbed. If later on you want to use the heatbed, you can still control it using a Sevenswitch on the PWM pin of the 14pin extension header.
If you decide to replace the ATmega644p/1284p, be aware that they normally don't come with a bootloader. So you will need an ISP programmer or some Arduino (with ArduinoISP sketch) to upload the bootloader, before you can use the Sanguinololu via USB.