AVR Freaks

While I do not have a schematic this info may help you... some of which is an educated guess..the USB interface in Manual Diagram 3.1 is more than likely the FTDI usb/uart chip FT232 www.ftdichip.com..that site will show you alot about how to interface USB to a micro...the info you may wish to focus on is how the ftdi chip can both regulate and switch using an external FET the power to the external circuit..in this case it probably powers the avr on the pcb and the voltage convertor as represented in the block diagram. My guess is that the lower left hand side of the pcb as illustrated in the manual is the circuit of interest and probably similar to the FTDI app notes. This is the side you want to grab the voltage you need for the external AVR but you should obviously take some precautions. Typically when you first plug in a usb device it should consume only a small amount of current so it can enumerate properly then after enumeration it can switch a larger load on (this is the very abbreviated version of the process)...this should not really be a problem if you grab the power at the right point in the circuit and you watch the current. I do not believe the VTG pin is the one you want to grab without modifying the device..Refer to 3.2 and 3.4.1 in the manual....VTG is used an input to the onboard voltage convertor to tell it what voltage the TARGET AVR is operating at..this programmer supports TARGET voltages from 1.8 to 5 volts ..it only draws a small amount of current FROM the TARGET SUPPLY probably just for the level shifters and the ref to the voltage converter....the picture in the manual is so fuzzy it is difficult to glean more info from the picture..a good digital pic of the pcb both sides would probably reveal more ...I feel sure what you want to do is possible , but it may take just a bit more discovery on your part before you actually heat up the iron...onward thru the fog

trace vtg pin to see where it goes..your measuremnt of 4.5vdc on this pin is somewhat curious..I can not see clearly the connections to what appears to be a tant cap by vtg pin...my guess is that the vtg input pin may go to an an analog input on the 8535 so the pc program can read the target voltage...possibly thru a resistor with a cap to dcc...if you can not tell which are caps and which are resistors refer to manual..in the color illustrations apparently brown squares are where caps are and blue squares are where resistors are for the most part....black squares...tant caps with a polarity indicator..the object is to see what components are connected to VTG since that is the pin I assume you wish to power the target VCC (+5) to..my next guess is that after all this you will find no problem..but you might want some protection on the vtg pin on the target isp pin....possibly a polyswitch, small res, but.... curious when this widget is not strapped to a target does the pc program screen fig 5.6 of manual that shows the target voltage give a reading? i think you are on the right track and you get extra * for researching before smoking it...most seem to smoke first and ask why later!!!! I am off in the fog........