Voltage out with or without jumper

[peterinaz]

I built my USBtinyISP without any trouble and it seems to work most of the time, except for an occasional 'broken pipe' error when I use it with Arduino IDE. But mostly it works great.

The problem I'm having is there seems to be a voltage at the output whether the jumper is connected or not. I had it connected to an unpowered Arduino UNO, and if the jumper is connected, there's 5vdc at the VCC pinout. But if I remove the jumper, there's about 3.6vdc at the VCC pinout on the unpowered Arduino. Is this normal? Will I run into trouble if I use external power on my target? Thanks in advance for your help!

[phild13]

Yes that is normal.
From the FAQ:

Version 2.0 which is almost certainly what you've got, uses a level shifter so that if the jumper is not in place, it will use whatever the target voltage is, a lot better for your low-voltage devices!

So, if you have a device that needs to run at 3.3V, don't have the jumper in place!

So it supplies 5v if the jumper is in place and 3.3v if the jumper is not in place.

[peterinaz]

Thanks! I did read that part in the F.A.Q., but misunderstood. I thought it meant that low-voltage targets had to be self powered! Now all is right with the world again!

[SB555]

Yes that is normal.
From the FAQ:

Version 2.0 which is almost certainly what you've got, uses a level shifter so that if the jumper is not in place, it will use whatever the target voltage is, a lot better for your low-voltage devices!

So, if you have a device that needs to run at 3.3V, don't have the jumper in place!

So it supplies 5v if the jumper is in place and 3.3v if the jumper is not in place.

I do not think it supplies 3.3V, the programmer only accepts it and can work with it... Correct me if I'm wrong though!
But that's my understanding of that explanation, meaning that the voltage that you have should not be there. Can't try it out right now. I'm pretty sure it shouldn't be supplied by the programmer, but by an external source.

[phild13]

@sb555
You are correct. I did not take the time to look first before answering.
See:
http://www.ladyada.net/images/usbtinyis ... sp2sch.png

Edit:
Pin 2 on both ISP connectors feed VCC (pin 14) to the 74AHC125N. When the jumper is not installed the 74AHC125N will be powered
by the target. So to use with a 3.3 volt target the target has to be powered

[SB555]

@sb555
You are correct. I did not take the time to look first before answering.
See:
http://www.ladyada.net/images/usbtinyis ... sp2sch.png

Edit:
Pin 2 on both ISP connectors feed VCC (pin 14) to the 74AHC125N. When the jumper is not installed the 74AHC125N will be powered
by the target. So to use with a 3.3 volt target the target has to be powered

Alright then