junebug and external power

sorry for all the questions later today i want to connect the junebug to a breadboard and drive 2 4017 counters that will be driving about 20 leds.. i would like to use 12v external power source to drive theese. do i have to connect the ground from the junebug and external ground together? i will try and do a diagram later when i am back home. thanks again in advanced

Yes, both grounds need to be joined.

whats the explanation for that? i am interested as to why

Otherwise, you'd have two circuits joined by one wire. There might be a +5v signal coming from the Junebug, but without a 'ground reference' the 4017s would not see it as +5v.

Not the best explanation, I'm sure someone else could explain it better.