Then you must know that it would take either a very large thermonuclear warhead, or several dozen (or tens of dozens) 100-400kt warheads burst at altitude to get the effect? Even Russia told us they 25MT warheads on their Mod3 SS18s were for taking out LCCs and they wouldn't waste one of them on an EMP.
NORKS may have the pistol, but there is only one round in the chamber.....and its wet.
EMP sending us back to caveman days is beyond hyperbole.
Altitude also matters.
On a more ...terrestrial...level, it isn't necessary to do the "one nuke over Kanas" scenario. Think of the French battling the majestic British Navy in centuries past: they couldn't go toe-to-toe, so they'd settle for cannonballs wrapped in chains aimed into the rigging, for a "mission kill".
One EMP over, say, New York, or Boston, or San Fran, could cause a lot of piss and consternation, enough to politically distract the US...say, if China wanted to move on Taiwan?