I recently had the rare pleasure to accidentally set my chip (328PB) to the wrong clock setting, right after spending hours carefully wiring everything up. All was working wonderfully well, but I wanted to switch over to the external xtal. Due to some Atmel process changes, the old high freq, large-swing mode is no longer avail (but I was gonna operate in a valid, not so high 10 MHz range). With one push of the fuse programming button everything ground to the proverbial halt. Of course I assumed I could inject a square wave "clock" into the XTAL 1 pin & all would be recoverable (after removing my xtal). I set up my generator (& used a scope to verify)....but no dice...I tried several different freqs, but the programmer would never connect to the chip. I thought I might have accidentally selected low freq (as in 32KHz), when I meant to select low power. So I went back & tried injecting 32KHz, 10KHz, 100KHz...still no dice (and I set the programmer to <1/4 of my clock).
After all this time messing around, I was gonna let my hot air out & just swap in a new chip, but had the thought---if I had selected the 32KHz watch xtal, that's nominally a pretty weak signal. So I hooked up my gen once again, but turned the amplitude way down. Amazingly the programmer saw the chip the very first try & I was back in business. Indeed, I had set the fuses to the 32KHz mode. I don't recall what the injection freq was, but it was at a rather feeble magnitude compared with a full logic swing. I verified that if I turned the generator voltage level back up, it would not detect the chip (using Atmel MKII programmer). At least/last I was able to change the fuse properly & get my xtal humming.