AVR damaged by ultrasound?

[geeheeb]

Hello:
First of all I must start with the classic "I'm new to this forum so excuse if this post is in the wrong section / this was already discussed etc" I am also inexperienced in electronics as well. By the way, I really did search for this subject and found nothing, so I feel free to post this.

I used an atmega168 programmed to toggle a pin on and off using a delay loop in order to obtain a 40 khz output. Something like:

ldi r20, 0xff
ldi r21, 0x00
loop:
out PORTB, r20
delay_loop
out PORTB, r21
delay_loop
rjmp loop

I tested it using different delay times with a LED and it was working ok.
I then set the delay so that the output was 40 khz (ultrasonic).
I attached a 100 nF capacitor for decoupling a cheap piezoelectric buzzer: PORTB1-----CAP----PIEZO----GND
when I connected the circuit both the atmega168 my programmer (an usbtiny which was being used for powering the AVR board) died.

While I wait for my programmer's replacement, I tried to obtain an ultrasonic signal using another piezo and a NE555 timer using a rheostat to manually increase the frequency, worked fine for a while and when I brought the NE555 to a theoretical 100 khz the chip died as well, using the same connections: NE555_OUT-----CAP----PIEZO----GND.

Is there any intrinsic limitation for such frequencies using general purpose IC's like the ones mentioned?
Could the CAP and PIEZO combination have generated high voltage spikes?
Could some internal crystal or cap have been damaged by using them this way?

I'd really appreciate any light that could be shed on this issue by any generous light bearer!

Thanks!

 

[FoxyRick]

First, don't cross post your questions to other forums; it's against the rules even if it seems relevant to both forums.

There is no inherent frequency limit that would do that, but there might be other explanations.

Piezo elements can generate quite high voltages (they are microphonic) if tapped or subject to vibration. That can easily be enough to damage a chip.

There is some good info here: **broken link removed**

 

[geeheeb]

First, don't cross post your questions to other forums; it's against the rules even if it seems relevant to both forums.

I sincerely apologise. won't happen again.

Thanks for your reply.

 

[FoxyRick]

No need to apologize - I didn't mean what I said to sound harsh, I tend to speak quite directly and it can sound harder than I mean. I was just letting you know the rule so that you didn't get in trouble from a moderator.

Another thing to keep in mind with piezo elements is that they are quite high capacitance. Trying to drive them from an MCU pin, especially at such a high frequency, might cause problems with the MCU pin's output circuit. I would not expect it to kill the MCU though.

Let us know how you get on with the problem or if you need more suggestions.