Maximum 555 astable oscillator output frequency


Joined Mar 6, 2009
The CMOS type I use include a maximum astable frequency figure in the data sheet as well as rise & fall times. Astable operating frequency values up to 2MHz seem to be the norm.

I would think the free running frequency nomographs included on data sheets for the older 555 timer types give an indication of achievable frequency - I notice most of these show values up to 100kHz. I suspect one can do better [500kHz ??] with care.

I'm not sure if there is a simple relationship between output rise / fall time and maximum operating frequency. One may have to also reference the minimum trigger pulse width requirements in relation to that matter.
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Joined Apr 5, 2008

In the following PDF (an exerpt from a philips databook), is given that the maximum frequency can be higher that 500 kHz.




Joined Aug 21, 2008
As an aside: The CMOS versions, which agreed are not technically NE555's do reach much higher frequencies. Whether stable or not, that depends upon the criterion.


Joined Feb 11, 2008
I remember a 555 being good for about 1MHz, and the CMOS 555 being much faster.

Even the old 555 which used a TTL output driver, running from +5v should give similar speed to the old TTL logic chips. So 1MHz should be ok and even a few MHz could be possible.

However if you wanted to run the 555 from 12v etc or over the full Vcc range of the IC then the top speed may be less.


Joined Feb 11, 2008
It should not be that hard to test, if you run a 555 astable at a high frequency and scope it.

The TTL output switching time (or pin7 discharge switching time) can be seen by slope, and the pin 2/6 threshold inputs delay can be seen as period between the time the cap waveform reaching 1/3 and 2/3 Vcc and the time the output (or pin7) switches.

Those two things (switching time, and delay period from threshold to switching) should both be visible and they set the max frequency.

I'm sure at 5v Vcc I could get at least a couple MHz, maybe a few MHz from a TTL 555 astable with some level of reliability.