Using an a555 Monostable Timer as an alternative to a crystal for a prop clock

AwesomeCronk

Thinking of using the a555 for a prop clock. Would an adjustable speed clock and the code

_clkmode = xtal1 + pll16x
_xinfreq = 6_250_000

conflict?

Peter Jakacki

Oh, the humanity....

Please just don't even go there and besides, a 555 is limited to 2MHz max and even then it will drift all over the place like flotsam and jetsam. I have a clock gen chip on my P2D2 that I can program to any frequency and it is very accurate.

But why would you need an adjustable clock?

Phil Pilgrim (PhiPi)

Yes/maybe. For this to work, the 555 would have to output a frequency between 4 and 10 MHz. But even then, it's really set up for a crystal, not an external oscillator. Also, your 555 would have to be 3.3V compatible. What are you trying to do with this?

-Phil

jmg

AwesomeCronk wrote: »

Thinking of using the a555 for a prop clock. Would an adjustable speed clock and the code conflict?

Why do you need an adjustable speed clock ?
There are better choices than a 555, depending on exactly what it is you are trying to do ?
What range do you need to adjust over, by what means ?

Phil Pilgrim (PhiPi)

Also, you said "monostable." You meant "astable," right?

-Phil

AwesomeCronk

Phil Pilgrim (PhiPi) wrote: »

Also, you said "monostable." You meant "astable," right?

-Phil

Umm... sure, I really can’t find out much about the thing.

As for the general information, I was simply curious. Thanks for the feedback.

Beau Schwabe

The only time I might consider running a processor like this would be using it somewhat like a giant ripple counter.
Suppose I had a sensor that I could convert it's output to a stream of pulses, but only the deviation in frequency from the sensor changes were so small that I couldn't accurately capture it with a standard micro without devoting tons of overhead processing. By using the stream of pulses from the sensor to drive the clock of a micro, I could divide out (within that micro) as many times as I wanted so that the small deviations compounded in my favor to become larger deviations that could be measured with another micro running with a more precise time base.

So unless your intent is to specifically use the clock input of a micro in some reciprocal timing scheme, I would advise against this method as the main time base of a micro.

Definitely turn off the PLL, otherwise the PLL will be constantly "hunting" for a frequency to lock onto.

JonnyMac

Oh, the humanity....

Umm... sure, I really can’t find out much about the thing.

Wait.. what?! Millions of 555 timers are deployed every year -- there is a wealth of information about it. What a bit of researching (hint, hint) would tell you is that it can be configured in one of three modes: bistable, monostable, and astable; the mode is dependent on the connection of external components. What you're after is astable.

AwesomeCronk

Hmm. I was thinking that an adjustable clock might be useful for complex propeller designs, to visually debug normally high speed code. Though further examination shows that a 6.25Mhz crystal to just go fast is likely wiser.

jmg

AwesomeCronk wrote: »

Hmm. I was thinking that an adjustable clock might be useful for complex propeller designs, to visually debug normally high speed code. Though further examination shows that a 6.25Mhz crystal to just go fast is likely wiser.

There are few cases where very slow clocks would make sense, but it is possible to do so.
See also this thread https://forums.parallax.com/discussion/170135/ultra-slow-clock-for-debugging#latest
I could see use in teaching, to show students the `wiggles per opcode` but that would be for half a dozen lines of demo code, to get the concept across.