SN74LV86A as oscillator

Paul Bryson

Other Parts Discussed in Thread: SN74LV86A

I have two spare XOR gates on a SN74LV86A and I need an oscillator which is asynchronous to my system clock. I am considering building an oscillator from the two XOR gates. However I am concerned that it may draw an excessive and unpredictable amount of supply current.

The supply is 5V.

I can't use a ring oscillator because the freq must be less than 16MHz.
Otherwise frequency is not very critical.

1. Is there any data available on the behavior of LV inputs at intermediate voltages?

2. How much feed through current should I expect in the transition region?

3. How wide is the input transition region?

4. Would adding external hysteresis feedback help? i.e. does the output begin changing as soon as the input enters the transition region or is there a gap?

5. Is there a circuit topology which is better than the others?

I have found these (imagine them with XORs):

over 12 years ago

0 Chris Cockrill over 12 years ago

TI__Mastermind 46480 points

These do burn some power. I have attached an app note. The AHC examples will be the same as LV.

oscillator TI.pdf

0 Paul Bryson over 12 years ago in reply to Chris Cockrill

Intellectual 490 points

Chris,

Thank you for taking time to reply. However the document you posted really does not apply to my question.

The document only describes Pierce oscillators - not relaxation oscillators that I was asking about. Also all of the graphs of "Icc vs Vi" and "Vo vs Vi" are for unbuffered inverters. The buffered parts in my question will likely be very different.

0 Chris Cockrill over 12 years ago in reply to Paul Bryson

TI__Mastermind 46480 points

Here is one other app note with different ways of creating the oscillators. It is not the same type of Cmos technolgy but may help.

Oscilator H.pdf

0 Paul Bryson over 12 years ago in reply to Chris Cockrill

Intellectual 490 points

Thanks, Chris.

The datasheet says:

"Support Mixed-Mode Voltage Operation on All Ports"

What does this mean? Does this imply any special tolerance for intermediate voltages on the inputs?

0 Chris Cockrill over 12 years ago in reply to Paul Bryson

TI__Mastermind 46480 points

"Support Mixed-Mode Voltage Operation on All Ports"

This means the inputs or outputs can except voltages above Vcc. However this statement is wrong.

Only the inputs can except voltages above Vcc.. There are no specs related to operating the part in the linear region although it can be done.

the LV family will have a small amout of hysteresis built in that is designed to prevent oscillation. (about 50mv typical) However it is not enough to prevent the part being used as an oscillator.

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SN74LV86A as oscillator