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oscillator startup time

O

Octavio

Jan 1, 1970
0
I'm using a 74HC4060(A) oscillator/divider with a 15MHz crystal in the
oscillator section. That oscillator must be able to turn on/off (using the
reset pin) with a 43uS ON / 8uS OFF duty cycle.

I'm not familiar with oscillator startup time stuff so I ask:
The oscillator must start oscillating within 1uS to give a reasonable number
of counts, say about 7 million cycles at the end of that microsecond.

Is this possible?

Thanks for any input!

Octavio
 
W

Winfield Hill

Jan 1, 1970
0
Octavio said:
I'm using a 74HC4060(A) oscillator/divider with a 15MHz crystal in the
oscillator section. That oscillator must be able to turn on/off (using the
reset pin) with a 43uS ON / 8uS OFF duty cycle.

I'm not familiar with oscillator startup time stuff so I ask: The
oscillator must start oscillating within 1uS to give a reasonable number
of counts, say about 7 million cycles at the end of that microsecond.

Is this possible?

Thanks for any input!

I don't understand your 7 million cycles at the end of a microsecond
question. Do you mean 7000GHz? Or do you mean 7MHz? ???

Anyway, quartz crystals have a very high Q, more than 10^4 or 10^5.
Their startup time is on the order of Q cycles, or possibly more than
5ms for a 15MHz crystal. That doesn't fit with your 43us / 8us
timing requirement.
 
J

John Fields

Jan 1, 1970
0
I'm using a 74HC4060(A) oscillator/divider with a 15MHz crystal in the
oscillator section. That oscillator must be able to turn on/off (using the
reset pin) with a 43uS ON / 8uS OFF duty cycle.

I'm not familiar with oscillator startup time stuff so I ask:
The oscillator must start oscillating within 1uS to give a reasonable number
of counts, say about 7 million cycles at the end of that microsecond.

Is this possible?
 
P

PeteS

Jan 1, 1970
0
Or an oscillator that can be gated on and off (they are available with
enable pins) although that is virtually guaranteed to generate runt
pulses at some time.


Cheers

PeteS
 
J

Jamie

Jan 1, 1970
0
Octavio said:
I'm using a 74HC4060(A) oscillator/divider with a 15MHz crystal in the
oscillator section. That oscillator must be able to turn on/off (using the
reset pin) with a 43uS ON / 8uS OFF duty cycle.

I'm not familiar with oscillator startup time stuff so I ask:
The oscillator must start oscillating within 1uS to give a reasonable number
of counts, say about 7 million cycles at the end of that microsecond.

Is this possible?

Thanks for any input!

Octavio
Don't do that, keep it running always, use Logic gates to switch it
on/off .. That way, you can precisely control it via count of
pulses if you wish using a counters. This will give you a perfect
timing on the edges.
something like 645 counts for the 43 us and 120 ct's for the 8Us.

i think i have the math correct there.
i could be off by 1 decimal point.
as far as the rest of your statement, i think you should read that
again.
 
O

Octavio

Jan 1, 1970
0
I could. I just wanted to save one chip.
Reading what you and other people answered, I believe it's the only way.

Thanks!
 
O

Octavio

Jan 1, 1970
0
Winfield Hill said:
I don't understand your 7 million cycles at the end of a microsecond
question. Do you mean 7000GHz? Or do you mean 7MHz? ???

I meant that, even if the oscillator is slow to start up, I could at least
get 7M cycles for the built-in counter to count by the end of the first
microsecond. This would have given a minimal error at start up for my
project.
*But* as far as I see, from you and other people who answered my question,
it is not possible. Startup time is much longer. I will use a separate
oscillator with a gate as a switch.

Thanks!

Octavio
 
J

Jamie

Jan 1, 1970
0
Octavio said:
I could. I just wanted to save one chip.
Reading what you and other people answered, I believe it's the only way.
Look at the High speed low pin count AVR's or PIC's, with careful
coding, you can get your self a Xbase clock with the scaling generated
signals you need from a single chip with the Xstyle
Of course, don' expect that uC to do any other processing because
it will up set the timing of course unless you use something like an
AVR that can be calculated easy to account for the CPU usage of the clock.
In any case, many of those uC have programmable counters that can
output directly to a pin with on/off duty cycle programming and clock
them self's directly from the base osc.
 
P

PeteS

Jan 1, 1970
0
Octavio said:
I meant that, even if the oscillator is slow to start up, I could at least
get 7M cycles for the built-in counter to count by the end of the first
microsecond. This would have given a minimal error at start up for my
project.
*But* as far as I see, from you and other people who answered my question,
it is not possible. Startup time is much longer. I will use a separate
oscillator with a gate as a switch.

For best results (not to say avoiding metastability) make sure it
switches the signal on and off synchronously.

Cheers

PeteS
 
O

Octavio

Jan 1, 1970
0
PeteS said:
For best results (not to say avoiding metastability) make sure it switches
the signal on and off synchronously.

The project is more or less like an ansynchronous receiver with a clock 16
times the input data rate. So the clock doens't need to be synchronous as
the error will be only few percent or 1/16.

Regards!
Octavio
 
W

Winfield Hill

Jan 1, 1970
0
Octavio said:
Winfield Hill wrote


I meant that, even if the oscillator is slow to start up, I could at
least get 7M cycles for the built-in counter to count by the end of the
first microsecond.

That's the problem: 7M means 7,000,000 cycles, and a 15MHz signal has
only 15 cycles in one microsecond. Or did you mean at least 7 cycles
in the first us, which would be like an immediate 7MHz signal?

A crystal oscillator starts up when the amplifier noise at 15MHz is
captured by the crystal and saved as resonate energy. This saved
signal is amplified and then again presented to the crystal, so its
signal builds up. But until the amplitude becomes high enough for
logic circuits to properly respond there is no output. Also, when
the logic output first begins, after the long no-output delay, it
has poor edge risetimes and isn't suitable to use for flip flops,
counters, or microprocessor circuitry. That's why microprocessors
have a long forced-reset interval. Otherwise they could perform
incorrect operations, or crash, from the defective clock signal.
This would have given a minimal error at start up for my project.

Many engineers use a gated oscillator circuit, which can start
instantly, and is trimmable to a reasonable frequency accuracy,
at least over a short time frame, like 43us (only 645 cycles).

If you need we can point you to such a circuit, or present one
as an ASCII drawing.
 
J

John Fields

Jan 1, 1970
0
A crystal oscillator starts up when the amplifier noise at 15MHz is
captured by the crystal and saved as resonate energy.

---
"Resonate energy"???

Not true. When the crystal is hit with noise which causes its
physical dimensions to change, it replies by "ringing" at its
resonant frequency and generating a voltage when it relaxes. If the
amplifier to which it's connected is biased around noise and has
enough gain, then that voltage will be enough to cause a transition
in the amplifier's output which will, if it's timed correctly, hit
the crystal again, squeezing it and, once that edge is gone, causing
the crystal to generate a voltage which will cause the cycle to
begin anew.
 
W

Winfield Hill

Jan 1, 1970
0
John said:
---
"Resonate energy"???

Not true. When the crystal is hit with noise which causes its
physical dimensions to change, it replies by "ringing" at its
resonant frequency and generating a voltage when it relaxes.

You don't think of that as the resonant energy building up in
the crystal? The crystal's increasing voltage appears across
it's resonate capacitance, with E = 0.5 C V^2. On the other
half cycle this energy is stored as current in the crystal's
inductance, E = 0.5 L I^2. That's increasing resonate energy,
driving the increasing voltage appearing across the crystal.
We're speaking, of course, of the very well-known electrical
resonate LCR models of the crystal's piezo-mechanical action.
 
R

Robert Baer

Jan 1, 1970
0
Octavio said:
I'm using a 74HC4060(A) oscillator/divider with a 15MHz crystal in the
oscillator section. That oscillator must be able to turn on/off (using the
reset pin) with a 43uS ON / 8uS OFF duty cycle.

I'm not familiar with oscillator startup time stuff so I ask:
The oscillator must start oscillating within 1uS to give a reasonable number
of counts, say about 7 million cycles at the end of that microsecond.

Is this possible?

Thanks for any input!

Octavio
It is possible to get one-cycle turn-on times, *but* you then must
accept a square-wave oscillator (well, as square as the IC will allow)
and, in this case an overdriven crystal.
I suggest you use a comparitor, using one input as the oscillator) gate.
 
R

Robert Baer

Jan 1, 1970
0
PeteS said:
Or an oscillator that can be gated on and off (they are available with
enable pins) although that is virtually guaranteed to generate runt
pulses at some time.


Cheers

PeteS
One "runt" pulse at the end of the gate time, at worst.
 
J

John Fields

Jan 1, 1970
0
You don't think of that as the resonant energy building up in
the crystal? The crystal's increasing voltage appears across
it's resonate capacitance, with E = 0.5 C V^2. On the other
half cycle this energy is stored as current in the crystal's
inductance, E = 0.5 L I^2. That's increasing resonate energy,
driving the increasing voltage appearing across the crystal.
We're speaking, of course, of the very well-known electrical
resonate LCR models of the crystal's piezo-mechanical action.
 
Or if the precision doesn't need to be perfect, use a RC oscillator.
It will start in one cycle. Alternatively add a crystal to calibrate
the RC oscillator

Regards

Klaus
 
R

Rich Grise

Jan 1, 1970
0
PeteS said:
Octavio said:
Octavio wrote:
I'm using a 74HC4060(A) oscillator/divider with a 15MHz crystal in the
oscillator section. That oscillator must be able to turn on/off (using
the
reset pin) with a 43uS ON / 8uS OFF duty cycle.
<snip>[use a free-running osc. and gate it]...
For best results (not to say avoiding metastability) make sure it switches
the signal on and off synchronously.

The project is more or less like an ansynchronous receiver with a clock 16
times the input data rate. So the clock doens't need to be synchronous as
the error will be only few percent or 1/16.

I think they mean that the gate edge should be in sync with the clock so
that you don't get fractional pulses, which could confuse the counter.

Good Luck!
Rich
 
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