SiTime, Silicon MEMS Oscillators and Clock Generators

Frequently Asked Questions

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What is the difference between the OE and ST features?

How does the load affect current consumption?

Can SiTime devices drive loads larger than 15pF?

Can I adjust the output rise and fall times of SiTime oscillators?

Are SiTime MEMS-based oscillators subject to "activity dip" issues?

Can I program SiTime oscillators?

Can I program a SiTime oscillator in my system?

Are SiTime products RoHS certified?

Where can I locate RoHS documents for SiTime products?

 

 

 

What is the difference between the OE and ST features?

The feature pin for most SiTime oscillators (pin 1) can be programmed to either "output enable" (OE) or "standby" (ST) functions. In both cases, pulling pin 1 Low stops the device's output oscillation, but in two different ways, as described below.

Applying logic Low to the OE pin only disables the output driver and puts it in Hi-Z mode, but the rest of the device is still running. Power consumption decreases due to the inactivity of the output. For example, for a 3.3V SiT8003 20 MHz device, the IDD decreases from 4 mA to 3.3 mA for a 15 pF load. When the OE pin is pulled High, the output typically enables in less than 1 us.

A device with an ST pin enters standby mode when the ST pin is pulled Low. All internal circuits of the device turn off and the current is reduced to a standby current, typically in the range of a few microAmps. When ST is pulled High, the device goes through the "resume" process, which can take from 3 ms to 10 ms. The standby current and resume time period are specified in the device datasheets. Some SiTime datasheets do not specify the resume time specifically; in those cases, the resume time is the same as the "start-up time."

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How does the load affect current consumption?

In most applications, the LVCMOS oscillator drives capacitive loads. During the rising edges, the device draws current from the power supply to charge up load capacitance. During the falling edges, the capacitance discharges to GND. The average current going through the load depends on the following parameters:

Output frequency (Fout). This determines how often current is drawn from the power supply.

  1. Load capacitance value (Cload). Larger capacitance values require more current to charge up load capacitance.
  2. Power supply voltage (Vdd). More current is required to charge up the load to higher voltages.

The additional power supply current from the load is computed as below:

I_load = Cload * Vdd * Fout

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Can SiTime devices drive loads larger than 15 pF?

Yes. SiTime devices with single-ended LVCMOS outputs are generally specified with a 15 pF capacitive load for rise and fall times. The device may drive a larger load, up to 60 pF, with slower rise and fall times. For the applications requiring both fast rise and fall times (~1 ns) and the ability to drive large capacitive loads, buffer devices with high drive strength output are available upon request. Contact SiTime for more details.

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Can I adjust the output rise and fall times of SiTime oscillators?

Yes. Users may adjust the output buffers of SiTime oscillator by changing the drive current strength. By increasing or decreasing the maximum drive current of the output stage, rise and fall times may be reduced or increased, respectively. A high drive current strength enables faster rise and fall times while driving a larger load. A low drive current strength reduces the clock edge slew rate and reduces potential EMI. Contact SiTime for ordering parts with modified drive strength.

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Are SiTime MEMS-based oscillators subject to "activity dip" issues?

No, they are not.

"Activity dips" are defined as abrupt changes of frequency in a quartz crystal-based oscillator. A crystal-based oscillator may often exhibit an activity dip at critical temperatures, and return just as abruptly to normal behavior for small deviations of temperature away from the critical value.

The most common causes of activity dips are:

  1. "Coupled modes" – the collision of different crystal oscillation modes with different temperature coefficients.
  2. Moisture inside the crystal package condensing onto the quartz plate.

All of these effects can rob the main oscillation mode of energy, effectively causing the crystal to drop out of oscillation or to oscillate temporarily in a different crystal mode.

SiTime MEMS-based oscillators are not subject to these effects for two reasons:

  1. SiTime MEMS oscillation modes are primarily determined by the material properties of silicon; all modes or spurious response characteristics change with temperature in exactly the same manner as the fundamental oscillation mode. Therefore, different modes can never interact at the same frequency and cause a dip.
  2. SiTime's MEMS First™ process uses standard silicon fabrication techniques to hermetically seal the MEMS in a very high-temperature, clean, vacuum environment. This creates an extremely clean, moisture-free environment for the MEMS, and eliminates the possibility of contaminant- or moisture-induced activity dips.

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Can I program SiTime oscillators?

For production volumes, most SiTime oscillator families are designed to be programmed at the factory. SiTime offers field programmable oscillators for use with the Time Machine™ II, a complete and portable programmer kit. This tool can program the frequency, voltage, stability and other functional characteristics of devices in our product families. The programmer and field programmable devices are ideal for fast prototyping and optimizing system performance by creating instant samples with custom frequencies or adjusting drive strenght. The field programmable oscillators have industry-standard footprints so they can be used as drop-in replacements for legacy quartz oscillators. See field programmable oscillators and Time Machine II for details.

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Can I program a SiTime oscillator in my system?

SiTime offers the following options for in-system programmability:

  1. For applications that only require two frequencies, SiTime offers the SiT8033 two-frequency programmable oscillator. The output of this device can be switched between two frequencies in a user system via a logic-level signal. The two frequencies are pre-selected, and are programmed at the factory.
  2. Digitally-controlled oscillators (DCXO) with LVCMOS outputs (SiT3907) and differential outputs (SiT3921 and SiT3922). These oscillators allow users to vary the output frequency dynamically within a narrow range (up to ±1600 ppm) and with very high resolution (1 ppb). These devices also replace the analog interface in many VCXO applications.
  3. Serially programmable frequency select devices (SiT3509). These devices allow users to select nine different, unrelated frequencies by writing to internal device registers through SiTime's 1-wire tri-level interface. This family also includes the SiT3519 DCXO that allows users to dynamically vary the output frequency up to ±1600 ppm. Contact SiTime for more information.

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Are SiTime products RoHS certified?

Yes. All SiTime products are RoHS certified.

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Where can I locate RoHS documents for SiTime products?

Please visit the quality and reliability section of the website. You will need to be a registered user before you can download documents from this section.

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