Digi-Key custom programming optimizes SiTime MEMS crystal oscillator selection

Silicon MEMS (Micro-electro-mechanical systems) crystals are becoming increasingly popular, mainly because designers need timing solutions with higher quality standards. Silicon MEMS crystals not only improve timing accuracy, but also develop products that will not wear out prematurely, can cope with a wider range of external environments, have longer battery life, and are smaller in size. So far, SiTime is the leader in silicon MEMS timing solutions, with a market share of 90% and shipments far exceeding 1 billion units.

MEMS oscillators are inherently superior to traditional quartz oscillators due to their design and construction. MEMS oscillators have a more standardized and simpler manufacturing process, so there is less chance of contamination. This results in a 30-fold improvement in DPPM (defective parts per million) and MTBF (mean time between failures) compared to quartz, which means higher quality and reliability. They are also more robust in most environmental conditions. Because MEMS oscillators have no exposed PCB between the resonator and the crystal circuit, and because the silicon crystal circuit is optimized for electrically noisy conditions, they are much less sensitive to EMI (electromagnetic interference). And because their mass is thousands of times smaller than that of quartz resonators, they are much less sensitive to shock and vibration. Similar design features make them less sensitive to operating temperature and circuit board noise.

There is a significant benefit to using quartz crystals, which is that they are much cheaper than MEMS crystals. This may be the reason why many people ignore the many advantages mentioned above and choose quartz, of course some applications simply do not require components with the highest quality, reliability and robustness. However, it is necessary to consider the BOM beyond the parts. First, quartz crystals require greater engineering design, not only to select the correct resonator, but also to avoid and overcome common problems such as cold start failure, crystal mismatch, EMI compliance failure, etc. MEMS crystals, on the other hand, greatly simplify system design through "plug and play" and do not have the above problems. Secondly, MEMS crystals have a smaller board area, can drive multiple loads, and do not require load capacitors; MEMS crystals use greatly reduced board space and may have a smaller overall BOM (bill of materials) cost. Finally, SiTime MEMS crystals are programmable, convenient for mass applications, and allow engineers more flexible designs, thereby accelerating time to market.

As a complete timing solution, SiTime crystal oscillators integrate MEMS resonators and programmable CMOS ICs (including oscillation circuits, phase-locked loops, temperature compensation, voltage regulators, and filters) in the same package. Quartz crystal oscillators must manufacture different parts to obtain different resonant frequencies, while MEMS crystal oscillators are programmed with different configurations to produce different output frequencies through programmable crystal oscillators or directly using "blank" crystal oscillators. In addition to frequency, there are many other parameters that can be configured, including supply voltage, frequency stability, output drive strength (i.e., rise/fall time), etc. SiTime produces multiple series of crystal oscillators, covering each crystal type (e.g., XO, TCXO, VCXO, DCXO, OCXO, and SSXO) as well as some special categories such as low power, automotive, rugged, etc. Currently, there are dozens of crystal oscillator series to choose from and hundreds of blank part numbers available. From this relatively small blank group, billions of products with different specifications can be generated.

As an example, consider the blank part number SIT2001 1BI-S1-XXX-0.0.FP000, which is the SIT2001 standard series SOT23 crystal in the industrial temperature range. The part number breakdown for this series is shown in Figure 1 below. Note that the frequency can be selected from a range of 1MHz to 110MHz with an accuracy of six decimal places! This means that this single physical part can produce 109,000,001 total frequencies. Also considering the three options for "Feature Pins", six options for "Supply Voltage", three options for "Frequency Stability", and nine options for "Output Drive Capability", there are a maximum of 52,974 million part numbers that can be derived from the same blank part.

Figure 1: Part number guide, taken from the datasheet for the SiT2001B series SiTime oscillators.

These blank parts can be programmed by SiTime at the factory in production quantities with a lead time of 3 to 5 weeks. Until now, Digi-Key has been ordering these pre-programmed parts from SiTime and keeping the most popular crystals in each series in stock. This is not unlike how Digi-Key handles non-programmable parts such as quartz resonators, and it works well for customers who order standard crystal configurations. However, any customer requesting a custom configuration must wait for a lead time of 3 to 5 weeks, depending on the quantity. When these custom requests are made, the part number is added to Digi-Key's catalog, but it is not usually kept in stock because they are rarely ordered. At last count, Digi-Key listed 176,509 different part numbers for SiTime crystals on its website, but only 5,913 parts (a mere 3.35%) were in stock.

To overcome these issues and begin to take full advantage of the programmability of SiTime oscillators, Digi-Key recently added an automated programming machine to its warehouse dedicated to programming SiTime parts. This machine (Figure 2) currently uses eight programming sockets and can program 1,500 units per hour. Most part numbers in our catalog are now available for order with lead times as short as 24 to 48 hours, with no minimum quantities! Digi-Key will simply program and ship from blank parts, rather than pre-programming from the SiTime factory. This also means that Digi-Key now has almost all SiTime oscillators in stock, with the exception of a few series that must be specially programmed at the factory. In other words, after switching to custom programming, Digi-Key now has 3.35% of the inventory, which is equivalent to 99% of the SiTime oscillator category.

Please note that while the process for ordering SiTime oscillators has not changed, custom orders must now be submitted through the SiTime Programmable Oscillators category on the Digi-Key Tech Forum. Once the request is submitted, we will immediately email our Application Engineering staff who will verify the new part number and add it to the Digi-Key website.

Figure 2: Digi-Key’s automatic programming machine (Image source: Digi-Key)

SiTime MEMS oscillators offer a compelling alternative to traditional timing solutions. Not only are they higher quality, more reliable, and more robust; their integrated and programmable design does not incur high engineering costs, takes up less board space, and provides greater design flexibility. From a few hundred basic part numbers across the oscillator family, billions of part numbers with unique configurations are available. However, without a programmer, only standard configurations are available. In fact, before automatic programming machines were dedicated to SiTime oscillators, uncommon custom configurations were only available with a fairly long production cycle. But now, with Digi-Key's fast shipping and custom programming capabilities, customers can get any number of SiTime oscillators in any configuration in a matter of days.

Matt Mielke received his Bachelor of Science in Computer Engineering from the South Dakota School of Mines and Technology in 2016. He then joined the Application Engineering department at Digi-Key Electronics to support customers through the development of reference designs and the generation of technical content. His areas of interest include embedded programming, low-power design, and digital signal processing.