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一开始，时钟数字电子设备很简单。您从一个基本的Pierce振荡器开始，该振荡器由压电石英晶晶体控制，将最终的正弦波从顶部剪掉，滤除了噪声，并将脉冲划分为可用的时钟速率。甚至还有一种可编程性。半导体时钟设备的制造商提供了可选择电阻的划分率。后来，一些“硅振荡器”使用了R-C多启动器代替石英。它们价格较低，但提供较少的温度稳定性。
That was the situation in the late 20th century and the early 21st. In 2006, SpectraLinear began offering greater programmability. The company’s EPro clock family incorporated up to four programmable phase-locked loops (PLLs) that allowed users to fine-tune output impedance (which in effect controlled drive strength), output skew, operating frequency, and spread-spectrum profile. Also, clock outputs often could be programmed to be in-phase or be 180° out of phase(see“Bulletproof Your System Timing With Programmable Clocks”)。
When Silicon Labs acquired SpectraLinear, it gained additional expertise in clock programmability. That capability has more recently grown with a new CMOS-MEMS technology developed inside Silicon Labs. This technology is a late addition, however. SiTime, an independent startup, offered the first devices based on silicon microelectromechanical-systems (MEMS) clock resonators in 2006. In 2007, Discera announced products based on its PureSilicon MEMS technology. The company has not discussed PureSilicon’s details in the media, though.
Complementing its acquisition of SpectraLinear last June, Silicon Labs introduced what it characterized as the industry’s first single-die MEMS oscillator, meaning that its MEMS structures are built directly on top of the same CMOS wafers that incorporate its analog circuitry. (SiTime uses separate die.) Silicon Labs designates the products “CMEMS oscillators” and says the process supports wafer probing of complete oscillator systems for quality and process control（看 ”Inside Silicon Labs’ CMEMS Resonators”)。更具体地，硅实验室保证产品将符合其数据表性能规格，以相对于焊接，拉力拉力，V，VDD至少10年的变化，工作温度范围，振动和冲击。
同样，在2010年，IDT收购了Mobius，这是一家由Michael McCorquodale创立的初创公司，该公司制造了硅LC谐波振荡器。但是次年，麦考夸尔左（McCorquodale（图。1）。随后,在2012年和2013年,踊跃参与了dvanced pMEMS products with multiple outputs and reduced jitter(see“pMEMS Oscillators Make Waves Without The Crystal”)。
MEMS Vs. Piezoelectric Quartz
SiTime might be considered the grandfather of the business. Since shipping its first products in 2006, it has moved 160 million units to more than 800 customers, with 40 to 50 programmable devices yielding about 200,000 part numbers. It also commands 80% of the MEMS timing market. The company was founded on the expectation that resonators based on silicon MEMS technology, unlike piezoelectric quartz crustal resonators, could utilize the economies of the silicon foundry infrastructure to make inroads into quartz dominance.
黎明前的MEMS-based计时设备,piezoelectric quartz-based suppliers included IDT, Texas Instruments, Analog Devices, Silicon Labs, and Microsemi making analog semiconductor-based chips. Epson, Kyocera, NDK, and Vectron had the quartz crystal technology. Quartz-based suppliers could buy analog ICs from the semiconductor companies and the semiconductor suppliers could offer timing devices, using an external quartz device as a reference. There was no successful unified solution until SiTime integrated MEMS regulators with its own electronics.
“The quartz guys had a lock on their expertise in precision machining of quartz crystals. They could take a quartz crystal and cut it and do all kinds of things to it to make sure that it operated at a standard frequency and stayed that way for 15 or 20 years. And they made a fabulous job of it,” says Piyush Sevalia, executive vice president of marketing at SiTime.
Meanwhile, the semiconductor-clock companies didn’t have the expertise in the special kind of packaging that quartz crystals require. Quartz needs vacuum-sealed ceramic packaging to ensure that that the resonator’s Q remains high. The need for separate packaging of resonator and analog electronics had several drawbacks, not least that in consumer products, a lot of board space was taken up with clocks, limiting opportunities for miniaturization. In those days, a device put out one clock output, and designers had to accommodate that.
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以.pdf格式下载本文 This file type includes high resolution graphics and schematics. |
SiTime was the first to successfully resolve the various problems, probably because its founders’ backgrounds embraced both analog and silicon MEMS resonator expertise. Building the resonator out of silicon and packaging it with the clock’s analog circuitry made it possible to provide multiple clock outputs from a single, compact semiconductor package, providing product designers with greater versatility in their own packaging(Fig. 2)。
For example, one of SiTimes’ MEMS wafers may look like any other semiconductor wafer. Yet it can hold from 30,000 to 50,000 or more MEMS die that are vacuum-sealed as part of the wafer processing. For an animation of the entire process, seewww.sitime.com/videos/mems-first.html。
MEMS Markets
如今，这些产品已成为最新的数码相机，平板电脑，摄像头，电子阅读器，机顶盒和消费者娱乐设备所必需的。在通信方面，它们进入开关，路由器，基地，固态驱动器和云存储。
The ability to use a conventional 0.18-µm silicon-foundry process and back-end technology was of some advantage, but it still left quartz a competitive edge when price was a buyer’s only criterion. However, silicon made inroads because of its supply chain advantages, reliability, and robustness.
“Because it’s silicon, we can get fully configured devices shipped in production in three to four weeks. With quartz, it might take eight to 10 weeks to get that. We can also be 10 times more reliable and 10 or 20 times more robust in terms of resistance to shock and vibration,” Sevalia says.
Sevalia市场渗透也有accelerat说ed as products for end users get smaller.
“Smart phones typically did not use MEMS timing components until we came out with a tiny 32-kHz device,” he says. “This could be a breakthrough, the proverbial hockey stick, that can drive revenues for the MEMS-timing companies dramatically. The industry, as a whole, could benefit.”
For instance, wearable electronics such as the personal health monitors use SiTime’s 32-kHz devices. Additionally, another set of new applications for silicon clocks is keeping networking systems synchronized.
“您的手机告诉时间。为此有一个本地时钟。但是，该本地时钟必须同步到全球中央时钟。在femtocells中，他们正在谈论与GPS同步，以及其他选择。这样做的方法是拥有本地时钟以及一些算法和软件来执行同步功能。” Sevalia说。
“通常，我们认为同步趋势将朝着更高的精度朝着更高的精度迈进，这意味着频率稳定性必须同样越来越好。如果时钟的频率不精确，则不确定性窗口会扩大，您将无法获得所需的性能水平。”他说。
Particularly in mobile, the emphasis on precision is a recent trend.
他说：“抖动的性能必须随着时间的流逝而变得更好，因为抖动减少了可以同步时钟和数据的不确定性窗口。”
添加功能
不包括MEMS谐振器，可编程性一直是所有这些公司定位的重要组成部分。Silicon Labs和Siteme每个都有一个程序员，可以使他们在几秒钟内编程这些设备并提供样品。对于其通信客户，Sitime还专注于高温（125°c）操作。大多数通信设备都在室外环境中运行，因此它需要能够承受40°C至45°C temperatures that are common in hot places. They can offer oscillators with guaranteed operation up to 125°C。
Each device provides temperature compensation to cancel out the frequency drift as temperature changes affect the resonator. Then, you have the output drivers and one-time programmable memory. SiTime has been able to design kilohertz devices with jitter as low as 500 fs, stability as low as 0.1 ppm, and very low (700 nA) power consumption.
Programmable parameters include drive strength; rise and fall times to reduce electromagnetic interference (EMI), improve jitter, or drive large/small loads; frequency to six decimals of accuracy; and stability, temperature, and signaling level (LVPECL or LVDS). For long traces, designers can make the drive stronger. If the trace is short, they can make the drive strength weaker and reduce EMI.
For a standard 15-pf load, standard drive strengths will result in a typical square wave. If designers wanted to drive a softer load, they would get pulses with fast leading edges. If they want to drive a harder load, they would get a softer leading edge. There is a significant difference in EMI between the two. So if designers are looking to their clock source to reduce EMI, a soft edge can help.
Latest SiTime Products
Sitime最近的32 kHzSIT15XXfamily of can be further divided into SiT153x devices that operate from a 1.2- to 3.63-V regulated supply, and SiT154x devices that operate from an unregulated 2.7- to 4.5V power source. Both come in comes in chip-scale packaging. The former are aimed at smartphones that use coin-cell or super-cap battery backup; the latter are for devices that use Li-Ion batteries.
Announced in March, they are presently the smallest timing devices for 32-kHz timekeeping functions, with a total footprint fully 85% smaller than the closest competing devices. None of the devices in the family require external decoupling, or load capacitors. (Quartz resonators typically require a load capacitor for excitation.) The small footprint makes the devices attractive for smart phones and tablets（看 ”Inside SiTimes’ Resonators,”)。
最初，Siteme MEMS谐振器表现出30 ppm/°C的频率变化。在–40°C至85°C的温度范围内，模拟电子设备温度补偿之前可能有4000-PPM的变化，将时钟输出变化降至低至0.1 ppm。相比之下，KHz石英设备本质上实现了0.4-PPM稳定性。
7月，该公司推出了用于智能手机的Tempflat Mems。这些与以前使用补偿电路以稳定温度稳定输出频率的MEMS振荡器不同。它们是最早在没有温度补偿的情况下超过石英共振器的最早的MEMS共振器。优点是，如果不需要补偿30 ppm/°C的谐振器变化，则电路消耗较小的功率，模具面积较小，电路的性能较好，并且系统设计更容易。(fig 3)
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