Design and implementation of a 2 μA temperature-compensated MEMS-based real-time clock with ±4 ppm timekeeping accuracy
Introduction
Silicon resonators are gaining increasingly more attention for use in modern electronics devices. They are especially valuable when low power consumption, high signal quality and high system integration level are considered. Currently, silicon resonators are the strongest competitors to quartz crystal resonators, which in spite of good quality, occupy more board area. MEMS devices, on the other hand, offer significant device area reduction. Moreover, the manufacturing process of silicon resonators is compatible with a standard CMOS line, giving the possibility of developing fully integrated, high quality frequency sources. The use of micromechanical devices in precision timing applications is being extensively studied and has been reported e.g. in [1], [2], [3].
Unlike their quartz counterparts, MEMS-based oscillators can be integrated in the same package with the interface electronics, or on the same die, as shown in [3]. One of the main drawbacks of MEMS devices is their inherently high Temperature Coefficient of Frequency (TCF). When targeting high accuracy and frequency stability, this temperature dependency needs to be compensated for. Several approaches to resolve this issue have been reported by other research groups, including material modifications [4], [5], [6], electronic circuitry or both [2], [7], [8], [9], [10].
The main focus of the design presented in this paper is in the use of a TIA-based oscillator with a low operating frequency to generate a temperature-compensated 1 Hz output signal. The presented system does not require high voltages to bias the resonator, the 1.2 V DC being sufficient for correct operation. This relaxes the requirement for a charge pump or a high-voltage tolerant silicon process.
The system presented in this work uses fractional-N division and an accurate on-chip temperature sensor to implement real-time clock with minimal timing variation over the process corners and −40 to 85 °C temperature range. The architecture described in this paper was chosen to allow for greater flexibility of the compensation system in order that the electronics interface could also be used with other resonator types.
Section snippets
System and circuit-level design
The core of the proposed system is an oscillator loop built on a MEMS resonator with a nominal frequency of 27 kHz. The correct operation of this block is ensured by a loop amplifier with an amplitude limiting circuit which prevents over-driving the resonator, without the need for any external control signals. The selection of oscillator frequency was dictated by two factors. Firstly, it should be high enough not to fall in the audio bandwidth and, secondly, low enough to minimize the active
Measurements
To characterize the system performance, several sets of measurements have been carried out. Due to the poor availability of the bonded circuits, only two sample systems have been characterized, and referred to in the following text as samples A and B. The timekeeping accuracy has been verified for each sample at 6 temperature points in the range −40 to 85 °C.
To minimize interconnect parasitics between the resonator and ASIC, both parts were bonded together and packaged on the same carrier, as
Summary
The design and measurements of a temperature-compensated real-time clock based on a micromechanical resonator were presented in this paper. The performance characteristics summarized in Table 1, Table 2 show that it can be a viable timing solution for accurate, low-power systems. The presented design was not optimized for power consumption, which can be further reduced by fine-tuning the analog electronics, especially the additional control circuitry and biasing network. For this prototype,
Acknowledgments
The authors would like to thank Murata Electronics Oy for providing MEMS elements and funding this work.
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Currently with Dialog Semiconductor, Istanbul, Turkey.