Driving Multiple Loads with 32 kHz Nano-Power MEMS Oscillators

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Optimizing board space and managing power consumption of wearable devices is critical in sustaining competitiveness within this fast growing consumer segment. With less power consumed, battery life is extended or provides the option to reduce the battery size. MEMS- based reference clocks offer an alternate to traditional quartz crystal timekeeping components, with advantages including a significant footprint reduction, improved accuracy, and lower system power. In addition, the capability to drive multiple loads with one ultra-small, nano-power oscillator is one way in which MEMS are supporting these improvements.

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A wearable device typically comprises a low-power MCU and a low-power RF transceiver (BLE1) and sensors as shown in the block diagram of Figure 1. The MCU and BLE devices consume a high percentage of the system power budget and are designed to spend most of the time in sleep mode to conserve power. In this mode all high-frequency oscillators and PLLs are disabled. The only continually operating clock sources are those derived from the always-ON 32 kHz resonator driven oscillators:

1. 32 kHz oscillator used by the MCU RTC and watch-dog timer 32 kHz oscillator as the BLE sleep clock reference*

* BLE – Bluetooth Low Energy is ubiquitous in today’s wearable devices.

An alternative solution which offers both space and power savings is to use one MEMS-based 32 kHz IM890 ultra-small, oscillator and TCXO to replace multiple 32 kHz quartz crystal-based oscillators. The output driver of one IM8XX can easily drive the 32 kHz crystal inputs of MCU and BLE devices without any signal reflections.

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Figure 2 shows how one IM890 oscillator can be used to replace two 32 kHz crystal resonators. The IM890 has typical rise/fall times of 20 ns when driving a 5 pF load. The IM890 output driver is capable of driving a combined trace and IC load of up to 100 pF without reflections, eliminating signal integrity issues.

Signal integrity can be demonstrated through IBIS simulations as shown in the following graphs with various transmission line conditions and output drive settings. The simulation setup is shown in Figure 3. IBIS models of IM890 LVCMOS output drivers with Vdd = 1.8V were used. To emulate the load seen by the IM890 a capacitor of various pF values is placed at the end of each transmission line.

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Figure 4 and Figure 5 show clock signal waveform at each load (C1, C2) obtained during IBIS simulations with the following setup:
1532 output driver = LVCMOS
50Ω transmission line length = 6 inches
C1 = 20 pF; C2 = 30 pF
The capacitance of each trace is ~17 pF and the total load seen by the 1532 output driver is 17pF*2+20pF+30pF = 84pF. As seen from the waveforms, the capacitive loads increase rise/fall time but does not degrade the clock waveform at either load.

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Figure 6 shows the waveform of a LVCMOS drive 532 at C1 at the end of a 100Ω 10-inch trace; C1 = C2 = 5 pF.

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In most applications, a 32 kHz IM890 oscillator provides an excellent clock source for several ICs within the system. This helps decrease the total power consumption of a system, increase its reliability and decrease the board size and BOM. ILSI MMD provides IBIS models for 15xx oscillators and strongly recommends running simulations to validate the signal integrity of the specific PCB configuration.