Energy-saving and very fast data communication or positioning after sleep mode is only possible with a highly accurate and fast 32,768 kHz system clock. With a 32,768 kHz silicon oscillator, a battery-operated solution based on hibernation technology can save over 50 per cent power. The specialists at PETERMANN-TECHNIK explain why 32,768 kHz silicon oscillators are gaining a foothold in battery-powered Hibernation Technology applications and what advantages they offer the user.
Many end products utilise hibernation technology, including wearables, Bluetooth Low Energy (BLE) communication units for commercial, industrial and automotive applications, IoT applications, GPS (commercial and automotive), M2M communication, personal trackers and medical patient monitoring systems, IoT, smart metering, home automation, wireless, etc...
How Hibernation Technology works
Hibernation Technology is mainly used in positioning applications and in end devices that exchange collected data with a receiver via Bluetooth Low Energy (BLE). In order to significantly increase battery life, particularly power-hungry circuit areas in these devices, such as the ICs for data transmission and positioning, are switched to energy-saving sleep mode whenever possible. As soon as the user searches for a new target or wants to transmit data via Bluetooth Low Energy, the switched-off areas must be woken up within a very short time and switched to energy-intensive working mode (Fig. 1).
Extremely short wake-up saves 50% system energy
For fast and energy-efficient data communication, the 32,768 kHz system clock must be highly accurate so that the application can run through the process shown in Fig. 1 very quickly and be put back into sleep mode immediately.
If the system clock is inaccurate, the energy-consuming process sequence shown in Figure 1 is repeated until the data has been transferred from the transmitter unit to the receiver device, for example from a wearable to a smartphone. The repetitions increase energy consumption and therefore significantly reduce battery life. In addition, the highly accurate 32,768 kHz reference frequency also eliminates the need for constant energy-consuming synchronisation of the system clocks between the transmitter and receiver. A very long autonomous operating time is a decisive factor in the market success of the transmitter unit. A patient monitoring device that does not allow a long period of use due to its high energy consumption will hardly be accepted. Users will ask themselves why they have to recharge their device or replace the battery and will not recommend the product or post a negative review on the Internet.
In GPS applications, another aspect of a very precise system clock is advantageous for saving energy: the extension of the hibernation periods while maintaining the quick start of less than one second.
Difference between 32,768 kHz crystal and crystal oscillator and 32,768 kHz ultra-low power oscillator
Due to the quartz cut, the temperature stability of a 32,768 kHz quartz crystal - unlike a MHz quartz crystal - cannot be restricted by changing the cutting angle. Over the temperature range from -40 °C to +85 °C, the most accurate temperature stability of a 32,768 kHz crystal is approximately -180 ppm (Fig. 2), while that of a MHz crystal is ±15 ppm.
For example, the 32,768 kHz silicon oscillator of the ULPPO series from PETERMANN-TECHNIK, which measures just 1.5 x 0.8 mm, has a temperature stability of ±5 ppm over the temperature range from -40 °C to +85 °C and is therefore 36 times more accurate than a 32,768 kHz quartz crystal. In addition, the ageing of the ULPPO is ±1 ppm after the first year and ±5 ppm after 10 years. The ageing of a 32,768 kHz crystal is ±3 ppm after the first year and well over ±20 ppm after 10 years. The frequency stability of a 32,768 kHz crystal at 25 °C, standard value ±20 ppm, must also be taken into account when considering the accuracy of the application. A 32,768 kHz crystal therefore only generates a very inaccurate 32,768 kHz system clock, which only enables very slow data communication and consumes a lot of power due to the described data communication repetitions.
Quartz oscillators with 32,768 kHz are also available on the market. These are larger (2.5 x 2.0 mm or 3.2 x 2.5 mm) and use different technologies. Quartz oscillators in which the 32,768 kHz is generated by dividing a MHz frequency (2.5 x 2.0 mm) are common. Such oscillators consume a few milliamperes and are therefore completely unsuitable for battery-powered solutions.
Other 32,768 kHz crystal oscillators (3.2 x 2.5 mm) are based directly on a 32,768 kHz crystal and consume less current if the frequency accuracy of the crystal is not compensated for by the oscillator IC. As a result, however, the frequency is just as inaccurate as that of a 32,768 kHz crystal, whereby this oscillator oscillates very slowly.
The third solution is based on a 32,768 kHz crystal and an oscillator IC that compensates for the very high frequency accuracy of the 32,768 kHz crystal, but starts very slowly at typically 3 seconds and therefore causes many current-consuming repetitions.
Power-saving solution from PETERMANN-TECHNIK
Most Bluetooth low-energy solutions use two 32,768 kHz crystals (one for the BLE IC's sleep mode and one for the MCU clock) and one MHz crystal as the reference frequency for the BLE chip (Fig. 3). In a typical wearable application, a 32,768 kHz silicon oscillator can simultaneously clock the sleep mode of the BLE and the MCU. This saves an enormous amount of space on the circuit board, as the ULPPO at 1.5 x 0.8 mm is only about half the size of the smallest 32,768 kHz crystal at 1.6 x 1.2 mm and 85 per cent smaller than a low-power crystal oscillator at 3.2 x 2.5 mm.
If the space requirement of a 32,768 kHz crystal with its two external decoupling capacitances to ground is taken into account, the ULPPO requires only 85 per cent of the space of a crystal-based solution. The ULPPO does not require any decoupling capacitors, as the integrated IC filters the supply voltage itself.
Extremely low power consumption
Even the standard version of PETERMANN-TECHNIK's 32,768 kHz silicon oscillators has an extremely low power consumption of less than 1 µA at a VDD of 1.8 VDC. In order to further reduce the current consumption, the output amplitude of the oscillator can be adapted to the ICs to be clocked. VOH can be set in the range from 0.6 to 1.225 V and VOL in the range from 0.35 to 0.8 V. A PMIC or MCU with a supply voltage of 1.8 VDC requires a VIH amplitude of 1.2 V or a VIL amplitude of 0.6 V. This allows the 32,768 kHz silicon oscillators to be optimally adapted to the MCU and the BLE in a power-saving manner: Another major advantage of next-generation silicon oscillator technology that a 32,768 kHz quartz oscillator does not offer.
High transient response reliability
As the 32,768 kHz crystals have very high resistances, they do not always harmonise perfectly with the oscillator stages of the ICs to be clocked. Sometimes the crystal oscillates, sometimes not. If it does, it is not always clear why. The negative input resistances of the oscillator stages of the ICs to be clocked often scatter enormously, or even capacitively. According to measurements by PETERMANN-TECHNIK specialists, it is not uncommon for over 25% capacitive dispersion to occur. This does not make the optimum wiring of a 32.768 kHz quartz crystal any easier and the frequency is also distorted in the circuit (frequency shift caused by the tuning sensitivity in ppm/pF of a quartz crystal). By using an ultra-low-power 32.768 kHz silicon oscillator, not only can several ICs be clocked simultaneously, but there are also no more transient problems and frequency shifts. Highest transient stability under all circumstances, at any temperature, at any time.
Enormous cost savings
A 32,768 kHz silicon oscillator from PETERMANN-TECHNIK saves two 32,768 kHz crystals and the circuit capacity, which reduces the space required on the circuit board enormously. A significantly smaller and more favourable circuit board is therefore sufficient for the application. Furthermore, the development, assembly, inspection and testing costs are also considerably reduced. Taking into account the lower component procurement and handling costs as well as more favourable component prices, the device manufacturer not only saves electricity, but also money.
Greener technology for a smarter world
Power-saving design starts with clocking. The ultra-low-power 32,768 kHz silicon oscillators from PETERMANN-TECHNIK are an example of how the right clock generators can extend the system energy of mobile devices based on hibernation technology by 50 per cent. The experts at clocking specialist PETERMANN-Technik advise on the selection of suitable components from the "Next Generation Clocking" product range with comprehensive technical support, design-in, fast sample and series delivery, enabling a fast time-to-market to be achieved.
Further information is available at:
SMD SPXO 32.768 kHz OSCILLATORS
or
Technical questions:
Phone: 0 81 91 / 30 53 95
E-mail: info(at)petermann-technik.de
