32768 crystal oscillator full series of technical details (32,768 crystal oscillator error and 32768 for BLE)

This is a real case of a certain power product: a batch of electric metering products existed in the warehouse and did not go online. It just happened to be condensed on the heavy PCB in the southern day, and there were a large number of 32768 vibration stops. So today we want to share with you is the 32768 crystal oscillator, we will mention several related cases and precautions later.

The Tuning Fork has been circled in the picture! This is the essence of the 32768 crystal oscillator to be said today. The basic nouns of piezoelectric effect, resonance, and overtone are not repeated here, and you can ask yourself if you don't.

The tuning fork is not just a scientific name, but a real existing structure (the fork in the figure is the cut quartz).

After reading this structure, it is easy to understand when I talk about a case (32768 in production is damaged).

Seriously speaking, this is a frequently-mentioned design misunderstanding, including many questions and answers about the 32768 crystal oscillator.

When I was studying (ignoring my age), I did this: I went to the electronics market and bought 32,768 crystal oscillators. I used 15pF with reference to other people's schematics. The 22pf capacitors are used directly. Looking at it now, this is just an entry-level experience.

For example, the 32768 crystal oscillator in the 2*6 package of the Japanese KDS brand has two kinds of load capacitance:

DT-26 32.768kHz 6PF 10PPM

DT-26 32.768kHz 12.5PF 10PPM

The load capacitance (English symbol CL or CLoad) is composed of two parts: one part is the material/actual capacitance of the solder, and the other part is the parasitic capacitance/stray capacitance Cstray (generally 2-5pF) caused by the PCB trace and the pad.

At this point, some people have already solved the doubt: Oh, the load capacitance parameter in the original crystal datasheet is not the capacitance value I want to paste.

The calculation formula and diagram are as follows:

You may have questions: the reference design provided by the original MCU I used does not have the RD and RF in the figure.

Yes! That's right! RD is rare in current popular MCUs. RF has a small number of MCUs or ICs still in use.

When the oscillation waveform is distorted and peaked, RD (tens of K to several hundred K) can be used.

RF is commonly used 1M ohms, generally refer to the IC's datasheet, related to IC design / internal drivers.

The problem is coming: I have seen some microcontrollers connected to the 32768 crystal oscillator, but the load capacitor is not connected. Is this the case?

The answer is yes, because some ICs can be configured with internal load capacitors, such as Ti's MSP430x2xxx series, which are equipped with 1pF, 6pF, 10pF, and 12.5pF four-speed load capacitors.

The crystal's error is expressed in PPM (parts per million), and the error consists of three parts: factory error, temperature drift, and annual aging rate.

Factory error: generally a range of screening control before leaving the factory, such as ± 10 PPM;

Temperature drift: As shown in the figure, the 32768 crystal oscillator has a typical temperature curve;

Aging rate: This is a parameter that is easily overlooked, but it is related to the life of the product. For example, if you use inferior crystal oscillator in RF communication and drift over time, you may not be able to communicate after a few years (especially narrowband communication). ).

Another problem mentioned here is the two capacitors CD and CG used in the load capacitor part of the previous figure. By fine-tuning these two capacitors, the frequency of the crystal can be fine-tuned. In demanding designs, it is generally recommended to use the most stable C0G (EIA standard, American Institute of Electrical Engineers) or NP0 (US military standard MIL) material.

For ordinary consumer electronics applications where clock requirements are not high, the 32768 crystal oscillator does not actually have to be compensated. With one exception, with the popularity of smartphones, BLE is now widely used in consumer electronics, but its 32768 crystal oscillator section is still well designed and controlled (as will be mentioned later).

From the typical values ​​of the pictures, 30 PPMs are less than 3 seconds per day. Is it necessary to compensate?

Friends who do the national grid meter will certainly not think so, because the standard requires a clock error of less than 0.5 seconds per day. This is actually a headache, because it requires the factory error heating and drift control to less than 5PPM. There are two solutions:

First, the SoC scheme requires the master MCU to be equipped with an RTC module that supports error correction and temperature acquisition. TI's msp430 series, some models with RTC module, support write calibration values ​​and temperature compensation values.钜泉meter MCU, HT6xxx is also equipped with RTC module with compensation, as shown in the figure.

Second, the use of external crystal module has been temperature correction, eg. M41TC8025, EPSON RX-8025T and so on.

BLE low-power Bluetooth technology, with the iphone4S hot sale has been greatly promoted.

BLE low-power implementation principle: Most of the time the slave keeps ultra-low power sleep, including turning off the high-frequency crystal used by RF (32M of TI cc254x series, 24M of cypress BLE chip), intermittently opening RF handshake Communication, maintaining the normal connection state of the link. Focus on this time interval is achieved by 32768 crystal oscillator. In the case of many actual cases, if the crystal oscillator is not quality controlled, it is easy to break the timeout.

For example, the cypress PSoC datasheet supports software configuration to compensate for this sleep clock:

There are similar mechanisms in TI chips.

Of course, this is not what the chip factory has come up with; to trace the source, this has clear requirements and conventions in the official specification of bluetooth.

A. Pay attention to the accuracy parameters, load capacitance, ESR and other indicators in the manual.

B, the 32768 crystal oscillator of the plug-in package, some people like the shell grounding. If soldering by hand, the soldering iron is too hot or the soldering time is too long, it is easy to cause permanent damage to the crystal. This is also a real production case.

C. At the beginning of the article, in the case of the wetness of the southern sky, the crystal oscillator area and related pins must be coated with three anti-paint protection.

D, ultrasonic welding plastic shell, it is very easy to cause damage of 32768 crystal oscillator (frequency is close, ultrasonic vibration caused by tuning fork in the crystal), this real case is also a lot, remember! Remember!

E, crystal layout and routing in the PCB layout. There are also online, not much to say, only a few reference designs are given here.

F, crystal oscillators, precision op amps and other devices are often afraid of solder paste. what is the reason? Solder pastes and the like have an effect on weak signals. We also have several cases of poor post-production testing (normal operation after cleaning the board).

Appreciation of Several Oscillation Circuits in CAMBRIDGE UNIVERSITY

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