考虑马克西姆实时时钟晶体的选择-Considerations

Abstract: This applicaTIon note describes crystal selecTIon criteria and proper layout techniques for connecTIng a 32kHz crystal to a real-TIme clock (RTC). Following these guidelines will prevent time errors and incorrect time due to crosstalk and improper crystal selection.

All Maxim real-time-clock (RTC) products require an external 32.768kHz watch crystal for use on their on-board crystal oscillator circuit. Except for this crystal, no other external components are required. Selection of the external crystal varies depending on the end application. Considerations are operating temperature range, package style/size, crystal load capacitance, and cost.

Crystal ManufacturersThere are many crystal manufacturers for 32.768kHz "watch crystals." A few of them are listed below, along with their web addresses for quick access to technical details regarding crystals.

Manufacturer Website Raltron http://www.raltron.com ECS INC International http://www.ecsxtal.com SaRonix http://www.saronix.com Abracon Corporation http://www.abracon.com Fox Electronics http://www.foxonline.com M-tron http://www.mtron.com Caliber Electronics http://www.caliberelectronincs.com Common Watch-Crystal SpecificationsWatch crystals at 32.768kHz are optimized for either 6pF or 12.5pF loads (refer to specific Maxim RTC data sheets for which crystal is required). There are common electrical characteristics for both, and these specifications are listed below. In addition, there is a common parabolic temperature curve for watch crystals, shown in Figure 1. Above or below +25°C, the frequency becomes lower with changes in temperature as determined by the frequency stability constant K, which has a maximum value of -0.040 PPM/(Δ°C)².

Parameter Symbol Min Typ Max Units Frequency f   32.768   kHz Equivalent Series Resistance (ESR) Rs 35   60 KΩ Frequency Stability K -0.028   -0.040 PPM/(Δ°C)² Turnover Temperature To +20   +30 °C Drive Level       1.0 µW Aging (first year @ 25 °C)   -3.0   +3.0 PPM Load Capacitance     6pF or 12.5pF     Q Factor Q 40,000   60,000  

Figure 1. Parabolic temperature curve.

In general, an RTC that uses a 12.5pF crystal will have a timekeeping current of 1.7X more than an RTC that uses a 6pF crystal (that is, 6pF RTC current = 300nA @2V whereas 12.5pF RTC current = 500nA@2V). Timekeeping current is that measured when there is no serial bus activity and the RTC is only using current to run its 32.768kHz oscillator and count real time. A standard, low-cost, 50ma-hr, lithium, backup battery powers an RTC with timekeeping current of less than 570nA for up to 10 years.

A 12.5pF load crystal oscillator is somewhat more stable and less susceptible to noise and PCB layout stray capacitance than a 6pF load crystal oscillator. This is partly due to the capacitance from each crystal pin to ground, internal to the RTC, which is 25pF per pin for the 12.5pF crystal RTC and 12pF for the 6pF crystal RTC.

There are also inventory issues for 32.768kHz watch crystals. Load crystals at 12.5pF are readily available through many distributors. Load crystals at 6pF are not as readily available and can require a minimum-quantity order to procure. Check with your local supplier for availability and price.

It is important to use the correct specified crystal with an RTC, for the wrong crystal can cause as much as a 100ppm error in the 32.768kHz oscillator frequency, which translates to a 4.3-minute error over a one-month period. In addition, the wrong crystal can cause excessive timekeeping current or failure of the oscillator to start properly. Through-Hole Leaded CrystalsThere are three common packages for through-hole leaded crystals. They are available in both 6pF and 12.5pF crystal loads. These package styles are shown below, along with manufacturers who offer such packages.


Figure 2. Package for through-hole leaded crystals.

REF TH1 TH2 TH3 A