Using the DS87C530/DS5250 Real

文章摘要：RTC Crystal ConsiderationsThe most important factor in the accuracy of the RTC (or any oscillator) is the characteristics of the oscillator crystal. The DS87C530 is rated for an accuracy of ±2 minutes per month over the full operating range of the device. Even higher accuracy can be obtained by cont

Using the DS87C530/DS5250 Real,标签：计算机控制技术,工厂电气控制技术,http://www.88dzw.com

RTC Crystal Considerations

The most important factor in the accuracy of the RTC (or any oscillator) is the characteristics of the oscillator crystal. The DS87C530 is rated for an accuracy of ±2 minutes per month over the full operating range of the device. Even higher accuracy can be obtained by controlling the temperature of the device and using the RTC calibration procedures described later. The DS87C530 has been designed to operate with 32.768kHz RTC crystals with a load capacitance (CL) of 6pF or 12.5pF. Unlike some crystal amplifiers, no external load capacitors are needed with the RTC crystal.

Dallas Semiconductor products are compatible with industry standard crystals. Table 2 shows a number of common 32.768kHz crystals. This list is by no means exhaustive, and the inclusion or exclusion of any vendor from this list is in no way a comment on the suitability of a specific crystal in a customer's application.

Table 2. Standard 12.5pF And 6pF RTC Crystals

MANUFACTURER	MODEL	CL	PACKAGE
Epson Crystal Corp.	MC-306 32.768K E	6.0pF	SMT
	MC-306 32.768K A	12.5pF	SMT
KDS America	DT-26S 32.768kHz	6.pF	Cylinder
	DT-26S 32.768kHz	12.5pF	Cylinder
	DMX-26 32.768kHz	6.pF	SMT
	DMX-26 32.768kHz	12.5pF	SMT
AVX/Kyocera	KF-38G-12P5200	12.5pF	Cylinder
	KS-309G-12P5200	12.5pF	SMT

Selecting Load Capacitance

The value of CL has the most bearing on the long-term accuracy of the RTC. This parameter specifies the capacitive load that the crystal needs to "see" across its pins to oscillate at its rated frequency. Note that CL is not the capacitance of the crystal itself, but rather the capacitance of the oscillator circuit and any capacitors connected to the crystal. Using a crystal that has a different CL than the actual load capacitance of the circuit will affect the frequency of the oscillator. In general, using a crystal with a CL that is larger than the load capacitance of the oscillator circuit will cause the oscillator to run faster than the specified nominal frequency of the crystal, and vice versa.

The DS87C530 defaults to a mode which makes it compatible with a 12.5pF crystal, but can be switched to 6pF by clearing the RTC Capacitance Select bit X12/6 (TRIM.6). Although both crystal types will remain within the specified accuracy, each has a different advantage. The reduced loading of a 6pF crystal will reduce the power consumption of the RTC crystal oscillator by 25 to 50 percent, increasing the life of the backup battery. A 12.5pF crystal, however, is less affected by noise and will maintain a higher accuracy over an extended time. Changing the capacitance of the RTC crystal amplifier has no effect on the system clock crystal attached to the X1 and X2 pins.

Fine-Tuning the Oscillator Frequency

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