（RTC芯片DS1307中的圆柱32.768KHz晶振）
关于实时时钟精准度RTC ACCURACY
The frequency accuracy of a crystal-based oscillator circuit is mainly dependent upon the accuracy of the crystal and the accuracy of the match between the crystal and the oscillator capacitive load. If the capacitive load is less than the crystal was designed for, the oscillator runs fast. If the capacitive load is greater than what the crystal was designed for, the oscillator runs slow.
C476310_实时时钟(RTC)
In addition to the errors from the crystal and the load match, crystals vary from their base frequency as the ambient temperature changes. RTCs use “tuning fork” crystals, which exhibit an error over temperature, as shown in Figure 2 . An error of 20ppm is equivalent to approximately 1minute per month.
OSCILLATOR BASICS
The oscillator used in RTCs is a CMOS inverter variation of a Pierce-type oscillator. Figure 1 shows a general configuration. These RTCs include integrated load capacitors (CL1 and CL2) and bias resistors. The Pierce oscillator utilizes a crystal operating in parallel-resonance mode. Crystals used in parallel-resonance mode will be specified for a certain frequency with a specific load capacitance. For the oscillator to run at the correct frequency, the oscillator circuit must load crystal with the correct capacitive load.
CRYSTAL PARAMETERS
Figure 3 shows the equivalent circuit for a crystal. Near the resonate frequency the circuit consists of a series circuit including motional inductance L1, motional resistance R1, and motional capacitance C1. The parallel component C0 is the shunt capacitance of the crystal.
Figure 3. Crystal equivalent circuit.
The load capacitance CL is the capacitive load of the oscillating circuit as seen from the pins of the crystal. Table 1 shows CL as a capacitance in parallel with the crystal. The load capacitors
used in an oscillator circuit, CL1 and CL2, plus any stray capacitance in the circuit, combine to create the overall load capacitance. All RTCs have integrated CL1 and CL2 capacitors. Care should
be taken to minimize stray capacitance in the PC board layout. The following formula shows the relationship between CL and load capacitor values:
CL=[(CL1×CL2)/(CL1+CL2)+CSTRAY]
OSCILLATOR STARTUP TIME
Oscillator startup times are highly dependent upon crystal characteristics, PC board leakage, and layout. High ESR and excessive capacitive loads are the major contributors to long startup times.
A circuit using a crystal with the recommended characteristics and proper layout usually starts within one second.
