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CDP6872 Datasheet(PDF) 5 Page - Intersil Corporation |
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CDP6872 Datasheet(HTML) 5 Page - Intersil Corporation |
5 / 15 page 5 CDP6872 Crystal Selection For general purpose applications, a Parallel Mode Crystal is a good choice for use with the CDP6872. However for applications where a precision frequency is required, the designer needs to consider other factors. Crystals are available in two types or modes of oscillation, Series and Parallel. Series Mode crystals are manufactured to operate at a specified frequency with zero load capaci- tance and appear as a near resistive impedance when oscil- lating. Parallel Mode crystals are manufactured to operate with a specific capacitive load in series, causing the crystal to operate at a more inductive impedance to cancel the load capacitor. Loading a crystal with a different capacitance will “pull” the frequency off its value. The CDP6872 has 4 operating frequency ranges. The higher three ranges do not add any loading capacitance to the oscillator circuit. The lowest range, 10kHz to 100kHz, auto- matically switches in two 15pF capacitors onto OSC IN and OSC OUT to eliminate potential start-up problems. These capacitors create an effective crystal loading capacitor equal to the series combination of these two capacitors. For the CDP6872, in the lowest range, the effective loading capaci- tance is 7.5pF. Therefore the choice for a crystal, in this range, should be a Parallel Mode crystal that requires a 7.5pF load. In the higher 3 frequency ranges, the capacitance on OSC IN and OSC OUT will be determined by package and layout parasitics, typically 4 to 5pF. Ideally the choice for crystal should be a Parallel Mode set for 2.5pF load. A crystal man- ufactured for a different load will be “pulled” from its nominal frequency (see Crystal Pullability). FIGURE 2. CDP6872 + - +5V VREG C1 C2 XTAL C3 2 OSC IN 3 OSC OUT 1 VDD Frequency Fine Tuning Two Methods will be discussed for fine adjustment of the crystal frequency. The first and preferred method (Figure 2), provides better frequency accuracy and oscillator stability than method two (Figure 3). Method one also eliminates start-up problems sometimes encountered with 32kHz tun- ing fork crystals. For best oscillator performance, two conditions must be met: the capacitive load must be matched to both the inverter and crystal to provide ideal conditions for oscillation, and the fre- quency of the oscillator must be adjustable to the desired frequency. In Method two these two goals can be at odds with each other; either the oscillator is trimmed to frequency by de-tuning the load circuit, or stability is increased at the expense of absolute frequency accuracy. Method one allows these two conditions to be met indepen- dently. The two fixed capacitors, C1 and C2, provide the opti- mum load to the oscillator and crystal. C3 adjusts the frequency at which the circuit oscillates without appreciably changing the load (and thus the stability) of the system. Once a value for C3 has been determined for the particular type of crystal being used, it could be replaced with a fixed capacitor. For the most precise control over oscillator fre- quency, C3 should remain adjustable. This three capacitor tuning method will be more accurate and stable than method two and is recommended for 32kHz tuning fork crystals; without it they may leap into an overtone mode when power is initially applied. Method two has been used for many years and may be pre- ferred in applications where cost or space is critical. Note that in both cases the crystal loading capacitors are con- nected between the oscillator and VDD; do not use VSS as an AC ground. The Simplified Block Diagram shows that the oscillating inverter does not directly connect to VSS but is ref- erenced to VDD and VRN. Therefore VDD is the best AC ground available. FIGURE 3. CDP6872 + - +5V VREG C1 C2 XTAL 2 OSC IN 3 OSC OUT 1 VDD |
Número de pieza similar - CDP6872 |
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Descripción similar - CDP6872 |
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