1, the passive crystals - passive crystal need to use the DSP chip oscillator, on the datasheet have suggested connection method. Passive crystal without the problem of voltage, signal level is variable, that is, according to the circuit of vibration to decide, the same crystal can be applied to a variety of voltage, can be used for a variety of different clock signal voltage requirement of DSP, and the price is usually lower, so if conditions permit suggestion for general application with crystal, it is especially suited for product line rich batch big producers. Relatively passive crystals for vibration, its shortcomings are the signal quality is poorer, usually need to exactly match the peripheral circuit (used in signal matching capacitance and inductance, resistance, etc.), replacing different frequency of crystal peripheral circuit configuration need to make corresponding adjustment. Suggest using high precision quartz crystal, try not to low precision ceramic alert.
2, active crystals, crystal active vibration does not require DSP internal oscillator, signal quality is good, stable, and the connection method is relatively simple, mainly completes the power filtering, usually using a capacitance and inductance type PI filter network, consisting of the output resistance filter with a small resistance signals), does not need complex configuration circuit. Active crystals usually usage: a feet dangling, two feet grounded, three feet to output, four feet by voltage. Active crystals, relative to the passive crystal defects is the signal level is fixed, need to choose the right output level, poor flexibility, and the price is high. For timing requirements sensitive applications, personally think or active crystals is good, because you can choose to compare the precision of crystals, and even high temperature compensated crystal vibration. Some internal no start up circuit, DSP can only use active crystals, such as TI, 6000 series and so on. Active crystals are usually larger than passive crystals, but now many active crystal vibration table is posted, and crystal size, some even smaller than many crystals.
Some matters needing attention:
1, need good need to configure the PLL frequency doubling of DSP peripheral circuit configuration, main is to isolate and filtering;
Under 2, 20 MHZ crystal crystal vibration basically is the fundamental frequency of the device, good stability, mostly in the form of harmonic of more than 20 MHZ (such as 3 times harmonics, harmonic 5 times, etc.), the stability is poor, so strongly recommend the use of low frequency components, frequency doubling with PLL circuit, after all need peripheral configuration is mainly capacitor, resistor and inductor, its stability and the price is much better than the crystal crystal vibration device;
3, the clock signal line length as short as possible, line width as large as possible, and other printing line spacing as large as possible, close to the device layout, can walk inside when necessary, and surrounded by land line;
4, by introducing the clock signal back from the outside from time to tome special design requirements, need to refer to the relevant details.
In addition to do a few instructions:
Good overall aspects such as the stability of the crystals in the crystal, precision measurement, and other fields, in particular, the vast majority are high-grade crystal vibration, so we can put all kinds of compensation technology together, to reduce the complexity of the design. Just think, if the crystal, then the wave shaping their own design, anti-interference, temperature compensation, the complexity of the case design will be what kind of? We here rf circuit design on the clock demanding situations, is to use high precision removing crystals, industrial-grade to hundreds of yuan a.
Application in the field of special if no suitable crystals, meaning that the complexity of the design is beyond the finished product on the market crystal vibration level, must be own design, this kind of circumstance will choose crystal, but these crystals is certainly not the ordinary crystals on the market, but a special high-end crystal, such as ruby crystal, etc.
Higher requirements of the field is more special, we here in high precision testing using the clock even atomic clocks, rubidium standard devices, through a dedicated rf connector connection, is a large equipment, and cumbersome
