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CC1050-RTR1 数据表(PDF) 26 Page - Texas Instruments |
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CC1050-RTR1 数据表(HTML) 26 Page - Texas Instruments |
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26 / 43 page  CC1050 SWRS044 Page 26 of 40 Crystal oscillator CC1050 has an advanced amplitude regulated crystal oscillator. A high current is used to start up the oscillations. When the amplitude builds up, the current is reduced to what is necessary to maintain a 600 mVpp amplitude. This ensures a fast start-up, keeps the current consumption as well as the drive level to a minimum and makes the oscillator insensitive to ESR variations. An external clock signal or the internal crystal oscillator can be used as main frequency reference. An external clock signal should be connected to XOSC_Q1, while XOSC_Q2 should be left open. The XOSC_BYPASS bit in the XOSC register should be set when an external clock signal is used. The crystal frequency should be in the range 3-4, 6-8 or 9-16 MHz. Because the crystal frequency is used as reference for the data rate (as well as other internal functions), the following frequencies are recommended: 3.6864, 7.3728, 11.0592 or 14.7456 MHz. These frequencies will give accurate data rates. The crystal frequency range is selected by XOSC_FREQ1:0 in the MODEM0 register. To operate in synchronous mode at data rates different from the standards at 1.2, 2.4, 4.8 kBaud and so on, the crystal frequency can be scaled. The data rate (DR) will change proportionally to the new crystal frequency (f). To calculate the new crystal frequency: DR DR f f new xtal new xtal = _ Using the internal crystal oscillator, the crystal must be connected between XOSC_Q1 and XOSC_Q2. The oscillator is designed for parallel mode operation of the crystal. In addition loading capacitors (C3 and C4) for the crystal are required. The loading capacitor values depend on the total load capacitance, CL, specified for the crystal. The total load capacitance seen between the crystal terminals should equal CL for the crystal to oscillate at the specified frequency. parasitic L C C C C + + = 4 3 1 1 1 The parasitic capacitance is constituted by pin input capacitance and PCB stray capacitance. Typically the total parasitic capacitance is 8 pF. A trimming capacitor may be placed across C4 for initial tuning if necessary. The crystal oscillator circuit is shown in Figure 16. Typical component values for different values of CL are given in Table 4. The initial tolerance, temperature drift, ageing and load pulling should be carefully specified in order to meet the required frequency accuracy in a certain application. C4 C3 XTAL XOSC_Q1 XOSC_Q2 C4 C3 XTAL XTAL XOSC_Q1 XOSC_Q2 Figure 16. Crystal oscillator circuit Item CL= 12 pF CL= 16 pF CL= 22 pF C3 6.8 pF 15 pF 27 pF C4 6.8 pF 15 pF 27 pF Table 4. Crystal oscillator component values Not Recommended for New Designs |
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