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參數(shù)資料
| 型號： | MQ80C52CXXX-20/883R |
| 廠商： | ATMEL CORP |
| 元件分類： | 微控制器/微處理器 |
| 英文描述： | 8-BIT, MROM, 20 MHz, MICROCONTROLLER, CQFP44 |
| 封裝： | CERAMIC, QFP-44 |
| 文件頁數(shù)： | 97/101頁 |
| 文件大小： | 3398K |
| 代理商： | MQ80C52CXXX-20/883R |
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118
7.7.2 Integrated Oscillator Setup
Using the internal oscillator, the oscillation frequency depends on the load capacitance
between the crystal pins XTAL1 and XTAL2. The total load capacitance CL must be
equal to the specified load capacitance of the crystal itself. It consists of the external
capacitors CX and parasitic capacitances connected to the XTAL nodes. Figure 7-15
shows parasitic capacitances, such as PCB stray capacitances.
Figure 7-15. Simplified XOSC Schematic with External Components
CX
16MHz
XTAL2
XTAL1
EVDD
CTRIM
CPCB
AT86RF212
PCB
XTAL_TRIM[3:0]
EVDD
VDD
XTAL_TRIM[3:0]
Additional internal trimming capacitors CTRIM are available. Values in the range from
0 pF to 4.5 pF with 0.3 pF resolution are selectable using the bits XTAL_TRIM of
register 0x12 (XOSC_CTRL). To calculate the total load capacitance, the following
CL = 0.5 (CX + CTRIM + CPAR + CPCB)
The trimming capacitors provide the possibility of reducing frequency deviations caused
by production process variations or by external components tolerances. Note that the
oscillation frequency can only be reduced by increasing the trimming capacitance. The
frequency deviation caused by one step of CTRIM decreases with increasing crystal load
capacitor values.
An amplitude control circuit is included to ensure stable operation under different
operating conditions and for different crystal types. Enabling the crystal oscillator in
P_ON state and after leaving SLEEP state causes a slightly higher current during the
amplitude build-up phase to guarantee a short start-up time. At stable operation, the
current is reduced to the amount necessary for a robust operation. This also keeps the
drive level of the crystal low.
Generally, crystals with a higher load capacitance are less sensitive to parasitic pulling
effects caused by external component variations or by variations of board and circuit
parasitics. On the other hand, a larger crystal load capacitance results in a longer start-
up time and a higher steady state current consumption.
AT86RF212
8168C-MCU Wireless-02/10
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