參數(shù)資料
型號(hào)： MC16S2CPU20B1
廠商： FREESCALE SEMICONDUCTOR INC
元件分類： 微控制器/微處理器
英文描述： 16-BIT, 20.97 MHz, MICROCONTROLLER, PQFP100
封裝： TQFP-100
文件頁(yè)數(shù)： 11/104頁(yè)
文件大?。?/td> 812K
代理商： MC16S2CPU20B1
MOTOROLA
MC68HC16S2
14
MC68HC16S2TS/D
3.3 System Clock
The system clock in the SIM provides timing signals for the IMB modules and for an external peripheral
bus. Because the MCU is a fully static design, register and memory contents are not affected when the
clock rate changes. System hardware and software support changes in clock rate during operation.
The system clock signal can be generated in one of two ways. An internal phase-locked loop can syn-
thesize the clock from a reference frequency, or the clock signal can be input directly from an external
source. Keep these clock sources in mind while reading the rest of this section. Figure 5 is a block
diagram of the system clock.
Figure 5 System Clock Block Diagram
3.3.1 Clock Sources
The state of the MODCLK pin during reset determines the system clock source. When MODCLK is held
high during reset, the clock synthesizer generates a clock signal from a reference frequency connected
to the EXTAL pin. The clock synthesizer control register (SYNCR) determines operating frequency and
mode of operation. When MODCLK is held low during reset, the clock synthesizer is disabled and an
external system clock signal must be applied. The SYNCR control bits have no effect.
The input clock is referred to as “fref”, and can be either a crystal or an external clock source. The output
of the clock system is referred to as “fsys”. Ensure that fref and fsys are within normal operating limits.
The reference frequency for this MCU is typically 32.768 kHz, but can range from 25 kHz to 50 kHz. To
generate a reference frequency using the crystal oscillator, a reference crystal must be connected be-
tween the EXTAL and XTAL pins. Figure 6 shows a recommended circuit.
32 PLL BLOCK
PHASE
COMPARATOR
LOW-PASS
FILTER
VCO
CRYSTAL
OSCILLATOR
SYSTEM
CLOCK
SYSTEM CLOCK CONTROL
FEEDBACK DIVIDER
W
X
Y
EXTAL
XTAL
XFC
VDDSYN
CLKOUT
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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