参数资料
型号: MC68334GCFC20
厂商: MOTOROLA INC
元件分类: 微控制器/微处理器
英文描述: 32-BIT, 20 MHz, MICROCONTROLLER, PQFP132
封装: PLASTIC, QFP-132
文件页数: 11/76页
文件大小: 444K
代理商: MC68334GCFC20
MC68334
MOTOROLA
MC68334TS/D REV. 1
19
PITM[7:0] — Periodic Interrupt Timing Modulus Field
This is an 8-bit timing modulus. The period of the timer can be calculated as follows:
where
PIT Period = Periodic interrupt timer period
PITM = Periodic interrupt timer register modulus (PITR[7:0])
EXTAL Frequency = Crystal frequency
Prescale = 512 or 1 depending on the state of the PTP bit in the PITR
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 three ways. An internal phase-locked loop can syn-
thesize the clock from either an internal reference or an external reference, or the clock signal can be
input from an external frequency source. Keep these clock sources in mind while reading the rest of this
section. The figure below is a block diagram of the system clock.
Figure 6 System Clock Block Diagram
3.3.1 Clock Sources
The state of the clock mode (MODCLK) pin during reset determines clock source. When MODCLK is
held high during reset, the clock synthesizer generates a clock signal from either an internal or an ex-
ternal reference frequency — the clock synthesizer control register (SYNCR) determines operating fre-
quency 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 — SYNCR control bits have no effect.
To generate a reference frequency using the internal oscillator a reference crystal must be connected
between the EXTAL and XTAL pins. The figure below shows a recommended circuit.
PIT Period
PITM
() Prescale
() 4
()
EXTAL Frequency
--------------------------------------------------------
=
32 PLL BLOCK
CLKOUT
PHASE
COMPARATOR
LOW-PASS
FILTER
VCO
CRYSTAL
OSCILLATOR
SYSTEM
CLOCK
SYSTEM CLOCK CONTROL
FEEDBACK DIVIDER
W
X
Y
EXTAL
XTAL
XFC
VDDSYN
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MC68334MFC20 32-BIT, 20 MHz, MICROCONTROLLER, PQFP132
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