MC68334 データシートの表示（PDF） - Freescale Semiconductor

部品番号

コンポーネント説明

メーカー

MC68334

32-Bit Modular Microcontroller

Freescale Semiconductor 

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:

PIT Period = -(--P-E---I-X-T---TM----A--)--L(--P---F-r--er--e--s--q-c--u-a--e-l-e--n--)-c-(--y-4----)

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.

EXTAL XTAL

XFC

VDDSYN

CLKOUT

CRYSTAL

OSCILLATOR

PHASE

COMPARATOR

LOW-PASS

FILTER

VCO

W

FEEDBACK DIVIDER

Y

X

SYSTEM CLOCK CONTROL

SYSTEM

CLOCK

Figure 6 System Clock Block Diagram

32 PLL BLOCK

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.

MC68334

MC68334TS/D REV. 1

19

Share Link: