MC68331TS datasheet(21/84 Pages) FREESCALE

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MC68331TS/D

PIV[7:0] —Periodic Interrupt Vector

The bits of this field contain the vector generated in response to an interrupt from the periodic timer.

When the SIM responds, the periodic interrupt vector is placed on the bus.

The PITR contains the count value for the periodic timer. A zero value turns off the periodic timer. This

PTP —Periodic Timer Prescaler Control

0 = Periodic timer clock not prescaled

1 = Periodic timer clock prescaled by a value of 512

The reset state of PTP is the complement of the state of the MODCLK signal during reset.

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 = [(PITM)(Prescaler)(4)]/EXTAL

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 MCU operation is fully static, register and memory contents are not affected when the

clock rate changes. System hardware and software support changes in the clock rate during operation.

The system clock signal can be generated in three ways. An internal phase-locked loop can synthesize

the clock from an internal or external frequency source, or the clock signal can be input from an external

source.

Following is a block diagram of the clock submodule.

PIRQL

Interrupt Request Level

000

Periodic Interrupt Disabled

001

Interrupt Request Level 1

010

Interrupt Request Level 2

011

Interrupt Request Level 3

100

Interrupt Request Level 4

101

Interrupt Request Level 5

110

Interrupt Request Level 6

111

Interrupt Request Level 7

PITR —Periodic Interrupt Timer Register

$YFFA24

PTP

PITM

RESET:

MODCLK

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