68HC05PL4 datasheet(63/98 Pages) FREESCALE

63 / 98 page

April 30, 1998

GENERAL RELEASE SPECIFICATION

MC68HC05PL4

8-BIT TIMER

REV 2.0

10-1

SECTION 10

8-BIT TIMER

This section describes the 8-bit count down timer module.

10.1

OVERVIEW

Figure 10-1. Timer8 Block Diagram

As shown in Figure 10-1 this timer contains a single 8-bit software programmable

countdown timer counter with a 3-bit software control prescaler. The counter’s

value may be preset under software control and counts down to zero. When the

counter decrements to zero, the timer8 interrupt request bit (T8IF in T8CR) is set.

Then if the timer interrupt is enabled (T8IE in T8CR is set) and the I-bit of the con-

dition code register are is cleared, the processor receives an interrupt. After com-

pletion of the current instruction, the processor proceeds to store the appropriate

registers on the stack and then fetches the timer8 interrupt vector in order to begin

serving the interrupt.

Interrupt Circuit

Timer8 Counter Register ($0E)

T8IF T8IFR T8IE

T8EN PS2 PS1 PS0

Timer8 Control and Status Register ($0D)

Overflow

Detect

Circuit

8-bit count-down timer counter

7-bit prescaler counter

Prescaler

Select

Logic

(8 to 1 MUX)

Internal

Bus Clock

To COP

Watchdog

Circuit

Freescale Semiconductor, Inc.

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