68HC705PD6 datasheet(83/165 Pages) MOTOROLA

83 / 165 page

July 7, 1997

GENERAL RELEASE SPECIFICATION

MC68HC05PD6

TIMER SYSTEM

MOTOROLA

REV 1.1

9-11

OLVL

The value of the output level (OLVL) bit is clocked into the output level register

by the next successful output compare and will appear at TCMP pin. This bit

and the output level register are cleared by reset.

0 =

low output

1 =

high output

9.1.5 Timer Status Register (TSR)

The timer status register is a read-only register and is illustrated below followed by

a deﬁnition of each bit. Refer to timing diagrams shown in Figure 9-3, Figure 9-4

and Figure 9-5 for timing relationship to the timer status register bits.

ICF

The input capture ﬂag (ICF) is set when a proper edge has been sensed by the

input capture edge detector. It is cleared by a processor access of the timer

status register (with ICF set) followed by accessing the low byte ($15) of the

input capture register. Reset does not affect the input compare ﬂag.

OC1F

The output compare ﬂag (OC1F) is set when the output compare register

contents matches the contents of the free running counter. The OC1F is

cleared by accessing the timer status register (with OC1F set) and then

accessing the low byte ($17) of the output compare register. Reset does not

affect the output compare ﬂag.

TOF

The timer overﬂow ﬂag (TOF) bit is set by transition of the free run from $FFFF

to $0000. It is cleared by accessing the timer status register (with TOF set)

followed by an access of the free running counter least signiﬁcant byte ($19).

Reset does not affect the TOF bit.

Accessing the timer status register satisﬁes the ﬁrst condition required to clear

status bits. The remaining step is to access the register corresponding to the

status bit.

A problem can occur when using the timer overﬂow function and reading the free-

running counter at random times to measure an elapsed time. Without

incorporating the proper precautions into software, the timer overﬂow ﬂag could

00000

6543210

TOF

OC1F

ICF

TSR

$0013

reset

68HC705PD6 Datasheet(PDF) 83 Page - Motorola, Inc

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