HC705JB2GRSSLASHH datasheet(60/128 Pages) NXP

60 / 128 page

GENERAL RELEASE SPECIFICATION

August 28, 1998

PROGRAMMABLE TIMER

MC68HC705JB2

9-2

REV 1.1

The basis of the 16-bit Timer is a 16-bit free-running counter which increases in

count with each internal bus clock cycle. The counter is the timing reference for

the input capture and output compare functions. The input capture and output

compare functions provide a means to latch the times at which external events

occur, to measure input waveforms, and to generate output waveforms and timing

delays. Software can read the value in the 16-bit free-running counter at any time

without affect the counter sequence.

Because of the 16-bit timer architecture, the I/O registers for the input capture and

output compare functions are pairs of 8-bit registers. Each register pair contains

the high and low byte of that function. Generally, accessing the low byte of a

speciﬁc timer function allows full control of that function; however, an access of

the high byte inhibits that speciﬁc timer function until the low byte is also

accessed.

Because the counter is 16 bits long and preceded by a ﬁxed divide-by-four

prescaler, the counter rolls over every 262,144 internal clock cycles. Timer

resolution with a 4MHz crystal oscillator is 2 microsecond/count.

The interrupt capability, the input capture edge, and the output compare state are

controlled by the timer control register (TCR) located at $0012 and the status of

the interrupt ﬂags can be read from the timer status register (TSR) located at

$0013.

9.1

TIMER REGISTERS (TMRH, TMRL)

The functional block diagram of the 16-bit free-running timer counter and timer

registers is shown in Figure 9-2. The timer registers include a transparent buffer

latch on the LSB of the 16-bit timer counter.

Figure 9-2. Programmable Timer Counter Block Diagram

TMRH ($0018)

TMR LSB

16-BIT COUNTER

÷ 4

INTERNAL

(fOSC ÷ 2)

TIMER CONTROL REG.

TIMER

REQUEST

OVERFLOW (TOF)

RESET

CLOCK

INTERRUPT

TMRL ($0019)

TIMER STATUS REG.

$0012

$0013

INTERNAL

($FFFC)

DATA

READ

TMRH

READ

TMRL

READ

LATCH

BUS

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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