HC705V8GRSSLASHD datasheet(88/172 Pages) NXP

88 / 172 page

SECTION 11: 16-BIT TIMER

Page 77

MC68HC705V8 Specification Rev. 2.1

accompany a successful output compare provided the corresponding interrupt enable bit

(OCIE) is set. After a processor write cycle to the output compare register containing the

MSB ($16), the output compare function is inhibited until the LSB ($17) is also written. The

user must write both bytes (locations) if the MSB is written first. A write made only to the

LSB ($17) will not inhibit the compare function. The free-running counter is updated every

four internal bus clock cycles. The minimum time required to update the output compare

The processor can write to either byte of the output compare register without affecting the

other byte. The output level (OLVL) bit is clocked to the output level register regardless of

whether the output compare flag (OCF) is set or clear.

11.3

INPUT CAPTURE REGISTER - $14:$15

Two 8-bit registers, which make up the 16-bit input capture register, are read-only and are

used to latch the value of the free-running counter after the corresponding input capture

edge detector senses a defined transition. The level transition that triggers the counter

transfer is defined by the corresponding input edge bit (IEDG). Reset does not affect the

contents of the input capture register.

The result obtained by an input capture will be one more than the value of the free-running

counter on the rising edge of the internal bus clock preceding the external transition. This

delay is required for internal synchronization. Resolution is one count of the free-running

counter, which is four internal bus clock cycles.

The free-running counter contents are transferred to the input capture register on each

proper signal transition regardless of whether the input capture flag (ICF) is set or clear.

The input capture register always contains the free-running counter value that corresponds

to the most recent input capture.

After a read of the input capture register MSB ($14), the counter transfer is inhibited until

the LSB ($15) is also read. This characteristic causes the time used in the input capture

software routine and its interaction with the main program to determine the minimum pulse

period. A read of the input capture register LSB ($15) does not inhibit the free-running

counter transfer since they occur on opposite edges of the internal bus clock.

Figure 11-2:

TCAP Timing

TCAP

tTLTL

tTL

tTH

See control timing specifications for TCAP timing requirements.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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