MC68340 datasheet(351/441 Pages) FREESCALE

351 / 441 page

MOTOROLA

MC68340 USER’S MANUAL

8- 7

to its location in the SR. See Figure 8-4 for a depiction of this mode. If the timing gate is

disabled (CR TGE bit is cleared),

TGATE

≈ has no effect on the operation of the timer;

thus the input capture function is inoperative. At all times, the

TGATE

≈ level bit (TGL) in

the SR reflects the level of the

TGATE

≈ signal.

COUNTER

CLOCK

COUNTER

TGATE

TOUT

Modex Bits in Control Register = 000

Preload 1 Register = 8

Compare Register = 7

TGE Bit of Status Register = 1

TG Bit in Status Register Initially = 0

OCx Bits in Control Register = 10

TG SET

TG CLEARED

TG SET

TIMEOUT

TC SET

ENABLE

Figure 8-4. Input Capture/Output Compare Mode

Since the counter is not affected by

TGATE

≈, it continues to decrement on the falling

edge of the counter clock and load from the PREL1 at timeout, regardless of the value of

TGATE

≈.

When the counter counts down to the value contained in the COM, this condition is

reflected by setting the timer compare (TC) and compare (COM) bits in the SR. TOUTx

responds as selected by the OCx bits in the CR. The output level (OUT) bit in the SR

reflects the value on TOUTx. Shadowing does not affect this operation.

If the counter counts down to $0000, a timeout is detected, causing the SR timeout

interrupt (TO) bit to be set and the SR COM bit to be cleared. On the next falling edge of

the counter clock after the timeout is detected, the value in PREL1 is again loaded into the

counter. TOUTx responds as selected by the CR OCx bits.

A square-wave generator can be implemented by programming the CR OCx bits to toggle

mode. The value in the COM should be one-half the value in PREL1 to cause an event to

happen twice in the countdown.

This mode can be used as a pulse-width modulator by programming the CR OCx bits to

zero mode or one mode. The value in the PREL1 specifies the frequency, and the COM

determines the pulse width. The pulse widths can be changed by writing a new value to

the COM.

Freescale Semiconductor, Inc.

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