MC68340 datasheet(354/441 Pages) FREESCALE

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8- 10

MC68340 USER’S MANUAL

MOTOROLA

successive timeout causes the counter to be loaded alternately with the values from

PREL1 and PREL2.

TOUTx behaves as a variable duty-cycle square wave when the CR OC bits are

programmed for toggle mode. The second timeout occurs after N2 + 1 periods (allowing

for the zero cycle), resulting in a change of state on TOUTx. The third timeout occurs after

N1 + 1 periods, resulting in a change of state on TOUTx, and so on (see Figure 8-6). The

OUT bit in the SR reflects the level of TOUTx.

COUNTER

CLOCK

COUNTER

TOUT

N1: N1 + 1

N2 + 1

N1 + 1

ENABLE

TIMEOUT

MODEx Bits in Control Register = 010

Preload 1 Register = N1 = 4

Preload 2 Register = N2 = 2

OCx Bits in Control Register = 01

TIMEOUT

N2 + 1

Figure 8-6. Variable Duty-Cycle Square-Wave Generator Mode

TGATE

≈ is negated when it is enabled (TGE = 1), the prescaler and counter are

disabled. Additionally, the TG bit of the SR is set, indicating that

TGATE

≈ was negated.

The ON bit of the SR is cleared, indicating that the timer is disabled. If

TGATE

≈ is

reasserted, the timer is re-enabled and begins counting from the value attained when

TGATE

≈ was negated. The ON bit is set again.

TGATE

≈ is not enabled (TGE = 0), TGATE≈ has no effect on the operation of the timer.

In this case, the counter would begin counting on the falling edge of the counter clock

immediately after the SWR and CPE bits in the CR are set. The SR TG bit cannot be set.

At all times, the TGL bit in the SR reflects the level of

TGATE

≈.

The duty cycle of the waveform generated on TOUTx can be dynamically changed by

writing new values into PREL1 and/or PREL2. If PREL1 or PREL2 is being accessed

simultaneously by the counter logic and a CPU32 write, the old preload value may actually

get loaded into the counter at timeout. If at timeout, the counting logic was accessing

PREL2 and the CPU32 was writing to PREL1 (or visa versa), there would be no

unexpected results.

8.3.4 Variable-Width Single-Shot Pulse Generator

This mode is used to produce a one-time pulse that has a delay controlled by the value

stored in PREL1 and a duration controlled by the value stored in PREL2. With TOUTx

programmed to change state, this sequence creates a single pulse of variable width. This

mode can be selected by programming the CR MODE bits to 011.

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