AT91 datasheet(125/153 Pages) ATMEL

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125

AT91X40 Series

1354D–ATARM–08/02

Figure 45. Clock Control

Timer Counter Operating

Modes

Each Timer Counter channel can independently operate in two different modes:

•

Capture Mode allows measurement on signals

•

Waveform Mode allows wave generation

The Timer Counter Operating Mode is programmed with the WAVE bit in the TC Mode

Mode, TIOA is always configured to be an output and TIOB is an output if it is not

selected to be the external trigger.

Trigger

A trigger resets the counter and starts the counter clock. Three types of triggers are

common to both modes, and a fourth external trigger is available to each mode.

The following triggers are common to both modes:

•

Software Trigger: Each channel has a software trigger, available by setting SWTRG

in TC_CCR.

•

SYNC: Each channel has a synchronization signal SYNC. When asserted, this

signal has the same effect as a software trigger. The SYNC signals of all channels

are asserted simultaneously by writing TC_BCR (Block Control) with SYNC set.

•

Compare RC Trigger: RC is implemented in each channel and can provide a trigger

when the counter value matches the RC value if CPCTRG is set in TC_CMR.

The Timer Counter channel can also be configured to have an external trigger. In Cap-

ture Mode, the external trigger signal can be selected between TIOA and TIOB. In

Waveform Mode, an external event can be programmed on one of the following signals:

TIOB, XC0, XC1 or XC2. This external event can then be programmed to perform a trig-

ger by setting ENETRG in TC_CMR.

If an external trigger is used, the duration of the pulses must be longer than the system

clock (MCK) period in order to be detected.

Whatever the trigger used, it will be taken into account at the following active edge of the

selected clock. This means that the counter value may not read zero just after a trigger,

especially when a low frequency signal is selected as the clock.

CLKSTA

CLKEN

CLKDIS

Stop

Event

Disable

Event

Counter

Clock

Selected

Clock

Trigger

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