AT91 datasheet(124/153 Pages) ATMEL

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124

AT91X40 Series

1354D–ATARM–08/02

The selected clock can be inverted with the CLKI bit in TC_CMR (Channel Mode). This

allows counting on the opposite edges of the clock.

The burst function allows the clock to be validated when an external signal is high. The

BURST parameter in the Mode Register defines this signal (none, XC0, XC1, XC2).

Note:

In all cases, if an external clock is used, the duration of each of its levels must be longer

than the system clock (MCK) period. The external clock frequency must be at least 2.5

times lower than the system clock (MCK).

Figure 44. Clock Selection

Clock Control

The clock of each counter can be controlled in two different ways: it can be enabled/dis-

abled and started/stopped.

•

The clock can be enabled or disabled by the user with the CLKEN and the CLKDIS

commands in the Control Register. In Capture Mode it can be disabled by an RB

load event if LDBDIS is set to 1 in TC_CMR. In Waveform Mode, it can be disabled

by an RC Compare event if CPCDIS is set to 1 in TC_CMR. When disabled, the

start or the stop actions have no effect: only a CLKEN command in the Control

in the Status Register.

•

The clock can also be started or stopped: a trigger (software, synchro, external or

compare) always starts the clock. The clock can be stopped by an RB load event in

Capture Mode (LDBSTOP = 1 in TC_CMR) or a RC compare event in Waveform

Mode (CPCSTOP = 1 in TC_CMR). The start and the stop commands have effect

only if the clock is enabled.

MCK/2

MCK/8

MCK/32

MCK/128

MCK/1024

XC0

XC1

XC2

CLKS

CLKI

BURST

Selected

Clock

AT91 Datasheet(PDF) 124 Page - ATMEL Corporation