PCINT13 datasheet(35/294 Pages) ATMEL

35 / 294 page

9219A–RFID–01/11

Atmel ATA5505 [Preliminary]

Notes:

1. Flash Fuse bits.

2. CLKSELR register bits.

3. Additional delay (+ 4ms) available if RSTDISBL fuse is set.

Note that the System Clock Prescaler can be used to implement run-time changes of the inter-

nal clock frequency while still ensuring stable operation. Refer to “System Clock Prescaler” on

page 41 for details.

4.2.7

Clock Output Buffer

If not using a crystal oscillator, the device can output the system clock on the CLKO pin. To

enable the output, the CKOUT Fuse or COUT bit of CLKSELR register has to be programmed.

This option is useful when the device clock is needed to drive other circuits on the system.

Note that the clock will not be output during reset and the normal operation of I/O pin will be

overridden when the fuses are programmed. If the System Clock Prescaler is used, it is the

divided system clock that is output.

4.3

Dynamic Clock Switch

4.3.1

Features

The Atmel

® AVR® provides a powerful dynamic clock switch circuit that allows users to turn on

and off clocks of the device on the fly. The built-in de-glitching circuitry allows clocks to be

enabled or disabled asynchronously. This enables efficient power management schemes to

be implemented easily and quickly. In a safety application, the dynamic clock switch circuit

allows continuous monitoring of the external clock permitting a fallback scheme in case of

clock failure.

The control of the dynamic clock switch circuit must be supervised by software. This operation

is facilitated by the following features:

• Safe commands, to avoid unintentional commands, a special write procedure must be

followed to change the CLKCSR register bits (See “CLKPR – Clock Prescaler Register” on

page 42.):

• Exclusive action, the actions are controlled by a decoding table (commands) written to the

CLKCSR register. This ensures that only one command operation can be launched at any

time. The main actions of the decoding table are:

–‘Disable Clock Source’,

–‘Enable Clock Source’,

–‘Request Clock Availability’,

–‘Clock Source Switching’,

Table 4-9.

Start-up Times for the External Clock Selection

SUT1..0(1)

CSUT1..0(2)

Start-up Time from

Power-down/save

Additional Delay from Reset

CC = 5.0V)

Recommended Usage

6CK

14CK (+ 4.1ms(3))

BOD enabled

6CK

14CK + 4.1ms

Fast rising power

6CK

14CK + 65ms

Slowly rising power

Reserved

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