Rainbow-electronics ATmega128RFA1 Manual de usuario descargar pdf (Pagina 148)

148

8266A-MCU Wireless-12/09

ATmega128RFA1

11.2.2 I/O Clock – clk

I/O

The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and

USART. The I/O clock is also used by the External Interrupt module, but note that some

external interrupts are detected by asynchronous logic, allowing such interrupts to be

detected even if the I/O clock is halted. Also note that start condition detection in the 2-

wire serial interface (TWI) module is carried out asynchronously when clk

I/O

is halted.

Similar the TWI address recognition in all sleep modes also occurs asynchronously.

11.2.3 Flash Clock – clk

FLASH

The Flash clock controls operation of the Flash interface. The Flash clock is usually

active simultaneously with the CPU clock.

11.2.4 Asynchronous Timer Clock – clk

ASY

The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked

directly from an external clock or an external 32 kHz clock crystal. The dedicated clock

domain allows using this Timer/Counter as a real-time counter even if the device is in

sleep mode.

11.2.5 ADC Clock – clkADC

The ADC is provided with a dedicated clock domain. This allows halting the CPU and

I/O clocks in order to reduce noise generated by digital circuitry. This gives more

accurate ADC conversion results.

11.3 Clock Sources

The device has the following clock source options, selectable by Flash Fuse bits as

shown below. The clock from the selected source is input to the AVR clock generator,

and routed to the appropriate modules.

Table 11-1. Device Clocking Options Select

(1)

Device Clocking Option CKSEL3:0

Transceiver clock 1111 – 0110

Reserved 0101 - 0100

Internal 128 kHz RC Oscillator 0011

Calibrated Internal RC Oscillator 0010

External Clock 0000

Reserved 0001

Notes: 1. For all fuses “1” means unprogrammed while “0” means programmed.

11.3.1 Default Clock Source

The device is shipped with internal RC oscillator at 16.0 MHz, the 1:2 prescaler enabled

and with the fuse CKDIV8 programmed, resulting in 1.0 MHz system clock. The startup

time is set to maximum time. (CKSEL = "0010", SUT = "10", CKDIV8 = "0"). The default

setting ensures that all users can make their desired clock source setting using any

available programming interface.

11.3.2 Clock Start-up Sequence

Any clock source needs a minimum number of oscillating cycles before it can be

considered stable.

1 2 ... 143 144 145 146 147 148 149 150 151 152 153 ... 523 524

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