參數資料
型號: MR83C154CXXX-25P883R
廠商: TEMIC SEMICONDUCTORS
元件分類: 微控制器/微處理器
英文描述: 8-BIT, MROM, 25 MHz, MICROCONTROLLER, CQCC44
文件頁數: 69/141頁
文件大?。?/td> 7628K
代理商: MR83C154CXXX-25P883R
26
2486AA–AVR–02/2013
ATmega8(L)
Asynchronous Timer
Clock – clk
ASY
The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly
from an external 32kHz clock crystal. The dedicated clock domain allows using this Timer/Coun-
ter as a real-time counter even when the device is in sleep mode. The Asynchronous
Timer/Counter uses the same XTAL pins as the CPU main clock but requires a CPU main clock
frequency of more than four times the Oscillator frequency. Thus, asynchronous operation is
only available while the chip is clocked on the Internal Oscillator.
ADC Clock – clk
ADC
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.
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.
Note:
1. For all fuses “1” means unprogrammed while “0” means programmed
The various choices for each clocking option is given in the following sections. When the CPU
wakes up from Power-down or Power-save, the selected clock source is used to time the start-
up, ensuring stable Oscillator operation before instruction execution starts. When the CPU starts
from reset, there is as an additional delay allowing the power to reach a stable level before com-
mencing normal operation. The Watchdog Oscillator is used for timing this real-time part of the
start-up time. The number of WDT Oscillator cycles used for each time-out is shown in Table 3.
The frequency of the Watchdog Oscillator is voltage dependent as shown in “ATmega8 Typical
Characteristics – TA = -40°C to 85°C”. The device is shipped with CKSEL = “0001” and SUT =
“10” (1MHz Internal RC Oscillator, slowly rising power).
Table 2. Device Clocking Options Select(1)
Device Clocking Option
CKSEL3..0
External Crystal/Ceramic Resonator
1111 - 1010
External Low-frequency Crystal
1001
External RC Oscillator
1000 - 0101
Calibrated Internal RC Oscillator
0100 - 0001
External Clock
0000
Table 3. Number of Watchdog Oscillator Cycles
Typical Time-out (VCC = 5.0V)
Typical Time-out (VCC = 3.0V)
Number of Cycles
4.1ms
4.3ms
4K (4,096)
65ms
69ms
64K (65,536)
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