參數(shù)資料
型號: MR80C32-20/883:D
廠商: ATMEL CORP
元件分類: 微控制器/微處理器
英文描述: 8-BIT, 20 MHz, MICROCONTROLLER, CQCC44
封裝: LCC-44
文件頁數(shù): 52/103頁
文件大?。?/td> 25028K
145
8272E–AVR–04/2013
ATmega164A/PA/324A/PA/644A/PA/1284/P
17.4
Counter unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure
17-2 shows a block diagram of the counter and its surrounding environment.
Figure 17-2. Counter Unit block diagram.
Signal description (internal signals):
count
Increment or decrement TCNT2 by 1.
direction
Selects between increment and decrement.
clear
Clear TCNT2 (set all bits to zero).
clk
Tn
Timer/Counter clock, referred to as clk
T2 in the following.
top
Signalizes that TCNT2 has reached maximum value.
bottom
Signalizes that TCNT2 has reached minimum value (zero).
Depending on the mode of operation used, the counter is cleared, incremented, or decremented
at each timer clock (clk
T2). clkT2 can be generated from an external or internal clock source,
selected by the Clock Select bits (CS22:0). When no clock source is selected (CS22:0 = 0) the
timer is stopped. However, the TCNT2 value can be accessed by the CPU, regardless of
whether clk
T2 is present or not. A CPU write overrides (has priority over) all counter clear or
count operations.
The counting sequence is determined by the setting of the WGM21 and WGM20 bits located in
the Timer/Counter Control Register (TCCR2A) and the WGM22 located in the Timer/Counter
Control Register B (TCCR2B). There are close connections between how the counter behaves
(counts) and how waveforms are generated on the Output Compare outputs OC2A and OC2B.
For more details about advanced counting sequences and waveform generation, see ”Modes of
The Timer/Counter Overflow Flag (TOV2) is set according to the mode of operation selected by
the WGM22:0 bits. TOV2 can be used for generating a CPU interrupt.
17.5
Output Compare unit
The 8-bit comparator continuously compares TCNT2 with the Output Compare Register
(OCR2A and OCR2B). Whenever TCNT2 equals OCR2A or OCR2B, the comparator signals a
match. A match will set the Output Compare Flag (OCF2A or OCF2B) at the next timer clock
cycle. If the corresponding interrupt is enabled, the Output Compare Flag generates an Output
Compare interrupt. The Output Compare Flag is automatically cleared when the interrupt is exe-
cuted. Alternatively, the Output Compare Flag can be cleared by software by writing a logical
one to its I/O bit location. The Waveform Generator uses the match signal to generate an output
DATA BUS
TCNTn
Control Logic
count
TOVn
(Int.Req.)
top
bottom
direction
clear
TOSC1
T/C
Oscillator
TOSC2
Prescaler
clk
I/O
clk
Tn
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