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ATMEGA8A-AUR 参数 Datasheet PDF下载

ATMEGA8A-AUR图片预览
型号: ATMEGA8A-AUR
PDF下载: 下载PDF文件 查看货源
内容描述: [RISC Microcontroller, 8-Bit, FLASH, AVR RISC CPU, 16MHz, CMOS, PQFP32, 7 X 7 MM, 1 MM HEIGHT, 0.80 MM PITCH, GREEN, PLASTIC, MS-026ABA, TQFP-32]
分类和应用: 闪存
文件页数/大小: 308 页 / 4674 K
品牌: ATMEL [ ATMEL ]
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ATmega8(L)  
Accessing 16-bit  
Registers  
The TCNT1, OCR1A/B, and ICR1 are 16-bit registers that can be accessed by the AVR CPU via  
the 8-bit data bus. The 16-bit register must be byte accessed using two read or write operations.  
The 16-bit timer has a single 8-bit register for temporary storing of the High byte of the 16-bit  
access. The same temporary register is shared between all 16-bit registers within the 16-bit  
timer. Accessing the Low byte triggers the 16-bit read or write operation. When the Low byte of a  
16-bit register is written by the CPU, the High byte stored in the temporary register, and the Low  
byte written are both copied into the 16-bit register in the same clock cycle. When the Low byte  
of a 16-bit register is read by the CPU, the High byte of the 16-bit register is copied into the tem-  
porary register in the same clock cycle as the Low byte is read.  
Not all 16-bit accesses uses the temporary register for the High byte. Reading the OCR1A/B 16-  
bit registers does not involve using the temporary register.  
To do a 16-bit write, the High byte must be written before the Low byte. For a 16-bit read, the  
Low byte must be read before the High byte.  
The following code examples show how to access the 16-bit Timer Registers assuming that no  
interrupts updates the temporary register. The same principle can be used directly for accessing  
the OCR1A/B and ICR1 Registers. Note that when using “C”, the compiler handles the 16-bit  
access.  
Assembly Code Example(1)  
...  
; Set TCNT1 to 0x01FF  
ldir17,0x01  
ldir16,0xFF  
outTCNT1H,r17  
outTCNT1L,r16  
; Read TCNT1 into r17:r16  
in r16,TCNT1L  
in r17,TCNT1H  
...  
C Code Example(1)  
unsigned int i;  
...  
/* Set TCNT1 to 0x01FF */  
TCNT1 = 0x1FF;  
/* Read TCNT1 into i */  
i = TCNT1;  
...  
Note:  
1. See “About Code Examples” on page 8.  
The assembly code example returns the TCNT1 value in the r17:r16 Register pair.  
It is important to notice that accessing 16-bit registers are atomic operations. If an interrupt  
occurs between the two instructions accessing the 16-bit register, and the interrupt code  
updates the temporary register by accessing the same or any other of the 16-bit Timer Regis-  
ters, then the result of the access outside the interrupt will be corrupted. Therefore, when both  
the main code and the interrupt code update the temporary register, the main code must disable  
the interrupts during the 16-bit access.  
79  
2486T–AVR–05/08  
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