ATMEga64L ATMEL Corporation, ATMEga64L Datasheet - Page 113

• Manufacturer Part Number: ATMEga64L
• Description: 8-bit AVR Microcontroller with 64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL Corporation

Accessing 16-bit
Registers
2490G–AVR–03/04
•
•
Interrupt flag and mask bits for Output Compare unit C are added.
The 16-bit Timer/Counter has improvements that will affect the compatibility in some
special cases.
The TCNTn, OCRnA/B/C, and ICRn 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. Each 16-bit timer has a single 8-bit register for temporary stor-
ing of the high byte of the 16-bit access. The same temporary register is shared
between all 16-bit registers within each 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 reg-
ister is read by the CPU, the high byte of the 16-bit register is copied into the temporary
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
OCRnA/B/C 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 OCRnA/B/C and ICRn Registers. Note that when using “C”,
the compiler handles the 16-bit access.
Note:
Assembly Code Examples
C Code Examples
FOCnA, FOCnB, and FOCnC are added in the new TCCRnC Register.
WGMn3 is added to TCCRnB.
...
; Set TCNTn to 0x01FF
ldi r17,0x01
ldi r16,0xFF
out TCNTnH,r17
out TCNTnL,r16
; Read TCNTn into r17:r16
in
in
...
unsigned int i;
...
/* Set TCNTn to 0x01FF */
TCNTn = 0x1FF;
/* Read TCNTn into i */
i = TCNTn;
...
1. The example code assumes that the part specific header file is included.
For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”,
and “SBI” instructions must be replaced with instructions that allow access to
r16,TCNTnL
r17,TCNTnH
(1)
(1)
ATmega64(L)
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