ATmega64A
• FOCnA, FOCnB, and FOCnC are added in the new TCCRnC Register.
• WGMn3 is added to TCCRnB.
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.
15.3 Accessing 16-bit Registers
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 opera-
tions. Each 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 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 register is read by the CPU, the high byte of the 16-bit register is copied into the tempo-
rary 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.
Assembly Code Examples(1)
:.
; Set TCNTn to 0x01FF
ldi r17,0x01
ldi r16,0xFF
out TCNTnH,r17
out TCNTnL,r16
; Read TCNTn into r17:r16
in r16,TCNTnL
in r17,TCNTnH
:.
C Code Examples(1)
unsigned int i;
:.
/* Set TCNTn to 0x01FF */
TCNTn = 0x1FF;
/* Read TCNTn into i */
i = TCNTn;
:.
Note:
1. See “About Code Examples” on page 8.
The assembly code example returns the TCNTn value in the r17:r16 register pair.
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8160C–AVR–07/09
ATMEL [ ATMEL ]
