Rainbow Electronics ATmega8HVD - Manual (Page 83)

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8052B–AVR–09/08
ATmega4HVD/8HVD
There is one exception in the temporary register usage. In the Output Compare mode the 16-
bit Output Compare Register OCRnA/B is read without the temporary register, because the
Output Compare Register contains a fixed value that is only changed by CPU access. How-
ever, in 16-bit Input Capture mode the ICRn register formed by the OCRnA and OCRnB
registers must be accessed with 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 access-
ing the OCRnA/B registers.
Note: 1. See ”About Code Examples” on page 5 .
The assembly code example returns the TCNTnH/L 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 dis-
able the interrupts during the 16-bit access.
Assembly Code Example
...
; 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 Example
unsigned int i;
...
/* Set TCNTn to 0x01FF */
TCNTn = 0x1FF;
/* Read TCNTn into i */
i = TCNTn;
...