Datasheet

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 that is currently stored in TEMP and the low byte being 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 TEMP register in the same clock cycle as
the low byte is read, and must be read subsequently.
Note: To perform a 16-bit write operation, 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.
Not all 16-bit accesses use the temporary register for the high byte. Reading the OCR1A/B 16-bit
registers does not involve using the temporary register.
16-bit Access
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.
Example 20-1. Assembly Code Example
(1)
...
; Set TCNT1 to 0x01FF
ldi r17,0x01
ldi r16,0xFF
out TCNT1H,r17
out TCNT1L,r16
; Read TCNT1 into r17:r16
in r16,TCNT1L
in r17,TCNT1H
...
The assembly code example returns the TCNT1 value in the r17:r16 register pair.
Example 20-2. C Code Example
(1)
unsigned int i;
...
/* Set TCNT1 to 0x01FF */
TCNT1 = 0x1FF;
/* Read TCNT1 into i */
i = TCNT1;
...
Note:
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 extended I/O. Typically LDS and STS combined with SBRS, SBRC,
SBR, and CBR.
Atomic Read
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 registers, then the result of the access
outside the interrupt is 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.
The following code examples show how to perform an atomic read of the TCNT1 register contents. The
OCR1A/B or ICR1 registers can be read using the same principle.
ATmega48PA/88PA/168PA
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40002011A-page 164