Datasheet

120
ATtiny828 [DATASHEET]
8371A–AVR–08/12
Figure 49. Timer/Counter Timing Diagram, with Prescaler (f
clk_I/O
/8)
12.10 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. 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 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 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.
OCFnx
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
clk
I/O
clk
Tn
(clk
I/O
/8)