ATmega16HVB datasheet(90/273 Pages) ATMEL

90 / 273 page

8042A–AVR–07/09

ATmega16HVB/32HVB

Figure 17-8. Timer/Counter Timing Diagram, Setting of OCFnx, with Prescaler (f

clk_I/O/8)

Figure 17-9 on page 90 shows the setting of OCFnA and the clearing of TCNTn in CTC mode.

Figure 17-9. Timer/Counter Timing Diagram, CTC mode, with Prescaler (f

clk_I/O/8)

17.9

Accessing Registers in 16-bit Mode

In 16-bit mode (the TCWn bit is set to one) the TCNTnH/L and OCRnA/B or TCNTnL/H and

OCRnB/A 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. The 16-bit

Timer/Counter 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. 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 cop-

ied 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.

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. However, 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.

OCFnx

OCRnx

TCNTn

OCRnx Value

OCRnx - 1

OCRnx

OCRnx + 1

OCRnx + 2

clk

I/O

clk

(clk

I/O/8)

OCFnx

OCRnx

TCNTn

(CTC)

TOP

TOP - 1

TOP

BOTTOM

BOTTOM + 1

clk

PCK

clk

(clk

PCK /8)

ATmega16HVB Datenblatt(PDF) 90 Page - ATMEL Corporation

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