ATmega16U4 datasheet(160/408 Pages) ATMEL

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7766D–AVR–01/09

ATmega16U4/ATmega32U4

15.11 Accessing 10-Bit Registers

If 10-bit values are written to the TCNTn and OCRnA/B/C/D registers, the 10-bit registers can be

byte accessed by the AVR CPU via the 8-bit data bus using two read or write operations. The

10-bit registers have a common 2-bit Timer/Counter4 High Byte Register (TC4H) that is used for

temporary storing of the two MSBs of the 10-bit access. The same TC4H register is shared

between all 10-bit registers. Accessing the low byte triggers the 10-bit read or write operation.

When the low byte of a 10-bit register is written by the CPU, the high byte stored in the TC4H

When the low byte of a 10-bit register is read by the CPU, the high byte of the 10-bit register is

copied into the TC4H register in the same clock cycle as the low byte is read.

To do a 10-bit write, the high byte must be written to the TC4H register before the low byte is

written. For a 10-bit read, the low byte must be read before the high byte.

The following code examples show how to access the 10-bit timer registers assuming that no

interrupts updates the TC4H register. The same principle can be used directly for accessing the

OCRnA/B/C/C/D registers.

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”.

The assembly code example returns the TCNTn value in the r17:r16 register pair.

Assembly Code Example

...

; Set TCNT

n to 0x01FF

ldi

r17,0x01

ldi

r16,0xFF

out

nH,r17

out

TCNT

n,r16

; Read TCNT

n into r17:r16

r16,TCNT

r17,TC

...

C Code Example

unsigned int

...

/* Set TCNTn to 0x01FF */

TCnH = 0x01;

TCNTn = 0xFF;

/* Read TCNTn into i */

i = TCNTn;

i |= ((unsigned int)TCnH << 8);

...

ATmega16U4 Datasheet(PDF) 160 Page - ATMEL Corporation

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