Rainbow-electronics ATmega128L Uživatelský manuál stáhnout pdf (Strana 109)

109

ATmega128(L)

2467B–09/01

• WGMn3 is added to TCCRnB.

Interrupt flag and mask bits for output compare unit C are added.

The 16-bit Timer/Counter has improvements that will affect the compatibility in some

special cases.

Accessing 16-bit

Registers

The TCNTn, OCRnA/B/C, and ICRn 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 stor-

ing 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 reg-

ister is read by the CPU, the high byte of the 16-bit register is copied into the temporary

Not all 16-bit accesses uses the temporary register for the high byte. Reading the

OCRnA/B/C 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 OCRnA/B/C and ICRn registers. Note that when using “C”, the

compiler handles the 16-bit access.

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 Examples

(1)

...

; 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 Examples

(1)

unsigned int i;

...

/* Set TCNTn to 0x01FF */

TCNTn = 0x1FF;

/* Read TCNTn into i */

i = TCNTn;

...

1 2 ... 104 105 106 107 108 109 110 111 112 113 114 ... 330 331

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