Rainbow-electronics ATmega8515L Manual do Utilizador descarregar pdf (Página 97)

ATmega8515(L)

2512A–AVR–04/02

Accessing 16-bit

Registers

TheTCNT1,OCR1A/B, andICR1 are16-bit registers that can beaccessedbythe AVR

CPU via the8-bit data bus.The16-bit register must be byte accessed using two read or

write operations. Each 16-bit timerhas a single8-bit registerfor temporary storing of the

highbyte of the16-bit access.The same temporary register issharedbetween all 16-bit

registers within each 16-bit timer.Accessing the lowbyte triggers the16-bit read orwrite

operation. When the lowbyte of a16-bit register iswritten by the CPU, the highbyte

stored in thetemporary register, and the lowbyte written are bothcopied into the16-bit

CPU, the highbyte of the16-bit register iscopied into thetemporary register in the

same clock cycleas the lowbyte isread.

Not all 16-bit accesses uses thetemporary registerfor the highbyte. Reading the

OCR1A/B 16-bit registers does not involve using thetemporary register.

To doa16-bit write,

thehighbytemustbewrittenbeforethelowbyte

. For a16-bit read,

the low byte must be read before the high byte

The following codeexamplesshowhow to access the16-bit timerregisters assuming

that no interrupts updates thetemporary register.The same principle can beused

directly for accessing the OCR1A/B andICR1 Registers. Note that when using “C”, the

compilerhandles the16-bit access.

Note: 1. Theexample codeassumes that the part specificheaderfileis included.

Theassembly codeexample returns theTCNT1 value in the r17:r16registerpair.

Itis important to noticethat accessing 16-bit registers areatomic operations. If an inter-

ruptoccurs between thetwoinstructions accessing the16-bit register, and theinterrupt

codeupdates thetemporary registerbyaccessing the same or any other of the16-bit

timerregisters, then the resultof theaccess outsidetheinterrupt will be corrupted.

Therefore,when both themaincodeand theinterrupt codeupdate thetemporary regis-

ter, themaincodemust disabletheinterruptsduring the16-bit access.

Assembly Code Examples

(1)

...

;

Set TCNT

to 0x01FF

ldi r17,0x01

ldi r16,0xFF

out TCNT

1H,r17

out TCNT

1L,r16

; Read TCNT

1 into r17:r16

in r16,TCNT

in r17,TCNT

...

CCode Examples

(1)

unsigned int i;

...

Set TCNT

to 0x01FF

TCNT

1 = 0x1FF;

Read TCNT

into i

i = TCNT

...

1 2 ... 92 93 94 95 96 97 98 99 100 101 102 ... 222 223

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