21.3 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. A 16-bit register must be byte accessed using two read or write operations. The 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 the 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.

Assembly Code Example(1)

   :.
; Set TCNT1 to 0x01FF
ldi    r17,0x01
ldi    r16,0xFF
out    TCNT1H,r17
out    TCNT1L,r16
; Read TCNT1 into r17:r16
in    r16,TCNT1L
in    r17,TCNT1H
   :.
unsigned int i;
   :.
/* Set TCNT1 to 0x01FF */
TCNT1 = 0x1FF;
/* Read TCNT1 into i */
i = TCNT1;
   :.
Note: 1. See About Code Examples.

The assembly code example returns the TCNT1 value in the r17:r16 Register pair.

It is important to notice that accessing 16-bit registers are atomic operations. If an interrupt occurs between the two instructions accessing the 16-bit register, and the interrupt code updates the temporary register by accessing the same or any other of the 16-bit Timer Registers, then the result of the access outside the interrupt will be corrupted. Therefore, when both the main code and the interrupt code update the temporary register, the main code must disable the interrupts during the 16-bit access.

The following code examples show how to do an atomic read of the TCNT1 Register contents. Reading any of the OCR1A/B or ICR1 Registers can be done by using the same principle.

Asesmbly Code Example(1)

TIM16_ReadTCNT1:
   ; Save global interrupt flag
   in    r18,SREG
   ; Disable interrupts
   cli
   ; Read TCNT1 into r17:r16
   in    r16,TCNT1L
   in    r17,TCNT1H
   ; Restore global interrupt flag
   out   SREG,r18
   ret
unsigned int TIM16_ReadTCNT1( void )
{
   unsigned char sreg;
   unsigned int i;
   /* Save global interrupt flag */
   sreg = SREG;
   /* Disable interrupts */
   _CLI();
   /* Read TCNT1 into i */
   i = TCNT1;
   /* Restore global interrupt flag */
   SREG = sreg;
   return i;
}
Note: 1. See About Code Examples.

The assembly code example returns the TCNT1 value in the r17:r16 Register pair.

The following code examples show how to do an atomic write of the TCNT1 Register contents. Writing any of the OCR1A/B or ICR1 Registers can be done by using the same principle.

Assembly Code Example(1)

TIM16_WriteTCNT1:
   ; Save global interrupt flag
   in     r18,SREG
   ; Disable interrupts
   cli
   ; Set TCNT1 to r17:r16
   out    TCNT1H,r17
   out    TCNT1L,r16
   ; Restore global interrupt flag
   out    SREG,r18
   ret
void TIM16_WriteTCNT1( unsigned int i )
{
   unsigned char sreg;
   unsigned int i;
   /* Save global interrupt flag */
   sreg = SREG;
   /* Disable interrupts */
   _CLI();
   /* Set TCNT1 to i */
   TCNT1 = i;
   /* Restore global interrupt flag */
   SREG = sreg;
}
Note: 1. See About Code Examples.

The assembly code example requires that the r17:r16 Register pair contains the value to be written to TCNT1.