at90pwm81-16se ATMEL Corporation, at90pwm81-16se Datasheet - Page 86

• Manufacturer Part Number: at90pwm81-16se
• Description: 8-bit Avr Microcontroller With 8k Bytes In- System Programmable Flash
• Manufacturer: ATMEL Corporation

12.1.2
12.2
86
Accessing 16-bit Registers
AT90PWM81
Definitions
The following definitions are used extensively throughout the section:
The TCNT1, and ICR1 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 sin-
gle 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 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 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.
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 ICR1 Registers.
Note that when using “C”, the compiler handles the 16-bit access.
BOTTOM
MAX
TOP
The counter reaches the BOTTOM when it becomes 0x0000.
The counter reaches its MAXimum when it becomes 0xFFFF (decimal 65535).
The counter reaches the TOP when it becomes equal to the highest value in the count
sequence. The TOP value can be assigned to be one of the fixed values: 0x00FF, 0x01FF,
or 0x03FF, or to the value stored in the ICR1 Register. The assignment is dependent of
the mode of operation.
7734M–AVR–03/10