This document discusses timers in AVR microcontrollers. It explains that timers count clock cycles and can be prescaled to achieve greater values at the cost of precision. Timer 0 is an 8-bit timer while Timer 1 is 16-bit. The key registers for Timer 1 are TCNT1 for counting, TCCR1A/B for control, and TIMSK/TIFR for interrupts. Accessing 16-bit registers requires using a temporary register to ensure precise timing of the high and low byte writes and reads.

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TIMERS

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Timers
• The AVR has differentTimer types.
• Not all AVRs have allTimers so look at the datasheet of your AVR before
trying to use a timer.

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What are timers?
• The timers basically only count clock cycles.
• The timer clock can be equal to the system clock (from the crystal or
whatever clocking option is used) or it can be slowed down by the prescaler
first.
• When using the prescaler you can achieve greater timer values, while
precision goes down.
• The prescaler can be set to 8, 64, 256 or 1024 compared to the system clock

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Timers in avr
• Timer 0 -- 8 bit
• Timer 1 -- 16 bit
• Timer 2 -- 8 bit

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8 BITTIMER

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Timer 0
• The timer clock (from System Clock, prescaled System Clock or External Pin
T0) counts up theTimer/Counter Register (TCNT0).
• When it rolls over (0xFF -> 0x00) the Overflow Flag is set and the
Timer/Counter 1 Overflow Interrupt Flag is set.
• If the corresponding bit inTIMSK (Timer Interrupt Mask Register) is set (in
this case the bit is named "TOIE0") and global Interrupts are enabled, the
micro will jump to the corresponding interrupt vector

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16 bit register

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TIMER REGISTERS
• TCNT1
• TCCR1A / TCCR1B
• OCR1A
• ICR1
• TIMSK andTIFR

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• All 16-bit registers have can only be accessed one byte at a time.
• To ensure precise timing, a 16-bit temporary register is used when accessing
the timer registers.
• Write:
When writing the high byte (e.g.TCNT1H), the data is placed in theTEMP
register.When the low byte is written, the data is transferred to the actual
registers simultaneously. So the high byte must be written first to perform a
true 16-bit write.

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• Read:
When reading the low byte, the high byte is read fromTCNT1 simultaneously
and can be read afterwards. So when reading, access the low byte first for a
true 16-bit read.

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TCNT
• Most important is theTimer/Counter Register (TCNT1) itself.
• This is what all timer modes base on.
• It counts System Clock ticks, prescaled system clock or from the external pin.
TCCR
• TheTimer/Counter Control register is used to set the timer mode, prescaler and other
options.

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Timer/counter control registers

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Clock select bits
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