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3. Timer0

3.1. TCNT0 (Timer/Counter 0 Register)

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The TCNT0 register is the core component of the 8-bit TIMER0 module. It acts as the actual counter that holds the current timer value. The value of TCNT0 increments (or decrements in certain PWM modes) based on the selected clock source and prescaler.

Users can read from or write to this register at any time. Note that manually writing a value to TCNT0 while the timer is running can cause the timer to "miss" a Compare Match with the OCR0x registers. This happens because the hardware comparison occurs in the clock cycle following a TCNT0 update.

In PWM modes, TCNT0 is constantly compared against OCR0A and OCR0B. When the values match, the output pins (OC0A/OC0B) toggle, clear, or set, depending on the configuration.

3.2. TCCR0 (Timer/Counter 0 Control Register)

In the ATmega328P, the control functionality is split into two registers: TCCR0A and TCCR0B. Together, they define the timer's behavior, including waveform generation, output modes, and clock scaling.

3.2.1. TCCR0A (Control Register A)

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  • Bit 7:6 - COM0A1:0 (Compare Match Output Mode A): Controls the behavior of the OC0A pin when TCNT0 matches OCR0A.
  • Bit 5:4 - COM0B1:0 (Compare Match Output Mode B): Controls the behavior of the OC0B pin when TCNT0 matches OCR0B.
  • Bit 1:0 - WGM01:0 (Wave Generation Mode): Combined with WGM02 in TCCR0B to select the timer mode (Normal, CTC, Fast PWM, Phase Correct PWM).

3.2.1.1. COM0x1:0 Description

This table shows the COM0x1:0 functionality when the timer is in a non-PWM mode (normal or CTC):

COM0x1 COM0x0 Description
0 0 Normal port operation, OC0x disconnected.
0 1 Toggle OC0x on Compare Match.
1 0 Clear OC0x on Compare Match (Set output to low).
1 1 Set OC0x on Compare Match (Set output to high).

3.2.1.2. WGM02:0 Description

This table shows how the WGM02:0 bits affect the counting sequence of the counter, the source for maximum (TOP) counter value, and what type of waveform generation to be used:

WGM02 WGM01 WGM00 Timer/Counter Mode of Operation TOP Update of OCRx at TOV Flag Set on
0 0 0 Normal 0xFF Immediate MAX
0 0 1 PWM, phase correct 0xFF TOP BOTTOM
0 1 0 CTC OCRA Immediate MAX
0 1 1 Fast PWM 0xFF BOTTOM MAX
1 0 0 Reserved — — —
1 0 1 PWM, phase correct OCRA TOP BOTTOM
1 1 0 Reserved — — —
1 1 1 Fast PWM OCRA BOTTOM TOP
Notes:
  • MAX: 0xFF
  • BOTTOM: 0x00
  • Update of OCRx at: When the hardware actually updates the value of the Compare Register if you change it while the timer is running.

3.2.2. TCCR0B (Control Register B)

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  • Bit 7 - FOC0A (Force Output Compare A): Only active in non-PWM modes. Writing 1 forces an immediate match on OC0A.
  • Bit 6 - FOC0B (Force Output Compare B): Only active in non-PWM modes. Writing 1 forces an immediate match on OC0B.
  • Bit 3 - WGM02 (Waveform Generation Mode): Works with WGM01:0 to set the mode.
  • Bit 2:0 - CS02:0 (Clock Select): Sets the prescaler or selects an external clock source.

3.2.2.1. CS02:0 Prescaler Settings

CS02 CS01 CS00 Description
0 0 0 No clock source (Timer/Counter stopped)
0 0 1 clk / 1 (No prescaling)
0 1 0 clk / 8 (From prescaler)
0 1 1 clk / 64 (From prescaler)
1 0 0 clk / 256 (From prescaler)
1 0 1 clk / 1024 (From prescaler)
1 1 0 External clock source on T0 pin. Clock on falling edge.
1 1 1 External clock source on T0 pin. Clock on rising edge.

3.3. TIFR0 (Timer/Counter 0 Interrupt Flag Register)

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  • Bit 2 - OCF0B (Output Compare Flag B): Set to 1 when TCNT0 matches the value in OCR0B.
  • Bit 1 - OCF0A (Force Output Compare A): Set to 1 when TCNT0 matches the value in OCR0A.
  • Bit 0 - TOV0 (Timer Overflow Flag): Set to 1 when the timer overflows (reaches its MAX value and restarts from 0).

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