Electronic – arduino – Need help understanding AVR ATMEGA / ATTINY timer mirrored output


I am trying to use Timer1 of Atmel AVR microcontroller, either AtMega328 as used in the Arduino, or the ATTiny85, to output two clock signals which are mirror images of each other. The frequency I am trying to generate is a variable 1 MHz to 2 MHz or more which are too high to do this using code to toggle the output pins unless I want to do almost nothing else in the controller. So I want to use the timer output directly on the associated pins. I am using GCC toolchain so not limited by arduino libraries or language.

Timer1 in the Atmega328 has two pins associated with it and I can get two identical 1MHz to 2MHz signals out of them. Though the datasheet seems to say I can get an inverted waveform, it is confusing me. I am also able to get two signals which are different duty cycles at 1 MHz, using the PWM settings with Timer1, but both signals go high at the same time, the shorter one goes low earlier. This does not serve my project. I do not even need the PWM pulse width variation, I just need two identical "clock" type signals of opposite phase, that's all.

I am not asking for anyone to write code for me to do this, I just need someone to tell me which mode / flags of the timer should give me a simple inverted waveform on one of the two pins associated with the timer. If possible I want to avoid using an external inverting circuit for one of the outputs unless that is only option.

If this is possible at all in the ATTiny, that will be even better. The ATTiny also has 2 pins associated with one timer, but I am not sure it has the same options as the ATMega.

I already have a 20 MHz crystal and capacitors connected on the PCB and 20 MHz clock is working reliably on the ATMega328. On ATTiny85 PCB I have a 8 MHz crystal and that is also working reliably.

Please help. Thank you.

UPDATE: There are some invalid assumptions in the answers and comments so far so maybe I should clarify: Note that in my original post I have stated that I am using a 20 MHz clock, not 8 MHz, and also that I do not need PWM.

The only mode that gives a high enough output frequency seems to be CTC mode because PWM modes are not working for 2 MHz output. Is there a way to invert either Timer 1 output A, or output B, in CTC mode?

I have now switched to a standard Arduino Uno (ATMega328, 16 MHz) instead of my own 20 MHz board to check my code, and this is my code for a nice steady 2 MHz clock in CTC mode from pins 9 and 10, the Timer 1 output pins:

#define tick 9
#define tock 10

void setup() {
  pinMode(tick, OUTPUT);  
  pinMode(tock, OUTPUT); 

  TCCR1A = _BV(COM1A0) | _BV(COM1B0) ;   // activate both output pins 
  TCCR1B = _BV(WGM12)| 1;                // set CTC mode, prescaler mode 1

  // various frustrating attempts to invert OC1B failed. What do I put here?

  OCR1A = 3;                             // set the counter max for 2 MHz


void loop() {

The oscilloscope traces for both pins are identical and in sync, how can I get either of the two signals inverted? The invert mode in the datasheet appears to do nothing in CTC mode. Am I reading the datasheet wrong, or will I be forced to use a lower frequency and PWM mode after all?

To add a specific "bounty" question to my original query:
So what changes do I need to make to my code above, to make it give perfectly inverted signals at pin 9 and 11 at the highest possible frequency for a 16 MHz clock, whether that is 2 MHz or not?

I will stick with a standard Arduino Uno for now, so that there is no error mode being introduced by my homespun board, and so that anyone with an arduino can try my code above and confirm that it works as I have mentioned and not as I need!

Best Answer

From the ATtiny85 Datasheet:

The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is defined by the combination of the Waveform Generation mode (WGM0[2:0]) and Compare Output mode (COM0x[1:0]) bits. The Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode bits do. The COM0x[1:0] bits control whether the PWM output generated should be inverted or not (inverted or non-inverted PWM).

Table 11-5 shows how to set the Mode.

Mode   WGM  WGM  WGM  Timer/Counter Mode    TOP      Update of    TOV Flag
c0     02   01   00   of Operation                   OCRx at      Set on
0      0    0    0    Normal                0xFF     Immediate    MAX(1)
1      0    0    1    PWM, Phase Correct    0xFF     TOP          BOTTOM
2      0    1    0    CTC                   OCRA     Immediate    MAX
3      0    1    1    Fast PWM              0xFF     BOTTOM       MAX
4      1    0    0    Reserved              –        –            –
5      1    0    1    PWM, Phase Correct    OCRA     TOP          BOTTOM
6      1    1    0    Reserved              –        –            –
7      1    1    1    Fast PWM              OCRA     BOTTOM       TOP

You want a Fast PWM mode (so either mode 3 or mode 7). If you want to vary the duty cycle, and it sounds like you do, you want mode 7 and vary duty cycle by setting OCRA.

Table 11-3 shows how to set the compare output mode for Fast PWM mode.

COM0A1/   COM0A0/
COM0B1    COM0B0     Description
0         0          Normal port operation, OC0A/OC0B disconnected.
0         1          Reserved
1         0          Clear OC0A/OC0B on Compare Match, set OC0A/OC0B at BOTTOM
                     (non-inverting mode)
1         1          Set OC0A/OC0B on Compare Match, clear OC0A/OC0B at BOTTOM
                     (inverting mode)

That is to say, you can set the OC0A output to go low when the Timer value == OCR0A and high when the Timer value == 0x00 by setting COM0A1:COM0A0 = 0b10. Or vise versa by setting COM0A1:COM0A0 = 0b11. And likewise for OC0B, OCR0B, COM0B0, COM0B1.

The PWM frequency is determined by the I/O Clock (8MHz it sounds like for you) and your timer prescaler setting. And the equation is given as f_clk_IO / (N * 256) for Fast PWM mode.

So you can use OC0A for "normal" polarity and OC0B for "inverted" polarity by setting OCR0A and OCR0B to the same value and setting COM0A1:COM0A0 = 0b10 and COM0B1:COM0B0 to 0b11.


Given you want to toggle the output as fast as possible and you are using the Mega328 operating at 16MHz, the CTC operating mode will allow you to obtain a switching frequency of:

f_OCnA = f_clk_IO / (2 * N * [1 + OCRnA) = 16e6 / (2 * 1 * [1 + 1]) = 4MHz

The Fast PWM mode will let you toggle the pin at:

f_OCnxPWM = f_clk_IO / (N * [1 + TOP]) = 16e6 / (1 * [1 + 1]) = 8MHz

So I still think you want Fast PWM mode. Specifically Mode 3 with OCR0A = OCR0B = 0x80 for 50% duty cycle. And set COM0A bits to 0x3 and COM0B bits to 0x2 to make the two waveforms on OC0A and OC0B inversions of one another.

Update #2 More the Mega328 Try this Arduino code:

#define tick 9
#define tock 10

void setup(){

  pinMode(tick, OUTPUT);  
  pinMode(tock, OUTPUT); 

  // Setup Waveform Generation Mode 15
  // OC1A Compare Output Mode = inverting mode
  // OC1B Compare Output Mode = non-inverting mode
  // Timer Prescaler = 1
  // TOP = OCR1A = 1

  //COM1A[1:0] = 0b11, COM1B[1:0] = 0b10, WGM1[1:0] = 0b11
  TCCR1A = _BV(COM1A1) | _BV(COM1A0) | _BV(COM1B1) | _BV(WGM11) | _BV(WGM10);

  //WGM1[3:2] = 0b11, CS1[2:0] = 0b001
  TCCR1B = _BV(WGM13) | _BV(WGM12) | _BV(CS10);

  OCR1A = 0x0001;
  OCR1B = 0x0001;

void loop(){