Electronic – Help understanding AVR execution timing

avrclocktiming

I am working with an Atmel ATMEGA32U4 microcontroller – datasheet here with a 16 MHz crystal for system clock.

From my understanding, this chip has a 'Divide clock by 8' fuse programmed from the factory, effectively making my system clock 2 MHz. (Page 348 of datasheet. CKDIV8 (bit 7) default value is 0, programmed).

I would like to confirm my system clock speed, so I wrote some code to output a pin low, delay one clock cycle, and then put the pin high again. Measuring the time the pin is low should equal one clock cycle.

Here is the code I used to accomplish this:

//Set PORT E as output
DDRE = 0xFF;

asm volatile("nop\n\t"::); 
PORTE |= 1<<2;  

asm volatile("nop\n\t"::); 
PORTE &=~ (1<<2);   

asm volatile("nop\n\t"::); 
PORTE |= 1<<2;  

asm volatile("nop\n\t"::); 
PORTE &=~ (1<<2);

A 'nop' is equal to one clock cycle, according to the AVR instruction set manual, page 108.

Based on this information, I would assume a 'nop' instruction to take 500 nanoseconds.
Is this assumption correct? (16 MHz/8 = 2 MHz. \$\frac1{2MHz}\$ = 500ns)

Here is a scope plot of my findings:
enter image description here

It would appear a 'nop' execution time is only 200ns (5 MHz)
I would expect there to be some additional overhead using C to set the port high/low, so 500ns is actually the minimum time I would expect the pin to be low. But as you can see from my measurement cursors 'a' and 'b', its not even close to 500ns.

Can anyone explain whats going on?
Is there an error in my method?
Am I missing something 'face-palming-ly' stupid? :p
Thanks for any help!

Best Answer

This is a bad approach. Ideally you would measure a very large number of NOP commands - say, one million - instead of just one. I think that's the first foolish thing I can find. I would not be very surprised if your number changes right after you implement that change.

Also, can you view the disassembly of your C code? I wouldn't even try to judge the timing without seeing exactly what assembly code was generated from my C code. I would try to work backwards from the assembly to calculate what the execution time should be (via the instruction set manual you found). That way you can see if the scope measurement is off by a little or by a lot. A little might mean you made a math error. A lot might mean one of your assumptions is wrong.

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

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.

UPDATE

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(){

}

Can someone help check the solution for this timing diagram

It looks like you are almost there, except that you are mixing up the behavior of the T input and the clock.

The way you have solved for the T flip flop looks like you believe that the high values of the clock specify "toggle requested" and the rising edge of the T input actually triggers the toggle.

But in fact it is exactly the other way around! A high value of the T input specifies a request to the flip-flop that "a toggle is requested", and the rising edge of the clock is the trigger. If T is high, while a clock edge arrives, then the flip-flop toggles. If T is low while the edge arrives, nothing happens.

It looks like you understand the concept, but you mixed up the roles of the signals.

Best Answer

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

Can someone help check the solution for this timing diagram

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