Electronic – Arduino PWM pin vs digital pin PWM simulation

arduinodigital-logicpinspwm

Do I understand it correctly from this thread, Why do some pins have `~` signs next to their numbers?, that the PWM pins generate a square wave signal independently from what the code uploaded to the chip does after starting the PWM output? In other words, if my code contains all sorts of delay() functions that pause longer than the period of the PWM signal, the PWM signal will still be generated properly, correct?

On the other hand, if I decided to implement my own PWM on a digital pin instead by flipping the state between HIGH and LOW in my code at regular intervals, the presence of other delay() calls in other parts of the code would directly impact my ability to generate the desired consistent PWM signal. Is this correct? I hope this question makes sense (a bit hard to ask the question).

Best Answer

You understood right. The pins with the tilde (~) symbol are pins which are linked to an hardware PWM module (i.e. timer). Since it is a hardware module, it is independent of whatever your CPU does. As such, if you are stuck in a delay, your hardware PWM will still continue to generate the proper output signal. On the other hand, a software PWM (one that you would generate with a manual pin toggling) would be interrupted by your delay.

Most of the time, you should prefer hardware PWM over software PWM for many reason:

It's more reliable;
It consumes less power (you could sleep your MCU while still generating PWMs);
It allows you to do something else instead of servicing the switching;
It allows for larger frequencies, because you can typically get frequencies up to your MCU's operating frequency, while a software emulation involves a few operations, so you are not able to keep up;
It's more accurate: the hardware PWM is simply a divider on your MCU's operating frequency, while software PWM uses CPU instructions, which require branching. Since it is compiled code, it is more or less deterministic, so your duty cycle would not be exactly the one you expect (near, but not perfect nor stable if doing something else at the same time).

Related Solutions

How to power an arduino controlled string of RGB LED’s from separate power source

Simply power the LED driver (i.e., its VCC pin) with a battery (or other higher-current-capable source), since the Arduino pins are only capable of sourcing 50 mA.

First of all, you won't even need a transistor, if you use a battery as I mentioned above. But to answer your question, if you were using a transistor: while there is lag, lag won't be significant enough to affect your application here.
And secondly, the TLC5940 is a constant-current driver, so you're fine. A constant current source/sink is exactly that -- it ensures a constant current flows independent of power supply fluctuations/other conditions. Thus your LEDs will light up with constant brightness.

Finally, a clarification regarding PWM with the TLC5940: You don't use PWM of the Arduino pin to directly create PWM on the TLC5940 I/O pins; that's NOT how it works. Instead, the TLC5940 uses serial communication with Arduino -- so you would send it instructions from the Arduino (via SPI or bit-banging, for example; see this TLC5940 Arduino library), and the TLC5940 reads those instructions and performs PWM on the desired I/O pins as instructed.

Electronic – One PWM timer to two outputs

It appears that there was some issue with predetermined values, by setting COM1A0 and COM1B0, Timer 1 can be configured to give independent signals. This is the setup code that works:

void setup_timers () {

TCCR1A |= (1 << WGM11) | (1 << WGM10);
TCCR1A |= (1 << COM1A1) | (1 << COM1A0) | (1 << COM1B1) | (1 << COM1B0);
TCCR1B |= (1 << CS11);
TCNT1 = 0;

DDRB = 0xFF;}

I'm still having trouble getting it to properly control the motors, but the servos work well, and it can be done.

Note: Timer 1 actually has two separate outputs, but Timer 0/2 do not. To get dual outputs, you need to toggle WGMx2 and the COMxB 0/1 registers, when WGMx2 is set, OCRxA will not output properly, but OCRxB will, with some fiddling. When WGMx2 is not set, it disable OCRxB, and OCRxA will work as normal, to get independent outputs, you will need to take this into account.

Best Answer

Related Solutions

How to power an arduino controlled string of RGB LED’s from separate power source

Electronic – One PWM timer to two outputs

Related Topic