[gnea/grbl-Mega Issue#52] Spindle PWM Pin Swap

#1 – chamnit 于 2018-01-26

Certain laser drivers run optimally at certain frequencies. If I recall correctly, OCR4A sets that frequency. Variable frequency is desirable for lasers, because material response to laser pulses is not uniform. Eventually I’ll get to add that option, but not right now.

#2 – knightmg1983 于 2018-01-26

@chamnit That does indeed make sense and so it seems that we would want to preserve the ability to vary the frequency. I believe that the changes I have made do preserve that ability with the exception of being able to change frequency on the fly since the ICR4 register is not double buffered.

Anyhow… My goal was to change the Spindle PWM pin to use Arduino Mega pin D6 so that I could feed the laser driver a 5V PWM signal. For whatever reason I couldn’t seem to find any reference on how to do this so I thought that I would share the changes that I made to the cpu_map.h file to achieve the pin swap:

“C
// Advanced Configuration Below You should not need to touch these variables
// Set Timer up to use TIMER4A which is attached to Digital Pin 6 - Ramps 1.4 Servo Pin 2
#define SPINDLEPWMMAX_VALUE 1024.0 // Translates to about 1.9 kHz PWM frequency at 1/8 prescaler, 15.6 kHz with a prescaler of 1
#ifndef SPINDLEPWMMIN_VALUE
#define SPINDLEPWMMIN_VALUE 1 // Must be greater than zero.
#endif
#define SPINDLEPWMOFF_VALUE 0
#define SPINDLEPWMRANGE (SPINDLEPWMMAXVALUE-SPINDLEPWMMINVALUE)

//Control Digital Pin 6 which is Servo 2 signal pin on Ramps 1.4 board
#define SPINDLETCCRAREGISTER TCCR4A
#define SPINDLETCCRBREGISTER TCCR4B
#define SPINDLEOCRREGISTER OCR4A //was OCR4C, the value of this register sets the duty cycle
#define SPINDLECOMBBIT COM4A1 //was COM4C1

// 1/8 Prescaler, 16-bit Fast PWM mode
#define SPINDLETCCRAINIT_MASK (1<TCCRBINIT_MASK ((1<OCRAREGISTER ICR4 // 16-bit Fast PWM mode requires top reset value stored here.
#define SPINDLEOCRATOPVALUE 0x0400 // PWM counter reset value. Should be the same as PWMMAX_VALUE in hex.

// Define spindle output pins.
#define SPINDLEPWMDDR DDRH
#define SPINDLEPWMPORT PORTH
#define SPINDLEPWMBIT 3 // MEGA2560 Digital Pin 6 (Ramps1.4 Servo 2)
“

Disclaimer: I am not an expert in this subject so errors may be possible. I just wanted to share what seemed to work for me so that others trying to do the same may benefit.

#3 – chamnit 于 2018-01-26

I’m not intimately familiar with the Mega2560, as I am with the 328p. I’d have to look at the data sheets, but changing pins is usually tricky, especially when using a built-in peripheral like hardware PWM. BTW, most of the spindle configuration stuff was done by several helpful users. I didn’t implement them personally. I just remember that the Mega2560 has lots of different ports and pins, each with a specific set of thing they can do and can’t. Enough to make my head spin. Sorry, I don’t think I can help you much more.

#4 – knightmg1983 于 2018-01-26

No problem at all. The changes posted above appear to work well enough. My intent was to better understand the original implementation and how the changes that I made might affect the rest of GRBL before posting them.

Thanks go to you and everyone else that has made this project into what it is today.