Fading LEDs using a PWM signal from the Edubot

[DanL]

[Edit]
For quick subject indexing purposes, this thread is turning into a discussion on timers and the use of custom PWM signals on the PIC18F8520
[/Edit]

A while ago, some of you may remember this thread where I presented an idea for a neat little car tachometer hack. I never really got around to finishing this, but for my high tech class, the opportunity has come up to make a demonstration of microcontrollers. I decided that it would be interesting to use the Edubot (both because of simplicity and familiarity) to create the tachometer hack mentioned in that thread.

Writing the code to gather the input signal and find the frequency (and thus, the RPM) is no problem for me. As for driving the LEDs, it was mentioned that the proper way is to use a PWM signal, just like was done for the IR beacons this year. Eventually I'm going to ask for help about using perhaps this IC (again, mentioned in the previous thread), but I don't really have time to learn about all the I2C bus stuff for this project. Instead, I have some questions about the PWM code used in this year's IR beacons.

First, can the Edubot generate a PWM signal with a frequency fast enough for the blink not to be 'detected' by the human eye? When I attached a status LED to this year's IR beacon, a flicker was recognized, much like some people can detect the 70-75Hz flicker of computer monitor.

Second, has there been a writeup explaining the custom PWM code used for the IR beacons? Specifically, I'd like to know how to adjust the duty cycle and frequency to create the best 'fade' effect possible.

Any of you advanced programming gurus have any suggestions?

[Astronouth7303]

Check User_Initialization(), step 5. there is an option (at least in the FRC) to set some pins up as custom.

[CrashZero]

I will gladly help but I am not sure just what you are trying to do. Are you trying to make the LED's blink fast enough that human eyes cannot detect it or do you want it to be slower so that it can be detected?

[Andrew Rudolph]

He wants to use the EDU bot to dim the leds, a common way to dim leds is to have them blink. so he wants it to blink fast enough so it just appears as the led is dim.

[CrashZero]

Ok i think that I understand what you want now, and I don;t think that I know a way to get it to do that. Sorry.

[Kevin Watson]

Dan,

This can be done using the PIC's dedicated PWM hardware. Have a look at the beacon code and note where the 40KHz duty cycle is set to 50%. You can get the effect you want by changing this value. The beacon drive circuit can be used to drive an array of LEDs too. If this isn't clear, I'll write some example code and post it.

-Kevin

[Alan Anderson]

Originally Posted by SuperDanman

First, can the Edubot generate a PWM signal with a frequency fast enough for the blink not to be 'detected' by the human eye? When I attached a status LED to this year's IR beacon, a flicker was recognized, much like some people can detect the 70-75Hz flicker of computer monitor.

Why not measure it and see?

The IR beacons aren't simple PWM signals. They are driven with short bursts of high-frequency pulses. The bursts repeat at a rate low enough that you can perceive as a flicker.

I don't know what the PWM frequency is, but I'd be very surprised if it were slow enough to detect with your eyes.

[NoRemorse]

PWM = pulse width modulation

its a serious of I's ans O's, and the higher the number, thr more of accurance of I's

the duty cycle is the total length between 2 leading edges

[FotoPlasma]

Originally Posted by NoRemorse

PWM = pulse width modulation

its a serious of I's ans O's, and the higher the number, thr more of accurance of I's

the duty cycle is the total length between 2 leading edges

The duty cycle is measured as a percentage of the total period of the square wave. You can also describe this as time between the rising edge and the falling edge of the square wave. I don't think that "the total length between 2 leading edges" was concise enough.

I'm sure there're other threads that explain the fundamentals of PWM, PPM, and whatever else.

[NoRemorse]

Your right, that was a typo on my part.

[Kevin Watson]

Originally Posted by NoRemorse

...the duty cycle is the total length between 2 leading edges

Actually, the amount of time between successive edges of the same type (i.e., rising or falling) is the period. One divided by the period is the frequency. Duty cycle, usually expressed as a percentage, is the amount of time the pulse is high divided by the total period.

-Kevin

[DanL]

Originally Posted by Alan Anderson

I don't know what the PWM frequency is, but I'd be very surprised if it were slow enough to detect with your eyes.

You can't detect it precisely (the human eye notices flashes of light only to about 25Hz-30Hz - coincidentally the framerate of television). However, some people can observe a flicker. Take your computer CRT - those are usually 70-75Hz. The problem with CRTs is this flicker can be detected and the subconcious detection of this flicker is one of the reasons prolonged exposure to a monitor leads to eye strain. The flicker is especially noticable from the corner of your eyes because the light detecting cells on the edges of the human eye are more sensitive to light vs. dark while the cells in the middle of the human eye are more sensitive to color. Proof: ever been stargazing and see a dim star in the corner of your vision but when you center your vision on it, it disapears? The corner of your vision is more sensitive to contrast while the center is more sensitive to color (if you think about this from an evolutionary standpoint, this also makes sense - our ancestors would have benefited from detecting moving objects in the corners of their vision). Anyways, the point of this extended tangent is that yes, the blinking is too fast to detect outright, but some people can observe the flicker much like the flicker of a computer monitor is observed.

Originally Posted by Kevin Watson

Dan,

This can be done using the PIC's dedicated PWM hardware. Have a look at the beacon code and note where the 40KHz duty cycle is set to 50%. You can get the effect you want by changing this value. The beacon drive circuit can be used to drive an array of LEDs too. If this isn't clear, I'll write some example code and post it.

-Kevin

Okay, so I've examined the use of timers with the help of your beacon code and interrupt template from kevin.org... however, I still have some questions

// initialize timer 2, which generates the 40KHz IR carrier frequency
	PR2 = PWM_TOTAL_PERIOD;		// set PWM frequency (defined in beacon.h) [154]
	TMR2 = 0;					// clear timer2 register [142]	
	T2CONbits.T2OUTPS0 = 0;		// setup 1:1 postscale [141]
	T2CONbits.T2OUTPS1 = 0;
	T2CONbits.T2OUTPS2 = 0;
	T2CONbits.T2OUTPS3 = 0;
	T2CONbits.T2CKPS0 = 0;		// setup 1:1 prescale [141]
	T2CONbits.T2CKPS1 = 0;
	T2CONbits.TMR2ON = 1;		// start timer 2 [141]

So, with a 1:1 prescale, one timer tick is 1e-7 seconds, 100 ns. Assuming PWM_TOTAL_PERIOD is 250, that means the timer rollover handler is called every 25e-6 seconds, or 25 microseconds. 1 / 25e-6 results in the 40KHz frequency described in the comment. I can now see how to use a timer to generate the interrupt frequency needed. However, I don't see how this is used to generate the PWM signal

beacon.h:
// This sets the PWM high period, which should be 
// half of PWM_TOTAL_PERIOD for a 50% duty cycle.
// Set to 10000000*desired_high_period.
#define PWM_HIGH_PERIOD 125

beacon.c:
	// initialize PWM 1, which generates type 0 beacon waveforms
	TRISCbits.TRISC1 = 0;		// make sure CCP2/PWM1 is setup as an output [110]
	CCPR2L = PWM_HIGH_PERIOD;	// set CCP2/PWM 1 duty cycle (defined in beacon.h) [

I don't see any information about what CCPR2L is, or what timer 2 is used for. My problem is I found where you initialized timer 2 to create a 40KHz pulse, but I'm not seeing how it goes about creating the signal. What does timer 2 do (I don't see any interrupt handling routine)? What is CCPR2L and how does that relate to a duty cycle?

[steven114]

There is some information in the IFI default code about how to enable a user PWM, along with assigning a duty cycle, etc. The rest of the information is in the PIC datasheet on Microchip's website.

[DanL]

Originally Posted by steven114

There is some information in the IFI default code about how to enable a user PWM, along with assigning a duty cycle, etc. ...

Any hints on where? ;-)

[steven114]

http://www.microchip.com/download/li...x20/39609b.pdf