Better PWM On The Raspberry Pi

February 10, 2013 by Brian Benchoff 13 Comments

[Thomas] wanted to play around with a few high-power LEDs and a RaspberryPi. LED controllers usually require some form of PWM to change the brightness of a LED, and unfortunately the Pi only has one PWM pin. [Thomas] could have gotten around this with a custom chip or even an Arduino hanging off the Pi’s USB port. He opted to go with software-based PWM, and did so in a way that is far superior to bit banging a pin.

Conventional wisdom says PWM without a real-time operating system is dumb – right up there with starting a land war in Asia. Turning a pin on and off in a while loop will eat up all the processor power in the Pi, so [Thomas] looked for a better way to do things. He came across the ServoBlaster project by [Richard Hirst] that creates pulses of different lengths by playing with direct memory access; [Richard] created a circular buffer that is read every 10μs. With 2000 values in the buffer, he can control eight different pins with very little impact on CPU usage.

For [Thomas], though, [Richard]’s project wasn’t enough. It was originally written for servos and is only able to drive PWM pins up to about 12%. A quick rewrite of [Richard]’s code allowed [Thomas] to control eight pins with PWM varying from 0% to 100% – and be able to do other things with his Pi in the process.

[Thomas] now has a 40 Watt RGB LED powered by a Raspberry Pi burned into his retina, and the satisfaction of a really clever way of giving the Pi more PWM pins.

Making Better Noises With Dual PWM

January 17, 2013 by Brian Benchoff 20 Comments

pwm_16b_sm

Although it’s technically possible to get 16 bits of resolution on a ATMega328, most implementations of PWM on everyone’s favorite ‘mega – including just about every Arduino sketch – are limited to 8 bit PWM. This means the pins can only output 256 different values, so if you’re playing around with music made on an Arduino don’t expect very high fidelity.

There is a clever way around this: use two PWMs, and use one pin for high bytes and another for low bytes. That’s what Open Music Labs did when working on a synthesizer project that needed very high quality audio.

The basic idea behind the build is that PWM pins can be used to create audio frequencies. Using two PWM pins and adding them together means it’s possible to add extra bits of resolution. This requires using different values of resistors on each pin. For example, using the same value of resistors on two PWM pins increases the resolution by one bit. Two pins with a resistor value ratio of 1:4 increases the resolution by four bits, and so on.

There’s a great tutorial for setting up these higher resolution, dual PWM outputs on an ATMega or Arduino, as well as a distortion analysis for this dual PWM setup.

Morse Code Flower Is Trying To Tell You Something

January 2, 2013 by Mike Szczys 5 Comments

morse-code-flower

To the casual observer this flower looks nice as its illuminated center fades in and out. But there’s hidden meaning to that light. Some of the blinks are longer than others; this flower is using Morse Code.

[Renaud Schleck] wanted to try a few different things with his MSP430 microcontroller. He decided on an LED that looks like a flower as it will be a nice piece of decor to set around the home. To add the Morse Code message he wanted something a bit more eloquent (and less distracting) than purely digital flashing. So he took the dots and dashes of the hard-coded message and turned them into fading signals by using Pulse-Width Modulation.

He free-formed the circuit so that it, and the coin cell that powers it, would fit in the flower pot. A reed switch is responsible for turning the juice on and off. When placed near a magnet the flower begins its gentle playback.

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MOD Player For The Stellaris Launchpad

November 28, 2012 by Mike Szczys 16 Comments

[Ronen K.] wrote in to tell us about the MOD playing Stellaris Launchpad project he recently completed. A MOD is a sound file for the computers of days long gone. But you’ll certainly recognize the sound of the 8-bit goodness that is coming out of this device.

To understand how a MOD file stores samples you might want to glance at the Wikipedia page. There are a ton of these files out there, but this implementation is meant for files with only four channels. For now the only external hardware used is an audio jack which needs a ground connection and a PWM signal on each of the two audio channels. [Ronen] is storing the files in flash memory rather than using an SD card or other external storage. This leaves 213k of space for up to six files that can be selected by the user buttons which cycle forward or backward through the list. See this demonstrated after the break.

The project ports existing code from an STM32 application. Since that is also an ARM microcontroller there’s not a ton of work that needed to be done. But he did have to write all of the PWM functionality for this chip. This PWM tutorial turned out to be very helpful during that process.

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PWM On The Stellaris Launchpad

October 31, 2012 by Brian Benchoff 20 Comments

[Joonas] has been following TI’s ‘getting started’ tutorials for their new Stellaris Launchpad. Everything had been going swimmingly until [Joonas] reached the fourth tutorial on interrupts. To the ire of LEDs the world over, implementing PWM on the new Stellaris Launchpad is a somewhat difficult task. After banging his head against the documentation for hours, [Joonas] finally cracked his PWM problem and decided to share his discoveries with the world.

The Stellaris has a PWM mode for its six hardware timers, but unfortunately there are no PWM units on the chip. Solving this problem required making two 16-bit timers out of a single 32-bit one. This allowed [Joonas] to specify a ‘load’ and ‘match’ value.

After coding this up, [Joonas] discovered the PWM timer only works on two of the Launchpad’s pins. Hours of Googling later, he had real PWM on his Stellaris Launchpad.

Given the amount of time [Joonas] spent on this problem, we’re glad to help all the other frustrated Stellaris tinkerers out there by sharing this.

The Inner Workings Of Servo Motors

October 18, 2012 by Mike Szczys 19 Comments

Servos seem to be the go-to option when adding motors to hobby projects. They’re easy to hack for continuous rotation for use in a robot, but with the control board intact they are fairly accurate for position-based applications. But do you know how the hardware actually works? [Rue Mohr] recently published an article that looks at the inner world of the servo motor.

As you know, these motors use a voltage, ground, and signal connection for control. The position of the horn (the wheel seen on the servos above) is dependent on that control signal. The duty cycle of a 20 ms pulse decides this. Inside the housing is a control board capable of measuring this signal. It’s got a chip that monitors the incoming PWM pulses, but that’s only half of the equation. That controller also needs feedback from the horn to know if its position is correct or needs to be changed. Integrated with the gear box that connects the motor to the horn is a potentiometer. It’s resistance changes as the horn turns. Knowing this, it is possible to fine tune a servo by altering that resistance measurement.

CPLD Motor Control

August 2, 2012 by Mike Szczys 4 Comments

[Chris] continues cranking out the tutorials, this time around he’s showing how to use a CPLD for simple motor control. The demo hardware is pretty basic, he built his own FPGA/CPLD demo board a few years back which used a PLCC socket for easy interfacing. You should be able to use just about any gear you have on hand.

Of course the thing about these chips is that you’re working with hardware that can be run in parallel. [Chris] mentions that this is what makes it perfect for timing-critical applications. Here he’s using a motor driver that monitors a PWM signal, using the duty cycle to actuate the direction and speed at which the motor turns. After the break you can see a demonstration of the CPLD reading from an ADC chip and converting the value to a PWM signal. [Chris] has also used the same hardware for VGA signals; something that is usually a timing nightmare if done with a microcontroller.

If this leaves you thirsting for more CPLD goodness check out our own guide on the subject.

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