Using PWM on AVR micro to generate DTMF
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From: gamma on 19 Feb 2010 03:25 Hi I'm wanting to generate DTMF tone dialling (to dial a phone number) using an AVR micro with the timer in PWM mode. There's a description of how it supposedly works here http://www.atmel.com/dyn/resources/prod_documents/doc1982.pdf and some source code here http://www.avrfreaks.net/index.php?func=viewItem&item_id=168&module=Freaks%20Tools From looking at the code in the example, the CPU runs at 8 MHz and timer 1 interrupts every 510 cycles (256 + 254) i.e. a counter counts from zero up to 255 and back to zero and an interrupt occurs every time the counter reaches zero. The output that generates the waveform changes when the counter matches an output compare register. The value in the output compare register changes on each interrupt, to generate a varying duty cycle. At 8 MHz and with a prescalar of one, an interrupt occurs every 64 microseconds or so. What I'm not sure I understand is what the idea of PWM mode with the AVR timer is. In non PWM mode, using output compare, you can also generate high/low pulses - the only problem is that for the short pulses, you have to be able to re-interrupt very quickly - too quickly - so PWM mode handles these short pulses. Is that right? Wikipedia has a picture of the waveform for the dial-tone of a "1" here http://en.wikipedia.org/wiki/File:MultiTone1.png Does anyone have any experience of using the AVR micro to generate dial tones and does it work? TIA
From: Stefan Brröring on 19 Feb 2010 05:51 > What I'm not sure I understand is what the idea of PWM mode with the > AVR timer is. In non PWM mode, using output compare, you can also > generate high/low pulses - the only problem is that for the short > pulses, you have to be able to re-interrupt very quickly - too > quickly - so PWM mode handles these short pulses. Is that right? PWM is used instead of a D/A converter, so it runs with about 16 KHz, thats about 10 times of the highest tone that you need for DTMF (about 1.6 KHz). So, you can see the PWM output as an anlog output. You only need to add a simple low-pass filter. The voltage at the output is nearly proportional to the pulse-lenght of the PWM signal. Then, with every int, you reload a value from a table which describes the wave-form. This can be calculated like: U = Umax (sin(f1*t) + sin/f2*t)) Because U has positive and negative values, you have to add an offset, so that you will only get positive values for the reload value. To eliminate the DC part, you need a high-pass filter. > Does anyone have any experience of using the AVR micro to generate > dial tones and does it work? No, but its an interesting topic...
From: Stefan Brröring on 19 Feb 2010 05:55 of course it should be like: > U = Umax (sin(2*pi*f1*t) + sin(2*pi*f2*t)) Stefan
From: Vladimir Vassilevsky on 19 Feb 2010 09:18 gamma wrote: > Hi > > I'm wanting to generate DTMF tone dialling (to dial a phone number) > using an AVR micro with the timer in PWM mode. > Does anyone have any experience of using the AVR micro to generate > dial tones and does it work? http://www.abvolt.com/products/telephone.htm This uses AVR Tiny 26; it generates tones by PWM. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Tim Wescott on 19 Feb 2010 11:14
On Fri, 19 Feb 2010 00:25:22 -0800, gamma wrote: > Hi > > I'm wanting to generate DTMF tone dialling (to dial a phone number) > using an AVR micro with the timer in PWM mode. There's a description of > how it supposedly works here > http://www.atmel.com/dyn/resources/prod_documents/doc1982.pdf and some > source code here > http://www.avrfreaks.net/index.php? func=viewItem&item_id=168&module=Freaks%20Tools > > From looking at the code in the example, the CPU runs at 8 MHz and timer > 1 interrupts every 510 cycles (256 + 254) i.e. a counter counts from > zero up to 255 and back to zero and an interrupt occurs every time the > counter reaches zero. The output that generates the waveform changes > when the counter matches an output compare register. The value in the > output compare register changes on each interrupt, to generate a varying > duty cycle. At 8 MHz and with a prescalar of one, an interrupt occurs > every 64 microseconds or so. > > What I'm not sure I understand is what the idea of PWM mode with the AVR > timer is. In non PWM mode, using output compare, you can also generate > high/low pulses - the only problem is that for the short pulses, you > have to be able to re-interrupt very quickly - too quickly - so PWM > mode handles these short pulses. Is that right? > > Wikipedia has a picture of the waveform for the dial-tone of a "1" here > http://en.wikipedia.org/wiki/File:MultiTone1.png Does anyone have any > experience of using the AVR micro to generate dial tones and does it > work? > > TIA I haven't _done_ it, but I see no reason that it shouldn't be bullet proof. If you can get unwrapped from all the details of the PWM generation process you'll find that a PWM output followed by a low pass filter acts pretty much like a DAC followed by a low pass filter. Get that firmly in your head, and then you can think of the PWM/filter combo as a simple 8- bit DAC. Eight bits aren't at all enough for decent audio quality, but it should be sufficient for solid DTMF signaling. -- www.wescottdesign.com |