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Variable-frequency PWMs available?

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pervasive

12 Posts

Posted - 31 Oct 2002 : 14:12:12

We're thinking of using a PWM output in our application. I see that the AVR(ADSmartIO) and SA-1111 have PWM outputs. Can we use them for our application? What's their resolution? Can we change the PWM frequency?

Thanks in advance, Bill

akidder

1519 Posts

Posted - 01 Nov 2002 : 15:10:13

Thanks for your question, Bill. The short answer is, Yes, you can use the PWMs on our systems for your application. Here are some details.

PWMs on ADS Systems

There are several potential sources of PWM outputs on ADS products. Here are some. Consult your product user manual for the PWM sources available on your device:

XScale PXA255 CPU
3.6864 MHz (system clock) base frequency, with a six-bit prescaler (divide by 1 to 64). PWM period can be 2 to 1024 PWM clock cycles. For PWM resolution of 1% (100 counts) or better, the PXA255 drives PWM frequencies of 56.25 Hz to 369 kHz.

Atmel ATMega microcontroller
Available PWM frequencies are 7Hz, 28Hz, 113Hz, 900Hz, 7.2kHz. We use 7.2kHz on our standard systems. Resolution is 8 bits.

SA-1111 Companion Chip
Fixed-frequency PWM at 14.4 kHz. Resolution is 8 bits.

We designate PWMs for use on our systems as backlight(brightness) and contrast(Vee) controls, but you may be able to expropriate them for other purposes.

Note that we may need to remove the filter capacitors from the outputs if you need straight PWM outputs. Note also that the SA-1111 PWM output isn't available on the connectors of some products.

If you need to set variable PWM frequencies different from the ones listed above, feel free to contact
ADS Sales with your requirements. Some of our products have hardware that could be used for PWMs with greater flexibility and/or resolution.

Non-ADS PWM Solutions

Depending on the frequency resolution, precision and number of PWMs you need, you might also consider an FPGA solution on your own board. Here are some links from the web that might be useful for your design:

App note for design of a 16-bit PWM
http://www.actel.com/appnotes/s06_04.pdf"

ABEL code for an 8-bit PWM
http://www.me.berkeley.edu/~dma/me235/syn_note.pdf

Atmel app note for creating PWMs with their FPGAs
http://www.atmel.com/atmel/acrobat/doc2324.pdf

General-purpose AHDL code for generating an arbitrary-resolution PWM (From http://links.epanorama.net/links/fpga.html):
PLD code creates PWM generators
This PLD (programmable-logic-device) code creates arbitrary-resolution, pulse-width-modulated (PWM) generators. PWM generators are useful as low-bandwidth D/A converters in hardware of microprocessor-based systems. When you pass it through a simple RC lowpass filter, a PWM waveform becomes a voltage that's approximately equal to the PWM duty cycle times the supply voltage. This code is written for Altera's devices, but you can quite easily translate the design structure and flow into VHDL or Verilog.

Drew Kidder
ADS Technology Transfer

Edited by akidder 15-Jul-2004: Add PXA255 PWM settings.

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