Generating high frequency signal with fine resolution using TMS320F2808

Alex Shpilsky

Other Parts Discussed in Thread: TMS320F2808, SPRC191

Hello,

The customer needs to generate a 50% duty cycle waveform to drive an ultasound transducer. They want to control the frequency resolution in 10Hz steps with a maximum frequency of 45KHz.. Can TMS320F2808 accomplish this?

Thank you.

over 14 years ago

0 Richard Poley over 14 years ago

TI__Mastermind 27200 points

Alex,

In order to support 10Hz resolution at the highest frequency (45kHz) you will need an edge accuracy of 4.88ns. The PWM module of F280x supports a maximum resolution of approximately 150ps, so yes, in principle the F2808 can accomplish this requirement. For more infomation on the high resolution PWM module, please refer to TI literature reference SPRU924f.

A code example can be found in the peripheral header files for this device (SPRC191)... http://www.ti.com/tool/sprc191

Hope this helps.

Best regards,

Richard

0 Richard Poley over 14 years ago in reply to Richard Poley

TI__Mastermind 27200 points

Alex,

It occurred to me after posting this that I might have misunderstood the question. You are asking about modulating PWM frequency with high resolution, not duty cycle. Unfortunately the F2808 does not have high resolution control of PWM period, so frequency resolution is always constrained by the maximum CPU clock, which on F2808 is 100MHz. Therefore you wouldn't be able to achieve the required 4.88ns resolution at 45kHz.

For high resolution control of PWM frequency you need a Piccolo class of device, such as F28069.

I'm sorry for the confusion.

Best regards,

Richard

0 Alex Shpilsky over 14 years ago in reply to Richard Poley

Intellectual 1000 points

Richard,

Are you referring to dual-edge high resolution mode period configuration? I found the following link http://www.ti.com/lit/ug/spruge8e/spruge8e.pdf

that describes it. It seems that F2802x will support this feature.

Thanks,
Alex

0 Richard Poley over 14 years ago in reply to Alex Shpilsky

TI__Mastermind 27200 points

Alex,

Yes, that's correct.

Any "Piccolo" class of device (F2802x, F2803x, F2806x) has high resolution PWM period. However, note that high resolution mode is not present at all on some very low cost devices in this series (e.g. F280200). The "Hardware Features" table in the relevant datasheet is the best place to check which device supports HR mode.

Best regards,

Richard

0 Cong Zheng over 13 years ago in reply to Richard Poley

Prodigy 20 points

Hi Richard,

Does F28335 has the high frequency resolution capability? Which doc should I look at if it has? Thanks!

0 Richard Poley over 13 years ago in reply to Cong Zheng

TI__Mastermind 27200 points

Hi Cong,

The F28335 has high-resolution available on six of its' PWM outputs. The literature reference for the relevant user's guide is "sprug02b.pdf", which you can type into the search engine on the TI web.

Best regards,

Richard

0 Cong Zheng over 13 years ago in reply to Richard Poley

Prodigy 20 points

Hi Richard,

I know that F28335 has HRPWM for high resolution pwm for duty and phase shift control. But I wonder if it has high frequency resolution capability? Thanks!

0 Richard Poley over 13 years ago in reply to Cong Zheng

TI__Mastermind 27200 points

Hi Cong,

Unfortunately the PWM module on F28335 does not support high resolution frequency modulation. For that you would need a newer device. You might like to look at the F28069, which is also floating-point but a little slower at 90MHz.

Sorry not to have better news.

Best regards,

Richard

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Generating high frequency signal with fine resolution using TMS320F2808