DDS (generating sine waves) with onboard DAC using latest PIC 16F chips?

[Ioannis]

Not quite relevant, but for the DAC that was discussed before, for the new PIC 1782/3, it needs 10usec for each sample to output a DC voltage. So it will reach about 390Hz maximum.

So even this solution is slow enough for the 5KHz target.

Can the HPWM be of some help here? I think it can for a 32MHz PIC.

Ioannis

[HankMcSpank]

Not quite relevant, but for the DAC that was discussed before, for the new PIC 1782/3, it needs 10usec for each sample to output a DC voltage. So it will reach about 390Hz maximum

Yeah, after my initial excitement ....while that's still a potentially very cool new PIC, a 28 pin solution seems excessive for a simple PIC DDS proposal!

Re going with HPWM.... alas, that would need someone who knows a whole more about filters than myself!

I'm obviously still not grasping something quite fundamental here (talk about being slow or what?!), cos this guy gets a lovely smooth 8 bit sinewave @ 8.9khz - and using a sampling (interrupt) rate of only 100khz .....

http://www.g4jnt.com/PIC_DDS.pdf (albeit he's LP filtering the PIC output & hasn't shown whether the prefiltered R2R ladder is ugly or otherwise)

[Ioannis]

I don't know how fast a PIC at 10, 15 or 20 MHz can output new values on a Port but sure he has a 3rd order filter at the output that makesthe difference.

Note that.

Ioannis

[mister_e]

Depends how much distortion you want to acheive. Sure enough you need a filter somewhere OR a way better granularity which imply a way bigger lookup table, but if you do so, you also limit your max Freq. Even with that, you'll always have some steps in your waveform.

Another thing, 1% (or less) precision resistor MAY help... but still.

[cncmachineguy]

Well that is not exactly what I was thinking about. but it does provide some data. with the tuning word = 1 I caculate it takes about 24 instrcution cycles to get through the main loop. thats 3 uS. or the 333K you referenced above. IMHO that seems plenty fast enough to make a 5K signal. at 5K, you will have enough time to run your loop ~66 times. so thats 33 loops for each half cycle. so every 5.4 degrees you change output values. that would be the same as skipping values in the table I suppose. I know I am not helping here, but something just feels better thinking about the values this way. the next issue is sine waves are not triangle waves. there is more action over time at the reversals. during the rise and fall they are almost vertical, well more then 45deg anyway. So it seems to me you need more of those samples there.

[Jumper]

Why dont you change to a 18F46K20 or similar that you can run on 64MHz. A 64MHz pic is not that much more expensive unless you plan to make lots and lots of your product.

Speed could be helpful sometimes.

Meep Meeep as the Road Runner would have said!

[mister_e]

[HankMcSpank]

Why dont you change to a 18F46K20 or similar that you can run on 64MHz. A 64MHz pic is not that much more expensive unless you plan to make lots and lots of your product.

Speed could be helpful sometimes.

Meep Meeep as the Road Runner would have said!

I'm not designing a product...this is just me learning about the benefits/constraints of DDS, think I need a break from this one for a few days now though!

RE going with an 18F PIC....the 16f1822 can run at 32Mhz (which is pretty fast!)....also smaller PICs just appeal to my sense of "wow, look what was squeezed out of that tiny thing!"