NCO Calculator

[mpgmike]

The basic formula reduces down to:

NCO Value = (Frequency * 2,097,150) / Fosc

Building the NCO Calc in Visual Basic, the behind-the-scenes math was:

NcoDec = (Freq * 2.09715) / Fosc
NcoDecR = Math.Round(NcoDec, 3)
FreqDn = Math.Round(((NcoInt * Fosc) / 2.09715), 4)
FreqUp = Math.Round(((NcoInt1 * Fosc) / 2.09715), 4)

There's a bit of other code in there, but that's the math.

[Mike, K8LH]

I think I found the problem... 2^21 equals 2097152, not 2097150. When I plug the 2097152 value into your formulas the phase increment and output frequency results match my calculations.

Thank you so much for helping me verify my calculations are correct.

Cheerful regards, Mike, K8LH

Fosc = 32000000
Freq = 1843200

NCO Decimal = 120795.9552
NCO HEX = 1D7DC
Result Rounded Up = 1843200.68359375
Result Rounded Down = 1843185.4248046875
Resolution = 15.2587890625-Hz

[Ioannis]

I never used the NCO up to this moment. What is the purpose of this module?

Ioannis

[Dave]

Well Ioannis, Here is an excerpt form the data sheet:

The Numerically Controlled Oscillator (NCO1) module
is a timer that uses the overflow from the addition of an
increment value to divide the input frequency. The
advantage of the addition method over simple
counter-driven timer is that the output frequency
resolution does not vary with the divider value. The
NCO1 is most useful for applications that require
frequency accuracy and fine resolution at a fixed duty
cycle.

I have recently built and coded a closed loop PID control for a stepper motor with the NCO.

[Ioannis]

Thanks Dave for the info.

Ioannis

[mpgmike]

With NCO you can target frequencies with spectacular accuracy as opposed to PWM or other methods. Many of the newer PICs with NCO also have the option of using LFINTOSC and MFINTOSC with NCO for a slower-than-Fosc(/4) clock. My current project targets frequencies as slow as 2 Hz, with accuracy to 0.02 Hz, and as high as 3.1 MHz. NCO lets me get that accurate.

[Mike, K8LH]

Hi mpgmike:

At the risk of seeming like a troll, may I make a couple suggestions, just in case you ever revisit your NCO Calc program, please?

(1) Allow entry of NCO Frequencies such as 8388608-Hz without rounding.
(2) Correct the NCO bit width constant (2097152) in your calculations.
(3) Display the NCO frequency resolution.
(4) Display the output frequency error.

I whipped up a quick spreadsheet to highlight the calculation problems I'm experiencing. Your NCO Calc app' introduces errors by rounding the NCO frequency and by using the incorrect NCO bit width constant in your calculations.

Have fun. Best wishes. Cheerful regards, Mike, K8LH