programming DDS core

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  1. - 19th October 2010, 15:57 #1
    harryweb's Avatar
    harryweb
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    Default programming DDS core

    Hi,

    I would like to program a DDS (AD9951).
    The frequency tuning word (FTW) must be calculated to have a good frequency output (Fo).
    Using also the frequency of DDS clock (Fs)
    My DDS clock is 360MHz = 360^6

    Fo = (FTW)(Fs)/2^32 (given by Analog device)

    - As I want to send FTW to the DDS for 10MHz (Fo)
    FTW = (2^32x10^6)/360^6 (this is true ?)

    but I don't know how to calc these big numbers... (with long ?)
    thanks for helping
  2. - 19th October 2010, 20:09 #2
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    Oh... it seems that formula is not good...

    The good formula:
    FTW = 2E32 x (Fo/Fs)

    Same pb with 2E32...
    Any idea ?
  3. - 19th October 2010, 23:31 #3
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    PicBasic code will work with that part?
    Dave
    Always wear safety glasses while programming.
  4. - 20th October 2010, 03:48 #4
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    Not sure if this will help but please consider;

    Fr (frequency resolution) = 360000000/2^32 = 0.08381903171539306640625 Hz

    FTW = INT(Fo / Fr)
    FTW = INT(10000000 / 0.08381903171539306640625) = 119304647

    or using reciprocal multiplication (with a constant 1 / Fr = 11.9304647);

    FTW = INT(Fo * (1 / Fr))
    FTW = INT(10000000 * (11.930464711111111111111111111111)) = 119304647

    Good luck on your project...

    Cheerful regards, Mike, K8LH
    Last edited by Mike, K8LH; - 20th October 2010 at 03:58.
  5. - 20th October 2010, 05:25 #5
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    PBP doesn't have decimals, and (2^32 * 10e6) takes more than 32-bit LONGs.

    But using some 64-bit math works out pretty good.
    With the N-Bit_Math module from ... http://www.picbasic.co.uk/forum/showthread.php?t=12433
    Code:
    <font color="#000000"><b>PRECISION  </b><font color="#008000"><b>CON </b></font><font color="#800000"><b>8 </b></font><b>SYSTEM         </b><font color="#0000FF"><b><i>' 8 bytes = 64-bit
</i></b></font><font color="#008000"><b>INCLUDE </b></font><font color="#FF0000">&quot;N-Bit_Math.pbp&quot;

</font><b>Base  </b><font color="#008000"><b>VAR BYTE</b></font>[<b>PRECISION</b>]
<b>Fo    </b><font color="#008000"><b>VAR BYTE</b></font>[<b>PRECISION</b>]
<b>Fs    </b><font color="#008000"><b>VAR BYTE</b></font>[<b>PRECISION</b>]
<b>FTW   </b><font color="#008000"><b>VAR BYTE</b></font>[<b>PRECISION</b>]

<font color="#008000"><b>ASM
   </b></font><font color="#000080">MOVE?CP  4294967295, _Base    </font><font color="#0000FF"><b><i>; 2^32 - 1
   </i></b></font><font color="#000080">MOVE?CP  10000000, _Fo        </font><font color="#0000FF"><b><i>; desired frequency out
   </i></b></font><font color="#000080">MOVE?CP  360000000, _Fs       </font><font color="#0000FF"><b><i>; DDS clock
   
   </i></b></font><font color="#000080">MATH_MUL _Base, _Fo, _FTW     </font><font color="#0000FF"><b><i>; 2^32 * Fo
   </i></b></font><font color="#000080">MATH_DIV _FTW, _Fs, _FTW      </font><font color="#0000FF"><b><i>; / Fs
</i></b></font><font color="#008000"><b>ENDASM

HSEROUT </b></font>[<font color="#FF0000">&quot; FTW=&quot;</font>,<font color="#008000"><b>HEX2 </b></font><b>FTW</b>(<font color="#800000"><b>3</b></font>),<font color="#008000"><b>HEX2 </b></font><b>FTW</b>(<font color="#800000"><b>2</b></font>),<font color="#008000"><b>HEX2 </b></font><b>FTW</b>(<font color="#800000"><b>1</b></font>),<font color="#008000"><b>HEX2 </b></font><b>FTW</b>(<font color="#800000"><b>0</b></font>),<font color="#800000"><b>13</b></font>,<font color="#800000"><b>10</b></font>]
<font color="#008000"><b>STOP
</b></font>
    Which shows a result of ...

    FTW=071C71C7

    Which is the hex value for 119,304,647

    The value is located in the lower 4 bytes of the FTW array and can be easily shifted out to the DDS.

    HTH,
    DT
  6. - 20th October 2010, 08:30 #6
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    WOW ! It works !

    Thank You Darrel
  7. - 20th October 2010, 10:06 #7
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    But... I have tried to use a variable for Frequency output and it doesn't work anymore...

    Code:
    PRECISION  CON 8 SYSTEM         ' 8 bytes = 64-bit
INCLUDE "N-Bit_Math.pbp"

define osc 4

FOUT    var LONG
Base  VAR BYTE[PRECISION]
Fo    VAR BYTE[PRECISION]
Fs    VAR BYTE[PRECISION]
FTW   VAR BYTE[PRECISION]

high portc.6

FOUT = 1000000

ASM
   MOVE?CP  4294967295, _Base    ; 2^32 - 1
   MOVE?CP  _FOUT, _Fo        ; desired frequency out
   MOVE?CP  360000000, _Fs       ; DDS clock
   
   MATH_MUL _Base, _Fo, _FTW     ; 2^32 * Fo
   MATH_DIV _FTW, _Fs, _FTW      ; / Fs
ENDASM

serout2 portc.6,84,[" FTW=",HEX2 FTW(3),HEX2 FTW(2),HEX2 FTW(1),HEX2 FTW(0),13,10]
STOP
  8. - 20th October 2010, 11:59 #8
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    MOVE?CP is for constants.

    Use MOVE?LP for LONG variables.
    DT
  9. - 20th October 2010, 12:06 #9
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    Darrel... you're wonderful !
  10. - 20th October 2010, 12:19 #10
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    ADDED:
    Since you are using PBPL, you can also move the result back to a LONG...
    Code:
    PRECISION  CON 8 SYSTEM         ' 8 bytes = 64-bit
INCLUDE "N-Bit_Math.pbp"

FOUT    var LONG
Base  VAR BYTE[PRECISION]
Fo    VAR BYTE[PRECISION]
Fs    VAR BYTE[PRECISION]
FTW   VAR BYTE[PRECISION]
FTWL  VAR LONG

high portc.6

FOUT = 10000000

ASM
   MOVE?CP  4294967295, _Base    ; 2^32 - 1
   MOVE?LP  _FOUT, _Fo           ; desired frequency out
   MOVE?CP  360000000, _Fs       ; DDS clock
   
   MATH_MUL _Base, _Fo, _FTW     ; 2^32 * Fo
   MATH_DIV _FTW, _Fs, _FTW      ; / Fs
   MOVE?PL  _FTW, _FTWL          ; move result to a LONG var
ENDASM

serout2 portc.6,84,[" FTW=",HEX8 FTWL," = ",DEC FTWL,13,10]
STOP
    DT
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