Hi, i am trying to program DDS AD9851, but i can’t start output frequency generation.
I use 20 MHz generator as reference clock and 6x multiplier to get 2 MHz output.
According to the on-line calculator http://designtools.analog.com/dt/dds/ad9851.html i got following values “W0 0xBA”, “W1 0x62” “W2 0x22” “W3 0x20” “W4 0x80”.
I made hardware configuration for serial load enable word W0.
To reset DDS default values i make this workflow:
1)pull up Reset line;
2) 10 microseconds delay;
3) pull down Reset line;
4)pull up W_CLK line;
5)pull down W_CLK line;
6)pull up FQ_UD line;
7)pull down FQ_UD line;
// reset device and enable serial load mod;
8) pull down Serial line and make for 40 times pull up/down W_CLK line;
9)pull up FQ_UD line;
10)pull down FQ_UD line.
To load calculated data (w0-w4) i make this workflow:
// i resetting device to clear phase accumulator +phase offset and then re enable serial load mode
8) transmit W0-W4 by right shift in series and sending current byte lsb.
10)pull down FQ_UD line;
Оn oscilloscope i see that the right data load to the DDS,but i don’t see any output generation. Could you check my workflow for errors please.
Upon transmitting of bits W0 to W4, did you provide a W_CLK on each bit before the FQ_UD pulse? Because in serial mode a W_CLK pulse is needed for each bit from W0 to W39.
Sorry I forget to tell about this. Yes I provide a W_CLK on each bit before the FQ_UD pulse.
How fast is the microcontroller operating, like it's clock frequency? My advise is, try introducing delays to the pulses if it is indeed running to fast. About enabling the serial mode, did you issue a XXXXX011 for W0? This should be done in parallel mode since serial mode must be entered from default parallel mode. XXXXX011 at W0 should be overwritten with a valid 40-bit serial word after entering the serial mode immediately to avoid unintended activation of the 6 X multiplier.
FQ_UD pulse will go twice for serial load after loading turning word.
(No it saying about in datasheet)
At Russian translation:
FQ_UD подавай дважды подряд при последовательном управлении после загрузки данных частоты и фазы.
Этой особенности нет в даташите.
Since the data transfers are done in serial, it is expected that information rate would depend on the serial clock and the number of bits needed for that information. The only way to speed it up is to use shorter pulses on the W_CLK, FQ_UD, and DATA, but make sure that it doesn't run faster than what is specified by the data sheet.