- 5 -

Rev. A

EVAL-AD9835EB

The SELSRC Bit

The SELSRC bit of the AD9835 is used to control the

selection of the frequency and phase registers used to provide

the output. If this bit is a zero the frequency and phase

selection comes from FSELECT, PSEL0 and PSEL1 pins

respectively. If the bit is a 1 then the registers are selected by

software. Pressing the SELSRC button toggles this bit on the

AD9835 and also enables or disables changing the frequency

and phase registers from software as required.

It should be noted that since there is no readback facility on

the AD9835 it is not possible to determine the contents of any

register which has not been programmed by the software. For

this reason selecting a frequency or phase register which

hasn't been programmed may give an unpredicted output.

The Sweep Facility

Pressing the Sweep button displays a new screen which can be

used to program the AD9835 to produce a continuously

increasing or decreasing frequency sweep. The user has the

option of entering a start frequency, a stop frequency and a

step frequency. There is also the option of setting the delay

between steps and the number of time the sweep is to be

repeated. If the user were to enter the sweep values shown

below then the frequency outputs also shown below will be

generated. Note that if the frequency span is not an integer

multiple of the step frequency then the span from the second

last frequency to the last frequency will be such that the stop

frequency is the last frequency outputed to the AD9835.

Start Freq.

1.00000MHz

Stop Freq.

1.70000MHz

Step Freq.

0.20000MH

Frequency Outputs:

1.00000MHz

1.20000MHz

1.40000MHz

1.60000MHz

1.70000MHz

Both Frequency Register 0 and Frequency Register 1 can be

loaded with seperate frequency, step, delay and loop

information but the output from the AD9835 will depend on

which frequency register is selected via the FSELECT pin.

If changing the frequency register is required then the user

should ensure that the SELSRC bit is zero before starting the

sweep.

Additional Information

Information about the current version of software can be

gathered from the About Screen (accessed by pressing the

About button on the main screen).

Features of the AD9835 Evaluation Board Software

The software allows the user to control all the functionality of

the AD9835. The main features are listed below.

• Access to both Frequency 0 and Frequency 1 registers

• Access to the Phase Reg. 0, Phase Reg. 1, Phase Reg. 2 and

Phase Reg. 3

• Access to Sleep, Reset and SELSRC bits.

• Ability to sweep through a frequency range using either the

Frequency 0 or Frequency 1 registers.

Each of these features is described in detail in this application

note.

The Frequency Registers

The AD9835 contains 2 frequency registers which can be

programmed individually. The software allows the user to

enter any frequency value between 0Hz and half the master

clock frequency. If a value outside this range is entered the

software picks the closest valid value. The frequency value is

converted to a 32 bit hexidecimal number which is loaded to

the appropriate frequency register when the enter key is

pressed. The hexidecimal number is also displayed on the

main screen.

The Phase Registers

The AD9835 contains four phase register which can be

individually loaded with different values. The phase numbers

are entered as integer values between 0 and 4095 (corresponding

to 0° to 360° phase shift). Once again the values are loaded

to the appropriate phase register when the enter key is

pressed.

Normal/Sleep Mode

The Control Register of the AD9835 contains a sleep bit to

put the part into a power-down mode. When this bit is a logic

1 the internal clocks are disabled and the DAC's current

sources and REFOUT are turned off. This bit can be

controlled by the two radio buttons in the Normal/Sleep

Mode box. The software defaults to Normal Mode reflecting

the reset state of the AD9835.

Reset Mode

The Reset bit in the Control Register of the AD9835 is

controllable by the RESET button of the AD9835 Evaluation

Board Software. Pressing this button toggles the state of the

bit in the control register. Note that it is assummed that the

RESET bit is low when the software is started. Setting the

RESET bit to 1 will set the phase accumulator to zero phase

corresponding to a full scale output.