What are the basic things to be checked when using the DDS evaluation board via external control?

Document created by sitti on Oct 4, 2013
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Q: Most of the DDS evaluation board comes with input/output headers for ease of accessibility to some digital input/output pins of the DDS. If I want to use these pins for external control, what are the basic things to be checked?

 

A: Most of the DDS evaluation board has available input/output headers that correspond to some of the digital input/output pins of the DDS. To name the DDSs of which their evaluation boards has this availability are the following:

 

  • AD985x
  • AD995x
  • AD991x

 

Most customers access these pins but problems arise due to incorrect handling of these pins. This will serve as a guideline on what the customers will check when using the output pins and/or input pins.

 

Output Pins

Some DDS output pins are provided for ease of access when the user wants to monitor the output of that pin. Or, it can be used to interface with external circuitry. These pins can be used directly without having to configure the logic state of other pins. Some of these typical pins may vary on each DDS board:

 

  • SYNC_CLK
  • SDO
  • DROVER
  • PLL_LOCK

 

Input Pins

Some DDS digital input pins through the I/O headers are provided to give the customers option in controlling the DDS. The default of the evaluation board is being set-up to be controlled by the corresponding DDS evaluation software. However, a partial or complete external control to the evaluation board can be done too. Some of these typical pins are the following and may vary on each DDS board:

 

  • MASTER_RESET
  • EXT_POWER_DOWN
  • SCLK
  • SDI
  • CSB
  • I/O_UPDATE
  • PROFILE PINS
  • FUNCTION PINS
  • PARALLEL PORT PINS

 

Partial External Control

This is the set-up in which the evaluation board is first controlled using the ADI software, before shifting to external control on the DUT. The basic things to be checked are the following:

  • Set-up the evaluation board so that it is controlled by the ADI software. The user may refer to the DDS Evaluation Board User’s Guide.
  • After the DDS registers is programmed using the evaluation software, the user may now proceed to control the DUT externally.
  • Locate the Master_Reset or simply Reset pin and set to logic 0 so as not to lose the DUT register settings.
  • Make sure the EXT_POWER_DOWN pin is set to logic 0.
  • Most of the DDS evaluation boards have logic buffers between the USB interface and the DDS. There are jumpers that enable/disable these logic buffers. Locate for these jumpers and set to disable to tri-state the ICs on the evaluation board. The user may refer to the evaluation board schematic to easily identify these jumpers.
  • Selecting the profile mode can be done two ways: hardwire each profile pin to logic 1 or 0, or connect interface cable from these pins to control it externally.
  • Connect ALL the required input pins to the external circuitry via interface cable to control the DUT.
  • Set the Function Pins (applicable only to AD9914/AD9915) to the desired parallel-port configuration.

 

Complete External Control

This is the set-up in which the evaluation board is 100% to be controlled externally. The basic things to be checked are the following:

  • Most of the DDS evaluation boards have logic buffers between the USB interface and the DDS. There are jumpers that enable/disable these logic buffers. Locate for these jumpers and set to disable to tri-state the ICs on the evaluation board. The user may refer to the evaluation board schematic to easily identify these jumpers.
  • Connect ALL the required input pins to the external circuitry via interface cable to control the DUT. Some critical inputs are the Master_Reset and the external power down pins.
  • Selecting the profile mode can be done two ways: hardwire each profile pin to logic 1 or 0, or connect interface cable from these pins to control it externally.
  • Set the Function Pins (applicable only to AD9914/AD9915) to the desired parallel-port configuration.
  • Apply power, reference clock and control ALL digital inputs. When controlling the digital inputs, make sure that there is a defined logic state for each input. The external power down must be set to logic 0 to ensure the DDS is not in power-down mode. 
  • Send Master Reset to the DUT. This places the DDS to a known state.
  • Program the registers to the desired configuration.

 

May it be partial or complete external control, make sure to apply power that is within specification. The reference clock must also be referred to the datasheet for specification. Some DDS has XTAL_SEL or REF_SELECT pin that is tied low or high depending if crystal or external clock is the source of clock input. Whether the external control is using serial or parallel communication, issuing an I/O_UPDATE is always necessary when programming the registers.

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