CONFIGURING THE AD7725

The AD7725 requires a configuration file to be loaded into the post processor to program the multipliers and accumulators to perform the intended function. The configuration file can either be loaded in Serial Mode or Parallel Mode.

Parallel Mode
In Parallel Mode an instruction set and a status register are used to communicate with the AD7725 and to source errors. There are two ways to configure the AD7725:

  • The default filter configuration file stored in internal ROM can be loaded into the post processor.
  • An external user-defined filter configuration file can be loaded into the post processor from a Microprocessor.
An instruction must be sent after power-up to inform the device whether the default filter or a user-defined filter is to be loaded into the post processor. The parallel interface is used to transfer the configuration file to the AD7725 and to output the conversion results.

Serial Mode
In Serial Mode, there are three sub-modes. These modes are selected by the values on the Serial Mode pins. These pins inform the device where configuration data is to be loaded from on power up. The sub-modes are:

  • Default Filter (SMODE0 = 0; SMODE1 = 0)
    The AD7725 can be configured to automatically load the default filter stored in internal ROM into the post processor on power up. In this mode, the AD7725 will operate as a standard sigma-delta ADC, outputting conversion results when the device has powered up and settled.
  • DSP (SMODE0 = 1; SMODE1 = 0)
    A user-defined filter can be developed externally and the resulting configuration file stored in a DSP. The user-defined filter data is loaded into the AD7725 from the DSP automatically on power-up. The DSP therefore loads data into the device and reads back the conversion results.
  • External EPROM (SMODE0 = 0; SMODE1 = 1)
    A user-defined filter can be developed externally and the resulting configuration file stored in an external EPROM. The filter data is loaded into the AD7725 post processor from the EPROM automatically on power up. Once configuration is complete, conversion results are output to a DSP.
In Serial Mode, several AD7725s can be daisy-chained together so they can all be configured from one external serial EPROM and conversion data from all devices can be read back by a single DSP. The daisy-chaining operation requires both configuration and conversion data to flow from device to device through FSO/SDO and FSI/SDI. To daisy-chain devices, a specific 'daisy chaining' configuration file needs to be developed using the filter design package 'FilterWizard', and loaded to the devices.

Downloading the Configuration File

Parallel Mode
After power up, the user needs to send an instruction to the AD7725 to inform the device whether the default filter is to be loaded or that an external configuration file is ready to be downloaded to the post processor. Following this instruction, the configuration file can be transferred.

The configuration file contains the device ID and the configuration data. The device ID is first loaded into the AD7725. The AD7725 checks the ID to confirm that this configuration data is to be loaded into the device. If the device ID is incorrect, the ID Error bit in the status register is set to 1 and the AD7725 abandons loading the configuration data. If the device ID is correct, the AD7725 accepts the ensuing configuration data.

The post processor can hold up to 8k bits of data. The configuration data is transferred to the AD7725 in "bursts" of 42 x 16 bits, that is, 42 16-bit writes occur. When this burst of information is transferred to the AD7725, the ADC checks the data for corruption. If the data is not corrupt, the 42 x 16 bits are loaded into the post processor and the AD7725 awaits the next burst of data. If the data is corrupt, the AD7725 does not transfer the data into the post processor. The CRC Error bit in the status register is set to 1 and the AD7725 abandons reading the configuration data.

The AD7725 generates an interrupt following every 16-bit transfer to indicate when it is ready to accept the next 16-bit word. The status register can be read during the downloading of the configuration file. Therefore, if an error occurs, the user can terminate the loading of the configuration file and begin the transfer again.

Serial Mode
In serial mode, no instructions are used to configure the device. The Serial Mode pins are used to select the source of the filter configuration file to be loaded into the device. The file can come from Internal ROM, a DSP or an external EPROM. In all cases, the filter data contained in the configuration file is loaded into the post processor automatically on power up. If no errors occur during the configuration, the CFGEND pin will go high. This pin is tied to the INIT pin in serial mode, which initiates conversions when driven high. Thus the device will start converting. If an error does occur during the configuration, the ERR bit will go low and CFGEND will not go high. The INIT pin will therefore remain low and not initiate conversions. RESETCFG should be pulsed low to reset the part and reload the configuration file.

沪ICP备09046653号
Send Feedback X
content here.
content here.

Send Feedback

Close