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.
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:
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:
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.
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.