The device has two data frame size options to set the number of device words per frame: fixed and dynamic-frame mode, controlled by the FIXED bit in the D_SYS_CFG register. By default, the ADS131A0x powers up in dynamic-frame mode.
In fixed-frame mode, there are always six device words for each data frame for the ADS131A04. The first device word is reserved for the status word, the next four device words are reserved for the conversion data for each of the four channels, and the last word is reserved for the cyclic redundancy check (CRC) data word. For the ADS131A02, there are two fewer words because there are two fewer channels reporting data words.
In dynamic-frame mode, the number of device words per data frame is dependent on if the ADCs are enabled and if CRC data integrity is enabled. The device words in a data frame of the command or status word, the data words for enabled ADC channels, and the CRC word if enabled.
Figure 53 shows the fixed-frame and dynamic-frame modes for the ADS131A04 in standby mode with CRC data integrity enabled and disabled. Figure 54 shows the fixed-frame and dynamic-frame modes for the ADS131A04 with ADC channels and CRC data integrity enabled and disabled.
Enabling the ADCs in the ADC_ENA register changes the SPI frame size when using dynamic-frame mode. This change may result in an F_FRAME error if the next command frame is not adjusted. The number of words in a frame is dependent on how many ADCs are enabled and if the CRC is enabled.