When the bus is idle, both the SDA and SCL lines are pulled high by the pull-up devices. The master generates a start condition followed by a valid serial byte containing High-Speed (HS) master code 00001XXX. This transmission is made in fast (400 kHz) or standard (100 kHz) (F/S) mode at no more than 400 kHz. The INA231 does not acknowledge the HS master code, but does recognize it and switches its internal filters to support 2.5-MHz operation.

The master then generates a repeated start condition (a repeated start condition has the same timing as the start condition). After this repeated start condition, the protocol is the same as F/S mode except that transmission speeds up to 2.5 MHz are allowed. Instead of using a stop condition, use repeated start conditions to secure the bus in HS-mode. A stop condition ends the HS-mode and switches all the internal filters of the INA231 to support the F/S mode. A bus timing diagram is shown in Figure 7-1.