Low power design using a scan bypass multiplexer as an isolation cell

An apparatus, comprising:

• a device having an input and an output;
• a multiplexer having a first input coupled to input of the device and a second input coupled to the output of the device;
• means for selecting one of the first input of the multiplexer and the second input of the multiplexer, the means for selecting having first input for receiving a scan mode signal having a first level indicating a scan mode is active and a second level indicating the scan mode is inactive, a second input for receiving a built in self test signal having a first level indicating built in self test mode and a second level indicating the built in self test mode is inactive, and a third input for receiving a power mode signal having a first level indicating normal power mode and a second level for indicating a lower power mode, wherein the means for selecting is coupled to the multiplexer and operable to control which input the multiplexer selects;
• wherein the means for selecting is configured to select the first input to isolate the device responsive to the scan mode signal being at the second level, the built in self test signal being at the second level and the power mode signal being at the second level.

A method for isolating an output of a device also having an input, where the input of the device is coupled to a first input of a multiplexer and the output of the device is coupled to a second input of the multiplexer, comprising:

• receiving a power mode signal, where a first level of the power mode signal indicating a normal operating mode and a second level indicating a low power level;
• receiving a built in self test mode signal, where a first level of the built in self test mode signal indicating a self test mode is active and a second level indicating the built in self test mode is inactive;
• receiving a scan mode signal where a first level indicates a scan mode is active and a second level indicates the scan mode is inactive;
• determining whether the power mode signal is at the first or second level;
• determining whether the built in self test mode signal is at the first or second level;
• determining whether the scan mode signal is at the first or second level;
• and
• selecting the first input of the multiplexer to isolate the output of the device responsive to determining the power mode signal is at the second level, the built in self test mode signal is at the second level and the scan mode signal is at the second level.