CLAMP

Putting a device into CLAMP mode forces the device to hold the last values written to it by JTAG (using EXTEST) whilst (like BYPASS mode) placing a 1-bit register in the boundary scan chain.

Using CLAMP mode is therefore useful because it allows the test system to drive a value and hold it, or to prevent a chip from booting up, whilst still allowing rapid access of other devices in the chain. It can also be used to prevent a device from responding to signals from other parts of a board under test.