A system for scan testing a device under test ("DUT") in which the clock
speed of the DUT differs from test equipment. A plurality of scan-flops in
the DUT form a scan-wheel, which defines a closed scan path. The Data bits
in the scan path are shifted through a scan-wheel controller based on the
DUT clock speed, so that a different bit passes through the scan-wheel
controller on each clock cycle of the DUT. Test data is only loaded by
replacing the data bit as it passes through the controller. The different
clock rates of the DUT and the test equipment define a scan wheel ratio,
which is used to determine the number of times that the scan-wheel must
rotate before all old data bits are unloaded and replaced by newly loaded
bits, and which also determines when data bits passing through the
controller will be unloaded and replaced.
