Patent ID: 7447962

Claim:
A test system comprising: a test target system operating according to a system clock; a test bus implemented using part or all of an input/output bus which is originally designed to transfer input and output data to and from the test target system; a plurality of input circuits, each said input circuit including an associated input port and a first shift register operating according to a shift clock independent of the system clock to supply first data input from said associated input port to the test target system via the test bus; and a plurality of output circuits, each said output circuit including an associated output port and a second shift register operating according to the shift clock to supply second data, which has been supplied from the test target system to the test bus, to said associated output port, the plurality of first shift registers and the plurality of second shift registers being connected in series to form a scan oath chain, wherein, when the test target system is tested, initial state setting is performed by circulating initial setting data in the scan path chain, and, after the system clock is activated, data of the input circuits is loaded into the associated first shift registers or other data of the output circuits is loaded into the associated second shift registers and the shift clock is activated to extract the data of the input circuits or the other data of the output circuits from the scan path chain, wherein enable data is circulated in the scan path chain, and the other data of the output circuits is sent to the test bus only when the enable data is active, wherein the test bus is m bits wide, m is an integer larger than 1, and the enable data has m consecutive active bits and has no active bit prior to and subsequent to the m consecutive active bits, and wherein m-bit data of the output circuits having the active bits is output to the m-bit test bus by shifting the first and second shift registers according to the shift clock every m bits.