| 摘要 |
<p>An apparatus and method for accessing hidden data in a boundary scan test interface is disclosed. The apparatus comprises a state detector, a shift register, a hidden register 37 and a comparison device. The invalid state transition loop, fig. 4, in the state transition diagram of the boundary scan test interface is used to transfer hidden signals without inputting signals through the standard input / output pins. An input, TMS, of the state transition diagram of the boundary scan test interface is initially monitored and an output of a first data is generated when a first predetermined input stream conforming to the invalid state transition loop is detected. An output of a second data is then generated when a second predetermined input stream is detected. The second predetermined input stream is different from the first one and also conforms to the invalid state transition loop. A combination of the first and second output data is stored in the shift register. An input key included in a combination of the first and second data is compared with a predetermined write key, and optionally a read key and a resume key. If the input key matches the write key specific write data is written to a hidden register. If the input key matches the read key a selector 38 is controlled to output the content of the hidden register. If the input key matches the resume key the selector is controlled to output the content of the ID code register 321. The boundary scan test interface may be a JTAG interface, IEEE 1149.1 interface or IEEE 1149.4 interface, and the state transition diagram may be in a test access port (TAP) controller, 31. The input to the state transition diagram may be a test mode select (TMS) input. The first predetermined input stream may be '0111' or a plurality of zeroes followed by '111'. The second predetermined input stream may be '1'. The first output data may be a '1' and the second output data may be a '0'. The odd bits of the combination of the first and second output data may be the input key and the even bits may be the specific write data. The access method is compatible with the boundary scan test interface and does not impact on state or data paths of the interface.</p> |
