US Patent 7805646 LSI internal signal observing circuit

An LSI internal signal observing circuit includes a pad; a monitor line connected to the pad and wired inside an LSI; a shield line wired adjacent to the monitor line and having a fixed potential; a buffer having an output enable terminal and connected to an internal node in the LSI; and a capacitor connected between an output of the buffer and the monitor line, wherein an output enable signal input to the enable terminal is controlled to set the buffer in an output enable state, and a change of a signal at the internal node is superposed on the monitor line through the capacitor and observed at the pad.