| 摘要 |
PURPOSE:To protect the secrecy of an internal ROM and to improve its function by inhibiting the internal ROM from being accessed externally by writing to a program element and making only a single-chip operation mode effective. CONSTITUTION:The mode of a test 2 is selected through external terminals P1 and P2, a system address supplied to an EPROM is specified, and logic 1 is supplied from a data bus and written in the EPROM. Then, NOR gate circuits G1- G3 output 0, but a gate G1 has an inverter IV1 provided at the output side and outputs are inverted into 1, so that a microcomputer MCU is placed in single- chip mode forcibly. Therefore, the internal ROM is inhibited from being accessed externally and its written software is protected. Further, a user when constituting a system in extended mode only does not select the writing of logic 1 to the EPROM. |
