| 摘要 |
PURPOSE:To prevent the run-away of a custom microcomputer by fixing the special bit value of a counter in accordance with the capacity of a ROM and limiting an output value in a single chip microcomputer having an instruction ROM and a program counter to access the ROM. CONSTITUTION:The output value upper limit of a program counter in accordance with the built-in ROM capacity is executed without changing a CPU internal part. For this reason, a counter output control part 2 composed of NAND gates 31-3n and reverse phase three state buffers 41-4n is connected to a program counter 1. When output fixing signals 71-7n are '1' at the gates 31-3n, respective output signals 51-5n of the counter 1 are outputted as they are and at the time of '0', the output signals 51-5n are fixed to the '0'. When an output control signal 8 is '1', the buffers 41-4n output the output of the gates 31-3n to signal lines 61-6n and at the time of the '0', the output is made into a high impedance. |
