| 摘要 |
<p>PURPOSE:To enable the L-level input of terminals fixed at H-level by effecting a source supply to terminals of a microcomputer through a pull-up resistance. CONSTITUTION:When a terminal 1 is connected to a Vpp terminal of an EPROM or is in an open state, a potential applied by a power source 2 is supplied to the terminal 1 as a Vpp power source for the Vpp by a transistor 5 for pull-up being in an ON-state. At the same time, this potential is inputted in an input buffer as 'H' level and is supplied to an operation switching circuit inside a microcomputer. At this time, a piggy-back microcomputer works in a first state. Meanwhile, when 'L' is inputted in the terminal 1, a potential of the terminal 1 becomes at 'L' level. Accordingly, 'L' is inputted in an input buffer 6 and 'L' input is further supplied to the operation switching circuit from the input buffer 6 so as to actuate the piggy microcomputer in a second state. Thus, the switching of an operation mode becomes possible.</p> |
