Minimum system (power supply circuit and I / O expansion and gating circuit)
design uses minimal system board is based on 80C52 microcontroller as the core, and has good scalability. CPU external crystal 11.0592MHz, mainly by 74LS373 latch circuit, 74LS138 decoder circuit and keys, display devices, ICL7135 its peripheral typical circuit, with 8255 foreign expansion of I / O interfaces. Minimum system circuit shown in Figure 2.
design uses minimal system board is based on 80C52 microcontroller as the core, and has good scalability. CPU external crystal 11.0592MHz, mainly by 74LS373 latch circuit, 74LS138 decoder circuit and keys, display devices, ICL7135 its peripheral typical circuit, with 8255 foreign expansion of I / O interfaces. Minimum system circuit shown in Figure 2.
2 minimum system schematic diagram
This circuit requires external transformer an AC220 / 9V, the secondary side of the transformer is rectified and filtered through CW7805 can get + 5V input voltage, this voltage power supply to do the smallest system.
System by 8255 to expand outside the PA, PB, PC total 24 I / O ports, so that as the system's input and output channels. 74LS138 output as decoded by the selection terminal of each chip, in addition to the minimum outer Y0 ~ Y3 used in the system, as well as Y4 ~ Y7 available to other extensions.
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