May 2005
Beginner to intermediate
398 pages
12h 12m
English
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Starting Simple
The PIC12C508 processor is a tiny 8-pin computer with just 512 words of internal program memory and just 25 bytes of internal RAM. It is intended for the simplest of control functions. It can be used in any small application for which you need to monitor digital inputs or turn something on or off. Its I/O pins can be used to synthesize a SPI or I2C interface, or to control a motor using PWM.
Figure 14-1 shows the schematic for a small computer based on the PIC12C508. The digital I/O signals of the PIC are brought out through a 7-pin connector. If the design were implemented using surface-mount components wherever possible, the connector would be the largest component on the PCB!
This particular PIC processor includes an internal RC oscillator that runs at 4 MHz, so we can use it without any external oscillator circuit. The design in Figure 14-2 shows the same PIC-based design, but this time using an external 32 kHz watch crystal for its oscillator. By running off a (slower) 32 kHz crystal, we have the advantage of greatly reducing the processor's power consumption. This is important for battery-powered applications.
Two 15 pF capacitors remove unwanted higher-order harmonics from the crystal's oscillation. The values for the capacitors vary depending on what speed and type of crystal you are using. The processor datasheet has tables showing recommended capacitor values for various crystal frequencies.
Figure 14-1. Minimal PIC12C805 computer
Figure 14-2. A basic PIC12C508 ...
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