tailieunhanh - Interfacing PIC Microcontrollers 21

Interfacing PIC Microcontrollers 21. Explore in detail microcontroller interfacing techniques using the popular PIC 16F877 Work through step-by-step examples interactively using circuit simulation software, supplied as assembly source code Gain the knowledge of a wide range of peripheral devices such as keyboards, displays, sensors and drives and serial communication with other processors, memory and more Use interactive simulation software to design and test circuits. | Interfacing PIC Microcontrollers Figure Power outputs interfaces since the PIC cannot provide enough current. When the coil is activated the contacts change over completing the load circuit which operates a lamp. Other high power loads such as heaters and motors can also be interfaced in this way as long as simple on-off but infrequent switching is needed. The transistor is selected for sufficient collector current handling and the base resistor to give plenty of base current which is calculated from the required coil current Coil current 40 mA collector current A Base current 40 mA 100 400 pA A Base resistor X 400 X 10 6 kQ 10 kQ The relay has a diode connected across the coil this is a sensible precaution for all DC inductive loads anything with a coil such as a motor or 186 Power Outputs solenoid . When the coil is switched off a large reverse voltage may be generated as the magnetic field collapses this is the way the spark is generated in a car ignition . The diode protects the transistor from the back EMF by forward conduction. In normal operation the diode is reverse biased and has no effect. The relay is selected for the load current and voltage requirements and the interface designed to provide the necessary coil operating current. However it is slow consumes a fairly large amount of power 40 mA X 5 V 200 mW and is relatively unreliable. The relay provides low on resistance and high off resistance. However it wastes a relatively large amount of power in the coil is slow and unreliable due to wear on the contacts. An alternative is the solid-state relay which is typically designed to switch AC loads from digital outputs with a solid-state device. It contains TTL buffering isolation and triac see below drive in one package with high reliability and switching speed. Triac Interface A relay can be used to control a DC or an AC load as it operates as a mechanical switch. However it has significant disadvantages as outlined above. A solid-state .