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Interfacing PIC Microcontrollers 19

Interfacing PIC Microcontrollers 19. 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 A specific type of op-amp is normally used for this type of application which has an open collector output. The output transistor switching circuit has to be completed by an external pull-up load resistor. This allows the output switching voltage to be different from the comparator supply voltage which is useful for interfacing circuits operating at say 15 V or with a 24 V single supply which must be connected to a TTL MCU input or output. The switching speed can be increased by using a lower value pull-up resistor at the cost of higher power consumption. Some MCUs have comparator inputs built in as a simple form of analogue input. Three types of comparator circuit are shown in Figure 7.13. The default chip type used here is the TLC339 a quad comparator. Simple Comparator The comparator detects whether the input is above or below the reference voltage. The circuit shown Figure 7.13 a has a reference voltage of 2.5 V applied to the - terminal. As the input changes the output switches at this voltage. The transfer characteristic shows the effect by plotting the output against the input voltage. The reference voltage can be changed as required giving a different switching level. The output of the comparator is connected to an LED indicator in the load circuit which is useful but not essential. The open collector output provides sufficient output current to drive an LED 10 mA without any additional driver stage. Trigger Comparator The output voltage in this circuit Figure 7.13 b is fed back to the terminal to set the reference level which changes depending on whether the output is high or low. The switching level therefore depends on the previous setting of the output. This gives two switching levels the output switches at a higher voltage when increasing from low to high and at a lower voltage when decreasing from high to low. In the circuit shown the LED circuit affects the switching level and may be omitted. Notice that the input is