Interfacing PIC Microcontrollers 31. 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. | Answers to Assessment Questions 12 Assessment 8 1 Emitter arrow-head shows direction of current NPN out from base PNP in. Vbe V. Typical current gain 100. 2 It is a voltage controlled current source with high input impedance. Zero and 5 V applied at the gate will switch it off and on. 3 Relay contacts have a low on resistance and high off resistance but the operating coil consumes significant power. 4 The DC motor needs a commutator to reverse the armature current on each half revolution so that the torque is developed in one direction only. 5 The thyristor switches direct current only while the triac switches alternating current. 6 The software option can be implemented by the MCU toggling an output with a delay. Alternatively a separate hardware oscillator based on the 555 timer chip can be switched on an off by the MCU. 7 Pulse Width Modulation uses a pulse waveform to control a current switch connected to the load. If the ON time increases as a percentage of the overall period the average current in the load and hence the power dissipated increases. 8 Bridge driver PSU 286 Answers to Assessment Questions The switches in the bridge FETs are turned on in pairs to allow the current to flow in either direction in the motor. 9 The stepper motor has four sets of coils which are activated in pairs to create a rotating magnetic field which operates the rotor. 10 360 15 24 steps rev Speed 100 steps sec 100 24 revs s 11 200 slots 100 ms 2000 slots s 2000 50 40 revs s 40X60 2400 rpm. 12 The DC motor drive is simpler in construction more efficient and higher speeds and torque are possible but it needs a feedback system for position control and a gearbox for low speeds. The stepper can positioned without feedback and holds its position but is less inefficient and is complex to drive. Assessment 9 1 No separate clock is sent with the data signal. 2 To increase the signal to noise ratio and the distance sent by increasing the signal amplitude. 3 10 8 data bits start
