Designing with a Microcontroller (v6)

Transcription

1 Designing with a Microcontroller (v6) Safety: In this lab, voltages are less than 15 volts and this is not normally dangerous to humans. However, you should assemble or modify a circuit when power is disconnected and don t touch a live circuit if you have a cut or break in the skin. Objective: This lab provides an opportunity to learn more about the use of a microcontroller (MCU) and its interfacing to other external devices. The lab has two parts. Part I of lab uses the PIC16F886 microcontroller to read an analog voltage across a potentiometer and display the measured voltage on two 7- segment displays. Assembly language will be used to program the 16F886 MCU. Part II builds on the first part and the student is to design a microcontroller solution to solve one of several tasks. Preparation: You should already be familiar with the architecture and instruction set of the 16F886. Study the 16F886 data sheet and refer to EE331 course notes for more information. To facilitate the lab experiment, sample codes to initialize the analog to digital converter (ADC), 7-segment display, and PWM module have been posted on the course website. Study the connection diagram given in Fig 2. You should determine the missing wiring box in advance by reviewing the segment drive code in the sample program and the 16F886 pin functions. Procedure (Part 1) - Analog to Digital Conversion Obtain from the technicians one 16F886 MCU and a four-digit 7-segment display module. Note carefully the connector pin-out of the display module and also the segment drive code in the sample program. Obtain the other components shown in Fig. 2 and connect the circuit. Follow the pin diagram shown in the PIC16F886 data sheet (pp. 3). For now, you may omit the dotted portion of the circuit marked Part 2. Fig. 1 Pin Functions of the PIC16F886 microcontroller Revised Feb. 01, 2012

2 This experiment uses assembly language which must be converted to machine code using an assembler. Using the ECE laboratory computers, launch MPLAB IDE under: All Programs/Electrical Engineering/Small Device/MPLAB IDE. When the development environment opens, select Project Wizard from the Project pull-down menu and create a new project by selecting the PIC16F886 device. Select the Microchip MPASM toolsuite. Using the sample code (provided on the course website), write an assembly program to repeatedly sample the voltage across the pot connected to port RA0. Note that display latches are connected to port B. Using the Latch Enable ( LE ) pin, your program can update the display digits one at a time. Fig. 2 Connection to 7-Segment Display (shows two latch enables). The PIC16F886 has a powerful 8-channel 10-bit ADC (Fig. 3) that is controlled by two special function registers (SFR): ADCON0 and ADCON1. The result of the conversion is placed into ADRESH and ADRESL. Other special function registers that have an important impact are a TRISA and TRISC (used to configure the data flow) and PIR1 and PIE1 which contain the ADC INT (interrupt) flag and INT enable bits. A procedure must be followed to properly complete one conversion. After the A/D module (see Fig. 3) has been configured, from the user s perspective digitizing a selected analog channel is relatively straightforward. Refer to the 16F886 data sheet (pp. 103) for the complete conversion procedure. Note that, we will be using 8-bit resolution. As a result, we need to read only the higher 8-bits from the ADRESH and ADRESL registers. [Setting the format as left justified would certainly help since we are required to read from ADRESH only (pp. 102).]. Vary the pot and observe the displayed output. Verify correct operation by creating a map of display output versus input voltage. Answer the following questions in your notebook: 1. What is the maximum error in this process? 2. What is the shortest possible complete measurement time using this setup? 3. What is the maximum sampling rate using one ADC channel? 2

3 Debugging Suggestions To check your circuit of Figure 2, borrow a pre-programmed 16F886 chip from the technical staff and vary the potentiometer voltage. Use the divide and conquer approach to software development. For example, initially just write the ADC voltage to Port B and check pin voltages as you vary the input. Figure 3: The PIC16F87X 10-bit 8-channel A/D module [2] Procedure (Part 2) Student s Own Design The second part of this experiment allows for more advanced design using a microcontroller and the results of Part I. Students should select one of the following tasks develop their own procedure for completion and verification. 1. Dimmer - Use the internal pulse width modulator (PWM) to control the light intensity of a LED (i.e. simulating a dimmer action). The dimmer control voltage should come from the potentiometer used in Part 1. The 7-segment display should be used in the same manner to display the control voltage. The output pulse waveform and duty cycle are to be displayed on an oscilloscope. Hints: Connect the remaining components of Fig. 2 (marked as Part 2). Extend the assembly program (written in part 1) to use the internal PWM. See PWM information given in the Appendix. 3

4 Once the program is compiled and downloaded into the MPU, vary the pot and observe the dimming action. Take measurements to verify correct system operation and the accuracy of your design. 2. Warning Lamp This task is to produce a visible warning system for tank pressure. You are given a voltage developed by a pressure transducer (for this lab, use the potentiometer of Part I) and you are to flash a lamp at 0.5 Hz for normal operation, 1 Hz for mild overpressure, 2 Hz for danger and 4 Hz for extreme danger. The duty cycle of the lamp should be 25% in each case. The nominal pressure transducer voltages for each of these cases are 0.5 V, 1.5 V, 2.5 V and 3.5 V. You should select appropriate voltage thresholds for your four cases. Develop a procedure to verify the accuracy. 3. Remote Control The task is to remotely control the intensity of a lamp in four steps. Hint: You might use PWM or you might control current using two or more output pins. The remote control might be one of the following: USB keyboard, TV remote control, flashlight (laser pointer not recommended), sound control, or any other suitable device. Develop a procedure to assess the range and reliability of your design. 4. Frequency Counter The task is to measure the frequency of a 0-5 V square wave input signal in the frequency range 100 Hz to 2000 Hz and use the two digit 7-segment display to indicate the frequency. The only requirement on the frequency display is that it be monotonic with input voltage. Develop a procedure to verify system operation and that the measurement is monotonic. 5. UPC Bar Code Reader This task is to read the first digit of the manufacturer code and display on one 7-segment readout used in Part I. Due to the precision of available photosensitive devices, test bar codes will be enlarged greatly. Some UPC bar code information can be found here and here 6. Stepper Motor Control This task is to operate a stepper motor using a microcontroller. Three (salvaged) 6-wire motors are available for use. 7. Student s Own Task Students are encouraged to propose their own experiment. Experiments with ARM or Nios-II processors may be considered. References 1. Course website: EE331: ( 2. The Quintessential PIC Microcontroller, Sid Katzen, 2nd edition, 2005, ISBN: (available for download from UofS library), Chapter 13 & PIC16F886 Data: 4. Helpful resources: 4

5 Appendix Within the PIC16F886, there are two CCP (Capture/Compare/PWM) modules. Each CCP module contains: A 16-bit Capture register A 16-bit Compare register A PWM Master/Slave Duty Cycle register The ECCP (CCP1) module has an associated control register (CCP1CON) to set the mode. When configured as PWM (see Fig. 4), (TMR2 + Prescale) is used to implement period (10-bits), while (CCPR1L + CCP1CON[5:4]) is used to set duty cycle (10-bits). The CCP1 pin (#13) is the PWM output. The equations are: Period = 4! t! PS ratio! ( PR2 + 1) OSC Duty = t! PS!{ extended CCPR1 H} OSC For an 8-bit resolution, load PR2 with h FF (the maximum value). CCPR1L is loaded with the converted digital voltage (read from ADRESH/L). Refer to the data sheet (pp. 128, [3]) for a detailed description of the operation. Figure 4: Timer 2 and the PWM CCP mode [2] 5