ECE 3724 Fall 2006 Test #4 – Jones / Reese (circle one to indicate your section) Net ID: (no names, please) You may use only the provided reference materials. You may use a calculator, either a four-function or a scientific calculator. You may not use a programmable calculator. The test is worth is 100 pts, you are given 1 pt for free. For any required I2C functionality, use subroutine calls i2c_start(), i2c_rstart(), i2c_stop, i2c_put(char byte), char i2c_get(char ackbit). If you use i2c_put, you must pass in as an argument the byte that is to be written to the I2C bus. If you use i2c_get, you must pass in an as argument the bit value to be sent back as the acknowledge bit value. You also have DelayUs() and DelayMs() functions available. Show all your work in any computations done or formulas used to receive full credit. Part I: (75 pts) a. (15 pts). The code fragment below performs I2C operations to a MAX517 DAC. Answer the questions in the box in the right (Hint: Read the datasheet!!!!) 1. What values are the A1, A0 lines on the MAX517 tied to (give as either VDD or GND for each)? 2. What do the instructions in operation 1 cause the DAC to do? If a voltage is output, give the voltage that appears on the DAC output assuming Vref = 5V. 3. What do the instructions in operation 2 cause the DAC to do? If a voltage is output, give the voltage that appears on the DAC output assuming Vref = 5V. // Operation 1 i2c_start(); i2c_put(0x5E); i2c_put(0x10); i2c_stop(); // Operation 2 i2c_start(); i2c_put(0x5E); i2c_put(0x06); i2c_put(0x5E); i2c_stop(); 1 of 7b. (15 pts) A gamepad controller containing two analog joysticks is connected to the PIC. Each joystick has an X axis (left/right) and Y axis (up/down). Internally, each axis of the joystick is a potentiometer connected between power (Vdd) and ground; joystick position up or left gives Vdd while down or right gives ground. The center position produces Vdd/2. The joystick axes (four total, two for each joystick) are connected to AN0 through AN3. Write the C function signed char whichDirection(unsigned char axis) which reads a particular joystick axis specified by the axis parameter (values of 0, 1, 2, 3 correspond to reading analog inputs AN0, AN1, AN2, AN3 respectively). The function returns UP_LEFT_DIR when the joystick is at least halfway between the center position and full up or left, DOWN_RIGHT_DIR when the joystick is at least halfway between the center position and full down or right, and CENTER_DIR otherwise. Assume Vdd=Vref+ is 5V, Vref– is 0V and that the ADC is already configured with left justification. Use only the upper 8 bits of the resulting A/D conversion. You must delay for 20 µs (use DelayUs) after selecting the A/D channel. Show all calculations, such as those used to convert voltages read from the joystick into UP_LEFT_DIR, CENTER_DIR, and DOWN_RIGHT_DIR, to receive full credit. PICJoystick axis 0 Joystick axis 3 AN3 VddVddAN0 #define UP_LEFT_DIR 1 #define CENTER_DIR 0 #define DOWN_RIGHT_DIR -1 signed whichDirection(unsigned char axis) { // your code here 2 of 7c. (15 pts) A second method for reading a joystick is to compare the voltages using analog methods. Assume the joysticks work in the way as described in problem b. Write the C function signed char whichDirectionAnalog(unsigned char axis) which returns UP_LEFT_DIR, DOWN_RIGHT_DIR, or CENTER_DIR, depending on the joystick position, as was specified for problem b. As before, the axis parameter (0,1, 2, or 3) is used to select a joystick axis; in this case use this parameter to control the RB2, RB1 digital outputs to steer a joystick axis through the analog mux to the V+ input of the comparator as shown in the figure below. Using the MAX 517 DAC, output an analog voltage V- to compare against the joystick’s current position (V+). The comparator output is ‘1’ if V+ > V– ; the comparator output is ‘0’ if V+ < V–. Assume that A0 and A1 of the MAX 517 DAC are both tied high. Assume that the PORTB pins RB0, RB1, and RB2 have already been correctly configured (RB0 as an input, RB1, RB2 as outputs). #define UP_LEFT_DIR 1 #define CENTER_DIR 0 #define DOWN_RIGHT_DIR -1 signed whichDirection(unsigned char axis) { // your code here 3 of 7d. (10 pts) Use the PWM module of the PIC18 to generate a square wave with a period of 15.2 µs and a high pulse width of 6.2 µs (the low pulse width is 9 µs). Show the calculations that you use to calculate the needed register values. Then write code for main() that configures the PWM for this operation. Your while(1){} loop should be empty. Use an Fosc value of 20 MHz. Assume the lower 2 bits of the 10-bit PWM pulse width value are already set to 0. Show all work. e. (10 pts) Write an ISR that responds to a CCPIF interrupt, which is being generated by the Timer1/CCP1 compare mode. On each CCPIF interrupt, increase the time until the occurrence of the next interrupt by 115.2 us. The main() code which configures the compare module is given below, you need to look at this to discover how Timer1 is configured. Assume Fosc = 25 MHz. Show all work necessary to determine the timer tick rate and CCPR1 update value. main() { T1CKPS1 = 1; T1CKPS0 = 1; RD16 = 1;TMR1CS = 0; TMR1H = 0; TMR1L = 0; CCP1M3 = 1; CCP1M2 = 0; CCP1M1 = 1; CCP1M0 = 0; T3CCP2 = 0; CCP1IF = 0; CCP1IE = 1; IPEN = 0; PEIE = 1; GIE = 1; TMR1ON = 1; while (1); } void interrupt myIsr() { // Your code here 4 of 7f. (10 pts) Write C code which reads two bytes starting at location 0x7A40 from the 24LC515 EEPROM, where the 24LC515 has A1 = 1 and A0 = 1. The 2nd byte read from EEPROM must be sent to one of two different MAX517 DACs connected to the I2C bus. If the first byte is zero, write the 2nd byte to a MAX 517 which has its A1,A0 pins as A0 = 0 and A1 = 1; if the first byte is not zero, write the 2nd byte to a MAX 517 where A0 = 1 and A1 = 0. Assume that the EEPROM is ready for a read operation, so that no polling is necessary. Part II: (24 pts) Answer 6 out of the next 8 questions. Cross out the 2 questions that you do not want graded. Each question is worth 4 pts. 1. When Fosc = 40 MHz and a 1:2 prescale is chosen for timer1, how much time occurs between TMR1IF interrupts? How can this time be increased? 2. What is the maximum possible PWM frequency? What is the minimum possible PWM frequency?
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