EE345M Final Exam Spring 2008 Page 1 of 8 Jonathan W Valvano First Name Last Name May 10 2008 9am 12n Open book open notes calculator no laptops phones devices with screens larger than a TI 89 calculator devices with wireless communication Please don t turn in any extra sheets 5 Question 1 If the CAN channel is noisy it is possible that some bits will be transmitted in error Assume there are four nodes one is transmitting and three are receiving What happens if a data bit is flipped in the channel due to noise being added into the channel 5 Question 2 Assume an analog input PAD0 is being sampled in the background Data is piped through a FIFO to a foreground thread and there is a real time OS with blocking semaphores We define time jitter t as the difference between when a periodic task is supposed to be run and when it is actually run The goal of a real time DAS is to start the ADC at a periodic rate t 1 fs Let tn be the nth time the ADC is started In particular the goal to make tn tn 1 t The jitter is defined as the constant t such that for all n t t tn tn 1 t t Consider the situation where the time jitter is unacceptably large Which modification to the system will cause the largest improvement in time jitter Just circle your selection A Run the ADC in continuous mode B Convert from blocking semaphores to spinlock to semaphores C Change from round robin to priority thread scheduling D Set the ETRIG bit in ATDCTL2 and clear the ETRIGLE ETRIGP bits specifying a falling edge hardware triggered ADC Use the PWM mode on PT3 of to create a squarewave with a period of t and connect PT3 to PAD7 E Increase the time you run with interrupts disabled EE345M Final Exam Spring 2008 15 Question 3 Consider the following motor interface A series wound DC motor like the ones in the robot kit can be modeled by a series combination of a resistor the ohmic contribution of the long thin wires used to wrap the electromagnets an inductor physically resulting from the cylindrical manner in which the electromagnets are wound and an emf a voltage caused by the load torque applied from the shaft back into the electromagnet The resistance is what you get when you measure the resistance of the motor with an ohmmeter a stopped motor disconnected from power In this case R 50 and L 100 H Hint L R is 2 sec Page 2 of 8 8 4V 1N914 R Motor L 9S12 Port Rb IB VOH IC VCE emf VBE Part a Under maximum load the emf becomes 40V and about 1A flows through the motor What transistor do you choose for this interface specify part number Why Part b What value do you choose for Rb Show your work using the symbols from the picture Part c Initially the motor is off no current to the motor At time t 0 the digital port goes from 0 to 5 and transistor turns on Assume for this section the emf is zero motor has no external torque applied to the shaft and the transistor turns on instantaneously derive an equation for the motor Ic current as a function of time EE345M Final Exam Spring 2008 Page 3 of 8 20 Question 4 Consider a robot powered by two DC motors on the rear wheels but the wheels are not perfectly matched One wheel has more friction than the other Experimental analysis shows the left wheel is slower than the right The goal of this design is to go as fast as possible in a straight line There are optical sensors on both wheels There are an equal number of stripes on each wheel Let NumLeft be the number of stripes counted on the left wheel Let NumRight be the number of stripes counted on the right wheel The approach will be to operate the left wheel at full speed and adjust the power to the right wheel so that the difference error NumRight NumLeft is driven to zero If error is positive right wheel is ahead of left wheel then reduce power to the right wheel If error is negative right is behind of left then increase power to the right wheel cog Passive Wheels Optical sensors Powered Wheels Passive front wheel Powered rear wheel Part a Assume the left wheel optical sensor is connected to PT0 and the right wheel optical sensor is connected to PT1 The hardware is given Write an initialization routine and two input capture ISRs that measure NumRight and NumLeft in the background EE345M Final Exam Spring 2008 Page 4 of 8 Part b Assume the left wheel motor is interfaced to PT2 and the right wheel motor is interfaced to PT3 Essentially there are two copies of the interface circuit shown in Question 3 Again the hardware is given You will set PT2 high and use 8 bit PWM on PT3 to adjust power to the right motor Write an initialization routine that sets PT2 1 and initializes a 100 Hz 8 bit PWM on PT3 Write a separate function that can be called to set the duty cycle of PT3 0 to 255 Assume a 4 MHz E clock PT3 does not have to be exactly 100 Hz just approximately 100 Hz Part c Let U be the 8 bit duty cycle to the right motor 0 to 255 Assume the time constant of the motor is 500 ms Develop equations using integer math you could use to calculate U from the measured error How often would you execute this equation If you need some controller constants define them as K1 K2 K3 etc specifying the sign of the constant without specifying the actual value Explain your rationale EE345M Final Exam Spring 2008 Page 5 of 8 15 Question 5 Consider the following implementation of a spinlock semaphore used with a round robin preemptive scheduler void OS Wait char semaPt asm sei Test and set is atomic while semaPt 0 disabled asm cli disabled asm nop enabled asm sei enabled semaPt disabled asm cli disabled enabled void OS Signal char semaPt semaPt Part a The compiler generated this code for OS Signal 0000 b745 1 TFR D X 0002 6200 3 INC 0 X 0004 3d 5 RTS Are there any critical sections in OS Signal I e can two threads both call OS Signal Justify your answer Part b Design a new op code for the 9S12 and use it to rewrite OS Wait so the spin lock semaphore can be executed without critical sections and without disabling interrupts Be VERY specific about what the new instruction does e g the INC 0 X instruction reads the 8 bit memory value pointed to by Register X adds one to the value stores the result value 1 back into memory sets the Z bit is the result is 0 sets the N bit if the result is negative and sets the V bit on a 127 …