# ANSFA - 1 5 H igh 102 U ppe r qua rte r 80 M e dia n 77...

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ECE 4175A Final 5-3-10 1 of 3 High 102 TEN PROBLEMS Upper quarter 80 Open book, open notes Median 77 Lower quarter 72 Low 42 1. the text. If the supply voltage for this board were changed to 5V, what would the requirements on the [ 5%] liquid crystal media be? The LCD controller chip derives voltages of 2/3VDD and 1/3VDD from its VDD voltage, so the waveforms would just be scaled up by a factor of 5/3, the ratio of the desired supply voltage of 5V to the 3V supply voltage of Chapter 16. The RMS values would likewise be scaled up by 5/3. a. The liquid crystal media would have to be turned off for an RMS voltage of less than 5/3 x 1.00 = 1.67V b. The liquid crystal media would have to be turned on for an RMS voltage of more than 5/3 x 1.73 =2.88V 2. Consider Brian Granaghan’s DevKP_BG.c file discussed in class. In his ScanKeys function, he first determines which column contains a depressed key. If he had coded a depressed key in the left column with [ 5%] COL=0 and a depressed key in the right column with COL=2, then which element of the KeyArray[ ] table (defined near the bottom of the previous page of the file’s listing) would be selected when the # key is depressed? KeyArray[2] 3. Dave Wyant, who used the 4.096V voltage reference, found that even when the ADCON1 register had been initialized to make the AN3/RA3/VREF+ pin serve as an analog input, it was also necessary to make [15%] TRISAbits.TRISA3=1 so that the digital circuitry for this pin was also set to be an input. The implication of this is that even though the pin’s internal circuitry leads to the ADC circuit, it also has a tristate buffer that is enabled when TRISA3=0 which will drive the pin low. The voltage reference datasheet specifies it has a typical short-circuit current of 110 mA to ground. This means that the voltage reference part has a Thevenin equivalent circuit consisting

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ANSFA - 1 5 H igh 102 U ppe r qua rte r 80 M e dia n 77...

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