## EE 476: Homework 4

Due At the start of your 4th lab period
1. Read all of Lab 4. Write the source code necessary to impliment the program for Lab 4. You must set up the UART control registers as shown in the example code in Lab 4. To use Timer1 capture mode, you will also need to set bits in TCCR1B and ACSR, and in TIMSK if you use the capture interrupt.

2. In terms of R2, R3, R4 and C as shown in lab 4, determine the time at which the voltage at PortB2 becomes greater than the voltage at PortB3. Assume that PortB2 starts at zero voltage at the instant that the pin is converted from an output to an input, and call that instant time=0. Also assume that the loading from the comparator inputs is negligable.

3. Select values for R2, R3 and R4 which are "good" as explained below.
1. As mentioned in Lab 4, it is handy if the voltage at PortB3 is (Vcc)(1-e-1) so that PortB2 will reach an equal voltage in a time equal to one time constant. There is a constraint that ` 5k<(R4+R3)<100k `. The low end constraint avoids loading the output too much. The high end constraint avoids noise sensitivity.
2. Once you pick a clock prescaler value, R2 can be adjusted so that the actual number of clock ticks which accumulate until the voltage at PortB2 equals that at PortB3 is equal to 2n times the capacitance. Scaling the count becomes just shift-right commands.

Jan 1999 Copyright Cornell University 1999