EE 264 Homework Assignment 7 Due 4/7/17
1. PORTB of an HCS12 is wired up so that bits 1 and 2 are connected to outputs and bit 0 is connected to an input. The input is wired to a switch, and the input is 0 if the switch is pressed and 1 if the switch is released. The two outputs are LEDs, and are wired so that writing a 0 turns OFF the LED and writing a 1 turn ON the LED.The schematic is shown below.
a. What setting should DDRB have?
b. Draw a flowchart to do the following: Test the input bit. If the switch is pressed, turn on both LEDs. If the switch is released, turn off both LEDs.
c. Write a subroutine to implement the flowchart. PORTB is at memory address $0001.
2. The HCS12 has several interrupt sources, almost all of which can be controlled via the I flag in the condition-code register.
a. Is the I flag active-high or active-low? That is, which state of the I flag indicates that interrupts are enabled?
b. Which instruction makes the interrupt flag active (enables interrupts)?
3. Watch the Podcast about digital interfacing, and answer the following short questions.
a. An HCS12 uses 5 Volts and is driving an LED. When the LED is on, it has a forward voltage of 1.8 Volts and draws 18 mA. What size current-limit resistor is needed?
b. What part turns on and off inside a relay in order to operate the switch?
c. Name an example of a chip or device that has enough drive strength to operate a relay that requires 100 mA of coil current.
d. Name an example of a device that protects circuits from too much current.
