ECE Lab2 Solution

Purpose:

*Review C programming/compiling.

*Learn how to address ports/pins and set i/o configuration.

*Gain experience at prototyping

*Learn how to correctly connect uC to peripherals.

*Understand I/O pin drive and input levels.

*Using multiplexed displays.

*Debouncing push button switches.

*Use a software timing loop to create delays.

*Gain experience creating a user interface.

*Understand the limitations of using software timed loops




Project Specifications:

*Build a four digit multiplexed LED display with eight push button switches.

*Program uC to display (on LEDs) values created by the push button switches.

*Display will operate in base ten.

*The i/o board push buttons will be utilized as follows:

Button S0 increments the value displayed on the 4 digit display by one.

Button S1 increments by two, button S2 by four, etc. Button S7 increments

by 128. The display top count is 1023 and the next count in sequence would

be 1. Initially, the count indicates zero.




Before Lab:

*Draw a complete schematic diagram of how you are going to hook up the

display and switches to port A and B. This is not a sloppy crayon

drawing. This is a fully complete schematic with all part numbers, values

and pin numbers. If spot checks of your working schematic find that your

drawings are not up to standard, you may loose points.

*Port A will be used to drive the segments (by pulling them to ground) and also to

sense the state of the push button switches.

*Determine the value of the PORTA LED segment current limiting resistors such that:

-current through an LED segment will not exceed maximum ratings

-Mega128 total port/device current capacity is not exceeded

-display is easily readable in lab light

*Port B bits 4-6 will select the digit to display via a 74HC138 3-to-8 decoder.

Look up how the 74HC138 works by reading its datasheet.

*Port B bit 7 in will be used to implement a PWM dimming control in another lab.

It will be driven continuously low for now to saturate the 2N4403 it drives

and keep the LEDs at full brilliance. Determine the value of the base bias resistor

to allow this transistor to source the necessary current for all the digits.

*Some of the 74HC138 outputs drive the digit driver transistors. Determine the value

of the base bias resistor to allow these transistors to source the necessary current

for each digit.

*PORTB bits 0-3 are not to be used at this time. They are reserved for other control

and for SPI use.

*The Y7 decoder output is used to enable the tri-state buffer on the push button board.

The purpose of this is to allow the pushbuttons to pull the PORTA inputs low...but only

under control of the uP. Otherwise, the push buttons could blank some

segments.




Assembly notes:

*Think ahead and carefully consider what you are doing.

*Place and solder in the LED display on the -BACK SIDE<- of the display board. This is

the board side -WITHOUT<- the IC.

*Use the ribbon cables to make connections to PORTA and PORTB. Connector goes on one end

and split out the necessary wires to make the connections to the boards.

*The 2x5 connection block (A_2-G_2) are best connected to the mega128 board.

*The 1x8 connection block (A-G) is best connected to the push button board.

*Mark your cables so you don't connect them to the wrong place.

*Make sure unconnected wires don't short out to anything.

*Remember to connect power to the push button board.

*Use the power and ground available on the mega128 board and through the connectors to get

power to your boards.

*Carefully check all your connections for bad solder joints and bridged pads.




Software Notes:

*Write some test code to test the display and the push buttons. This will check your wiring

and avoid you spending too much time fiddling with software that is exercising broken hardware.

(recommendation only)

*Write out pseudocode or flowchart for lab2 code. Don't just start writing code.

(recommendation only)

*Write C code to perform desired function.

*Use of interrupts or timers is not allowed.




Testing:

*Test and debug code.

*Measure the actual current flowing through one segment. Try and account for any differences.

*Measure the duty cycle (on time versus off time) for each digit. Is it what you expected?




What to turn in (at beginning of your lab section)

*Professional quality schematic diagram you used to wire up your circuit. Hand drawn schematics

are entirely acceptable and even preferable.

*Sheet(s) showing calculations done for:

-segment current limiting resistors

-base resistors for digit driver transistors (Q1-Q5)

-base resistor for PWM transistor (Q6)

-show that Iol spec for mega128 is not exceeded by driving the segments

-show that neither Iol or Ioh spec for mega128 is not exceeded by the push buttons

*Printout of your code. Use a2ps to make easily readable.

*Sheet(s) with actual measurement of:

-segment current and its deviation from calculated values

-duty cycle for display digits

 

Grading:

*Circuit function +70% (demo system in lab)

-performs as expected,switches debouncing cleanly

-if count "jumps" -deduct 10%

-dim display/flickers -deduct 5%

-other abnormality -deduct 5%




*Documentation +30%

-complete, neat, correct

-code well written, commented

-sloppy documentation -deduct 10%

-poorly commented code -deduct 10%




Lab weighting = 0.15