ECE 271                         LAB 7                        Spring 2012

PURPOSE:  More flowcharting/programming practice, table lookups, 
bit operations, using the 7-segment LED display.

EXERCISE: Write a stopwatch program. The count is displayed in the four 
7-segment LEDs. The program will count as long as PC0 is pushed and will 
stop counting when PC0 is released. Counting will occur every 1/10 second, 
so the leftmost three digits gives the number of seconds, and the rightmost 
digit gives tenths of a second. Pushing PC1 will reset the count.

PRELAB DUE WHEN YOU COME TO LAB: FLOWCHART AND ALL YOUR CODE (UNDEBUGGED
AT THIS POINT). We will check for this at the beginning of lab.

Your data area will include a double byte COUNT (the current time in 
1/10ths of a second) and perhaps several other counters. The display 
portion of the code will require a "Display Mask", a temporary double 
byte value (TempVal on flowchart), and a display table indicating which 
segments are lit for a given digit to be displayed.  

Your program will operate as follows: Clear the counter and initialize 
register DDRD (PORTD data direction register in memory location $1009) 
to %00111100, making the middle four bits of this port outputs. Then in
a loop do the following: 1) execute the "Display Value" code 5 times 
taking a total of 1/10 second (each execution of "Display Value" code 
takes 20 ms), 2) clear the count if PC1 is pushed, and 3) increment the 
count if PC0 is pushed.

Displaying is complicated by the fact that the LEDs can only display one
digit at any given time. The digits are therefore continuously scaned,
displaying each for a short time (we'll use 5 ms) and then moving on to
the next. PORTD (location $1008) controls which digit is currently 
displayed as follows
                                                             PORTB bits
  PORTD pattern    Displayed Digit                              --0--	
    00111000	    1 (rightmost)                              |     |
    00110100	    2 (second from right)                      5     1
    00101100	    3 (second from left)                       |--6--|
    00011100	    4 (leftmost)                               4     2
                                                               |     |
                                                                --3--

Write the value to PORTD and then write a bit pattern to PORTB (the LEDs
port you have been using) and the given segments of the specified digit
will light. Delay (e.g., for 5ms), then display the next digit. 
Continuously cycle through the digits and all four digits will appear as 
if they are all lit simultaneously.

NOTES:
The pattern you write to PORTB indicates which LED segments will be lit for
that digit. A data table for this is given on the web page.
 
The "Display Value" flowchart is given on the back of this sheet. You must 
provide a flowchart for the remainder of the program. Include "Display 
Value" as a simple block on your flowchart. Code should follow flowcharts 
exactly.  Program comments will reflect what is found in the flowchart.

Note that your stopwatch will only count while PC0 is pushed. Such operation 
is worth 9 points. Ideas for a full 10 points: 1) have the stopwatch change 
state each time PC0 is pushed. (First press starts, second press stops, etc.)
2) blank the leading zero digits of the display.