CSC270  Lab #12

Experiment #1: Programming the Kit's display.

As we saw in class, the circuits that drive the 7-segment displays are 74LS259 circuits, and contain 8 D-flipflops. Each one has an address associated with it:

           Leftmost display digit:  0C16Fh
                    second digit:   0C15Fh
                    third digit:    0C14Fh
                    fourth digit:   0C13Fh
                    fifth digit:    0C12Fh
           rightmost display digit: 0C11Fh

Note that we do not know for sure which LED these addresses correspond to, yet...

Checking the segments

As we saw in class, there should be a jumper, or two posts on the mother board of the kit allowing us to clear the flipflops all at once. This will/should turn all the LEDs ON at once.

Look for two posts labeled ST₁ and ST₂ on the board, and connect them temporarily with a wire. Do you observe all the segments turning on?

First programming task: Blinking LED

Your next task is to write a small program that will turn one of the LED segments of the first LED block ON and OFF. We will start with Address 0C16Fh.

The idea is to store a 1 (01h) at that address, wait about 1/10th of a second, then store 0 (00h), wait 1/10th of a second and repeat. The result is an LED blinking at a rate of 1/0.1s = 10 Hz.

Which LED responds to Address 0C16Fh? _______________

Modify your program and use Address 0C16Eh instead of 0C16Fh. Do not modify anything else. What LED does the program activate now?

Continue on decrementing the addresses and figure out the assignment of addresses to LEDs:

            Address of LED a:  ___________
            Address of LED b:  ___________
            Address of LED c:  ___________
            Address of LED d:  ___________
            Address of LED e:  ___________
            Address of LED f:  ___________
            Address of LED g:  ___________
            Address of LED DP: ___________

Next programming task: Displaying 3 on the leftmost digit

Your next task is to write a program that starts with an 8-bit binary pattern in Acca representing the display of the decimal 3 on the leftmost 7-segment display. The program then stores this pattern, one bit at a time, on all 8 segments (Hints: look up the rotate and shift instructions).

Be efficient and use the Index register to hold the address of the LEDs!

Also, use a procedure to display the contents of Acca on a 7-segment digit. Here is the skeleton and specification of such procedure:

            ;------------------------------------------
            ;OUTPUT: takes the pattern stored in Acca
            ;        and stores in on the 7 segment
            ;        display whose address is passed in
            ;        the X register.
            ;------------------------------------------
            Output:   ...
                      ...
                      ...
                      RTS
     

With this procedure, displaying a character should be a fairly simple task. Look at the following example (which might or might not work with your procedure, depending on how you coded it):

                      LDAA     # 5Bh       ;pattern for digit '5'
                      LDX      # 0C158h    ;address of second digit
                      JSR      Output      ;output pattern on digit

Name that task: Displaying your name on the digits

Now that you have your output procedure working, write a program that uses it to output your name on the display. You may shorten your name if you wish!

Experiment #2: A 2-bit output port

Figure 1: The main players.

Figure 1 shows the main components of the two-bit output port setup. The 6800, a decoder, two bidirectional buffers controlled by RE-bar, and two D-flipflops (the sn54/74LS74), the outputs of which are connected to two LEDs.

Your job is to figure out how to wire them up so that the processor can store two bits in the port, and activate the LEDs according to the values of the bits.

As with the RAM lab, you have to make two types of connections: static and dynamic. The static connections correspond to the data path: How do you get the bits from the processor to the flipflop?

The dynamic connections correspond to timing-specific issues: How can you be sure that the data comming from the processor will be latched in the flipflop at the right time, and only when the processor is outputing these two bits, and not at other times?

Let's start with the static connections first:

Static connections: Data Path

Two data bits, originally in the accumulator must at some point end up in the two flipflops. How can you make this happen?

Go ahead and add lines to Figure 1 to create a data path from the processor to the flipflops.

Static connections: Address Selection

The next action item is to figure out an address (or several) that is not assigned to any of the other devices on the kit, and to reserve it for your 2-bit port.

The table below shows the assignment of addresses used by the Kit. The only addresses that are available to you are the ones marked as Available.

Address (Binary)	Address (Hex)	Device
1111 1111 1111 1111 1111 1100 0000 0000	FFFF FC00	ROM
1111 1011 1111 1111 1100 0010 0000 0000	FBFF C200	Available
1100 0001 1111 1111 1100 0001 0111 0000	C1FF C170	Not usable
1100 0001 0110 1111 1100 0001 0001 0000	C16F C110	Display
1100 0001 0000 1111 1100 0000 0000 1111	C10F C00F	Not usable
1100 0000 0000 1110 1100 0000 0000 0011	C00E C003	Keyboard
1100 0000 0000 0010 1100 0000 0000 0000	C002 C000	Not usable
1011 1111 1111 1111 0000 0010 0000 0000	BFFF 0200	Available
0000 0001 1111 1111 0000 0000 0000 0000	01FF 0000	RAM

Table 1: Memory map of 6800 Kit.



Figure 2: The 7442 used as a 3-to-8 decoder.

You will use the 7442 as a 3-to-8 decoder to help you select a free address range. The 7442 is shown in Figure 2.

Looking at Table 1, select an address that is available, and wire up the 7442 to activate one of its outputs when the address is present on the address bus.

Can you use the same address you used for the RAM during last week's lab? Why?

Dynamic Connection: latching the 2 bits in the flipflops

Complete the timing diagram shown below and show how the different signals behave during a processor cycle.

Of interest is a cycle when the 6800 outputs the 2 data bits to the port, and at the same time outputs on the address bus the address you selected in the previous section.

Yi represents the output of the decoder that is energized by the address of the fliplfops. Dm and Dn are the two data bits from the data bus that are connected to the flipflops. Clock is the clock signal fed to the two flipflops. For this signal, indicate when a low-to-high transition would be appropriate for it to capture the two data bits. In other words, indicate the range of time during which it would be okay for it to go from low to hight.

Figure 3: Timing diagram of the main signals

Once the timing diagram is complete, figure out how to generate the clock signal of the two flipflops.

Testing your circuit

To test your circuit, write a program that shows all combinations of two bits on the LEDs. Your program should be an endless loop that stores 00, 01, 10, 11, 00 and so on in the flipflops (which now act as a 2-bit I/O controller, or 2-bit port).

Write your program so that the LED patterns remain visible for about 2 tenths of a second before a new pattern is stored in the 2-bit port.