CSC 270    Lab #10

Experiment #1: Adding a 2114 RAM chip to the Kit.

The 2114 is a 1Kx4 RAM circuit. It contains 1024 nybbles. Its pin-out is shown in Figure 1 below.



Figure 1: 2114 pin-out.

Your goal is to attach one such circuit to the 6800 processor.

We will use just one, although two are required to implement a memory of bytes. But the wiring of the second RAM chip is identical to the wiring of the first one, except that it connects to four different bits of the data bus. You'll simply have to be careful which bits of the data bus you connect to, so that you'll know which nybble (high or low) the processor accesses.

Block Diagram

Figure 3 shows the block diagram that you have to complete before you can wire up the kit. Although not much is required to connect the 6800 to the 2114, this can be tricky, and we will take each step one at a time.

Decoding

The CE' signal controls access to the RAM chip (Chip Enable, or Chip Select), and it must be activated only when the processor is sending a valid address to the RAM circuit. You need to figure out how to select appropriate bits of the address bus to enable the RAM, knowing that the 64K address range of the HeathKit is divided into several fields corresponding to different devices, as shown below.

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

the 7442, a 3-to-8 decoder, is available to decode blocks of addresses.



Figure 2: the 7442 3-to-8 decoder. Note:The 7442 is in fact a 4-to-10 decoder without an enable input, but we can make it work as a 3-to-8 decoder by using the most-significan input bit as an enable.

For the decoder you need to determine the logic signal to be used to enable the decoder chip itself, and you also need to determine the inputs that will, in turn, give you an output from the decoder that is used for the RAM chip select signal.

To begin, add the decoder to Figure 3. Identify which signals from the 6800 should be used to energize the (active-low) enable input of the decoder. Wire up our enable input (i.e., the A3 decoder input).

Figure out what address bits to connect to the three inputs of the decoder, based upon your identification and selection of an output that energize an address range that doesn't map to anything already associated with memory or I/O devices (RAM, ROM, Keyboard, Display, Unavailable). Connect this signal to the Chip-Select signal (CS') of your RAM circuit.

Since the 2114 contains 1024 different memory words, you have to make sure that the output line you select for the RAM enable signal can be energized by 1024 different addresses. Furthermore, these 1024 addresses shouldn't overlap with anything already in use (RAM, ROM, I/O).

Verify that this is the case, and indicate why in your report.



Figure 3: Incomplete block diagram of 6800 to 2114 connection.
You have to add the decoder and the hardware controlling the tri-state buffers.

Data Bus Connection

Connecting to the data bus must be done via the tri-state buffers. Although the RAM has tri-state buffers internally, and we could have connected its data Input/Output pins directly to the data bus, the kit is designed in such a way that the blocks marked D0, D1, D2, ..., D7 into which you connect wires are connected to additional tri-state drivers and not to the Data Bus. The main reason for this is security. If your design is flawed, all you will destroy is the tri-state drivers, but not the processor!

Complete Figure 3 by connecting the data pins of the 2114 to the tri-state drivers on the kit, which are the white receptacles in the Data I/O block.

R/W' signal

The R/W' input signal of the 2114 [ (WE)' ] just happens to be of the same polarity as the R/W' signal of the processor. A 1 indicates a processor read operation, a 0 a processor write. Connect the two together on Figure 3.

Address lines

The 2114 contains 1024 words of memory, hence its 10 address input lines. Which address bits from the processor should you connect to the address inputs of the 2114? Why? With your connection established in Figure 3, what is the address of the first word of your RAM? What is the address of the last word of your RAM?

Energizing the Read Enable, R/E', signal

The signal RE' controls the tri-state drivers. Energizing it is the trickiest and most crucial element of the design. If it is not implemented correctly, RE' will activate the tri-state drivers at the wrong time and will force them to output information on the data bus at the same time the processor uses the bus for some other purpose. This conflict will make the processor read erroneous information, and the computer will very likely crash!

Moreover, a few seconds of this bad treatment would be long enough to make some circuits die.

To find the exact formula for activating RE', use the information in Table 2 to complete Table 3.

R/E'	Action
0	Lets information flow from Data Bus Block, through tri-state drivers, and onto Data Bus, so that 6800 can read information in.
1	Lets information on data bus go through tri-state drivers to Data Bus Block (where you can connect wires).

Table 2: How RE' works.

Enable' from decoder (to RAM)	R/W' from processor	Behavior of 2114	(RE)'
0	0
0	1
1	0
1	1

Table 3: Energizing RE'

Once you have found the truth table for RE', generate its boolean equation in terms of the enable' signal from the decoder (going to the RAM), and R/W'. Add the necessary logic gates to the circuit in Figure 3.

Before wiring this circuit, double check it with me again!

Wire-up the whole circuit.

Testing

How can you test that your memory is working properly? Run a test to show that you can actually store information in the RAM and retrieve it. Make sure you describe in your lab report how you ran the test. Are all 1024 nybbles accessible to the processor? What happens when you try to write at the address that is one higher than the last address of your RAM?