EXPERIMENT #2 Data Storage

2.1
ECE 385
EXPERIMENT #2
Data Storage
I. OBJECTIVE
In this experiment, you will design and construct a simple 2-bit, four-word shiftregister storage unit.
II. INTRODUCTION
Conceptually, random access memory (RAM) is a storage device arranged as a set
of binary words that can be individually identified and accessed using unique addresses
(see Figure 1).
STORAGE
address contents
SAR
101 word 0 0110
word 1 1100
SBR word 2 0000
1110 word 3 0000
word 4 0000
FETCH word 5 1110
word 6 1101
STORE word 7 0000
Figure 1: An Eight-word Storage Unit Using 4-Bit Words
2.2
To fetch a word from storage, the unique word address is placed in the Storage
Address Register (SAR) and a FETCH signal is sent. The binary string or a content of the
specified word appears in the Storage Buffer Register (SBR) a short time later (exactly
how much later depends upon the technology used for the storage).
To store a word into storage the unique word address is placed in the SAR, the
binary data to be stored is placed in the SBR, and a STORE signal is sent. The binary
data in the SBR is stored in the word whose address is specified in the SAR. The
previous contents of the word are destroyed by the STORE operation.
Cathode ray tubes, delay lines, and magnetic cores were once used for storage. In
the 1970s, this was replaced by semiconductor RAMs, which are common now. You
should be able to construct a storage unit from parallel-in/parallel-out shift registers,
multiplexers, counters, and combinational logic.
One storage technique uses serial-in/serial-out (SISO) shift-registers shifting
synchronously. A single 1024-bit SISO shift register could be used to provide 1024
words of storage, where each word is a single bit (e.g. a 1024x1 RAM). Note that with a
1024-bit SISO shift register, only the output of the rightmost flip-flop and only the input
to the leftmost flip-flop are available. In theory, a shift register can be built at much lower
cost compared to a RAM. This is because there are far fewer pins and interconnections in
shift register than in RAM. In addition, the storage cell of a shift register could be very
simple, a capacitor for example. The shift operation is simply moving charge from one
capacitor to the neighboring capacitor. Such shift registers are called Charge Coupled
Devices (CCDs). Today, CCDs are primarily used in imaging applications, such as in
digital cameras. The charge in a cell slowly decays and therefore must be refreshed
before it is lost. For this reason a SISO memory based on CCDs must be continuously
shifted to keep the information from being lost.
Words larger than a single bit can be constructed by using more 1024-bit shiftregisters clocked synchronously. Typically, 16 such SISO shift registers would be used to
construct 1024 words of storage, where each word is 16 bits long. More generally, an
n-bit, m-word shift-register storage consists of n m-bit shift-registers shifting together
(see Figure 2).
2.3
•
•
•
storage for
m n-bit words
m words
n bits
Figure 2: Configuration of a Shift-register Storage
As mentioned earlier, an alternative to the above storage devices are those devices
that are built with "static" logic elements (SRAM). This is a setup where the storage
device can retain data so long as a specified supply voltage is maintained. These SRAM
chips are readily available from a number of manufacturers with varying features and
parameters.
III. PRE-LAB
A. Design, document, and build a 2-bit four-word shift-register storage unit using
two 74LS194 shift-registers without using the parallel load or parallel output
capability. Of the 74LS194 data (non-control) inputs and outputs, you may use
only the serial input and the rightmost (Qd) output. For the purposes of this
experiment, imagine that the maximum and minimum clock period is specified as
1 millisecond for the 74LS194. The registers must be shifted on each clock pulse.
The clock must run continuously – do not gate the clock (this is bad practice in
digital design, why?) You may use clock enable or inhibit pins on the chips.
Signal Definitions:
LDSBR When LDSBR is high, the SBR is loaded with the data
word DIN1, DIN0.
2.4
FETCH When FETCH is high, the value in the data word
specified by the SAR is read into the SBR.
STORE When STORE is high, the value in the SBR is stored into
the word specified by the SAR.
SBR1, SBR0 The data word in the SBR; either the most recently
fetched data word or a data word loaded from switches
(note that when none of the LDSBR/FETCH/STORE
switches is set, SBR should maintain the data in it)
SAR1, SAR0 The address, in the SAR, of a word in the storage
DIN1, DIN0 Data word to be loaded into SBR for storing into storage
Use flip-flops for the SBR. DIN1, DIN0, SAR1, and SAR0 should be obtained
from switches. FETCH, STORE and LDSBR should also be obtained from
switches. Display SBR1 and SBR0 on LEDs. You may also wish to include the
display of other signals in your design for convenience when debugging your
circuit. You can assume that only one of the FETCH/STORE/LDSBR switches
will be set at any given time. Do not combine the FETCH/STORE switches!
To design the shift-register storage unit, we first need to look at the required
specs. The most crucial requirement is for the shift registers to shift continuously,
while using the serial input and output to store and fetch the data. We can break
down our circuit operation into four operations: load, read, write, and do nothing.
Let’s first imagine the scenario where the circuit is turned on, but we are neither
loading, reading nor writing. This is the most common state of the circuit, where
no action is taken from the user – do nothing. Our requirements dictate that the
shift registers must continuously shift, where any potentially stored data will be
shifted out of the registers and into the void. To prevent losing any data, we will
need to redirect the data shifting out of the registers back by connecting the serial
output of the registers to their serial input, where the stored data will now be
looping continuously in the shift registers. However, during a write operation, we
do want to replace the old data with new data. To serve both purposes, a 2-to-1
multiplexer (MUX) can be placed at the serial input of the shift registers, taking
either the new data or the old data depending on the current operation.
The rest of the circuit operation hinges upon the SBR, which serves two purposes:
loading new data from DIN during a load operation and reading from the shift
2.5
register during a read operation. Note that the SBR must also behave as a register
(that is, be able to synchronously maintain its previous contents). There are two
ways to approach this. In the first, a simple D-flip flop may be used to implement
the SBR. The input of the SBR takes in these three different choices by using a 3-
to-1 MUX (or a 4-to-1 MUX with one input ignored), where one of the inputs is
used to loop back when the SBR needs to maintain previous data. Alternatively,
you may use a register chip which has a load enable to implement the SBR. This
allows you to use a 2-to-1 MUX here instead.
But what is the ‘current operation’ at any given moment? Surely the desired
operation is dictated by the user using the switches, but the inputs alone is not
sufficient to tell each part of the circuit what to do. For example, if you would like
to read from a specific address in the shift registers, you would first set the SAR
to the specific address then you would hit the FETCH switch. But since the shift
registers are constantly shifting data in and out of their serial ports, when exactly
do you load the data into the SBR? How do you exactly tell what input the MUX
should choose from? To solve the various problems associated with controlling
and timing, it is generally not a good idea to use the inputs to directly control the
various circuit components. Rather, it is almost always desired to have a
centralized control logic that takes in all the inputs, process the request, and sends
out various signals to control the circuit components. Figure 3 shows a general
block diagram for the proposed circuit design. The most common form of a
control logic is a state machine, which we will discuss in the next experiment. In
this experiment, we will improvise a simpler control logic based on the
requirements of our specific circuit.
First, notice that our shift registers are four word long, that is, each data will take
exactly four shifts/clock cycles to loop back to its original location. We can
exploit this property by employing a 2-bit counter (four distinct values) to keep
track of the internal data address, then use a comparator to match the internal
address with the SAR. Note that since the register is always shifting, it is
meaningless to indicate "absolute" storage addresses. Rather, all addresses are
"relative." If you wish to store data X in address Y, you can write the data into a
random cell Z when the internal data address from the 2-bit counter matches the
SAR. This (previously arbitrary) cell Z will now be associated with the address Y.
Later, when we wish to fetch from address Y, we wait for the internal counter to
2.6
match the SAR again - that is when cell Z once again becomes available for
reading or writing. Another interpretation that might be useful is that the counter
always keeps track of the address associated with data to be shifted out from
serial output/into serial input of the shift register array at the up-coming clock
edge. Note that to control the MUXs, the ‘select’ signals generated by the control
logic has to take into account of the input switches and the comparator output (to
indicate if we are currently looking at the correct address for reading/writing).
Figure 3: Block diagram of the shift-register storage unit.
Your pre-lab writeup should contain a written description of your circuit
operation, a block diagram, operation of the controller, a logic diagram and layout
documentation.
HINT: Use the Pulse Generator to provide a basic clock. Continuously clock
the shift-register and a counter that keeps track of which word is
currently available. Use combinational circuitry (or 74LS85) to check
for a match between the available word and the SAR.
B. Meet with your lab partner and wire up and test your design before coming to the
lab. Use either the mini-switchbox circuit that you built at the end of Lab 1 (detailed in
the General Guide) or attend an open lab session to test your circuit with the real
switchbox. Only the clock input needs to be de-bounced to strep through your circuit
(why?).
2.7
Demo Points Breakdown:
1.0 point: When LDSBR is high, the data in DIN is loaded from the switches into SBR
1.0 point: When STORE is high, the contents of the SBR are stored into the location
specified in SAR
3.0 points: When FETCH is high, the data word specified by the SAR is read into the
SBR
Note: you may get 1 point of partial credit from these 3 for demonstrating that your shift
register can shift right on each pulse of the clock. If you get the entire assignment
working, you do not need to demonstrate this independently (since you may need to
rewire a shift register to demo this).
IV. LAB
Finish testing and demonstrating your circuit to your TA. Correct the design if
necessary and document your changes for the lab report.
Follow the Lab 2 demo information when debugging is completed.
V. POST-LAB
1) Your post-lab writeup (notes) should contain a corrected version of your prelab writeup and an explanation of any remaining problems in the operation of the
circuits. This will aid the writing of your lab report.
2) Discuss with your lab partner and answer at least the following questions in
your lab report:
• What are the performance implications of your shift register
memory as compared to a standard SRAM of the same size?
• What are the implications of the different counters and shift
register chips, what was your reasoning in choosing the parts you
did?
2.8
VI. REPORT
In your lab report, should hand in the following:
• An introduction;
• Written description of the operation of circuits from the pre-lab;
• Block diagrams for part A;
• Design steps taken for all circuits. This includes but not limited to design
considerations on the SBR MUX, the Shift Register MUX, and the control logic.
Truth tables/K-maps leading to the final circuit design should be included (if
any);
• One (1) component layout sheet, with the package layout of all circuits (DO NOT
draw the interconnections! Refer to GG.20 for the proper documentation);
• Circuit diagrams for all circuits;
• Requested documentation from the lab;
• A conclusion regarding what worked and what didn’t, with explanations of any
possible causes and the potential remedies.
• See also, refer to the report checklist linked on the course website.