DIGITAL LOGIC FAMILIES:
REGIS TER TRANSFER LEVEL ( RTL)
The modules of a digital system are
best defined by a set of registers and the operations that are performed on the
binary information stored in them. Examples of register operations are shift, count, clear. and load. Registers are assumed to be the
basic components of the digital system.
The flow and processing performed on the data stored in the registers are
referred 10 as register transfer
Operations, We'll see subsequently how a hardware description language
includes operation; that correspond to the register transfer operations of a
digital system.A digital system is represented at the register transfer level (R1l..) when it is specified by the
following three components:
1. The set of registers in the
system.
2.
The
operations that are performed on the data stored in the registers.
The control that supervises the
sequence of operations in the system. A register is a group of flip-fops that
stores binary intonation and has the capability of performing o ne or more
elementary operations. A register can load new information or shift the
information to the right or the le ft. A counter is. considered a regjster that
increments a number by a fixed value (e .g.• I ).A flip-flop is considered a
one-bit register that can be set. cleared. or complemented. In fact. the flip
-flops and associated gates of any sequential circuit can be called registers
by this definition. The operations executed on the information stored in
registers are elementary operations that are perfumed in parallel on a data wor
consisting of bits during one clock cycle. The data produced by the ope ration
may replace the binary information that was in the register before the
operation executed. Alternatively. me result may be transferred to another
register (i.e.• an operation on a register may leave its contents unchanged).
Th e digital circuits introduced in registers that implement elementary
operatio ns. A counter with a parallel load is able to perform me incremcnt-by-one
and load operations. A bidirectional shift register is able to perform me
shift-rig ht and shift-left operations.
The operations in a digital system
are controlled by timing signals that sequence the operations in a prescribed
manner. Certain conditions that depend on results of previous operations may
determine the sequence of future operations. The outputs of the control log ic
are binary variables that initiate the various operations in the system's
registers
.Information trasfer from one
register to another is designated in symbolic form by mean s of a replacement
operator. The denotes a transfer of the contents of register RJ into register R2---that is. a
replacement of the contents of register R2
by the contents of register RJ. By
definition. the contents of the source register RI do not change after the transfer. They are merely copied to RI . symbolizes the transfer and its
direction: it points from the register whose con tents are being transferred
and towards the register that will receive the contents . A control signal
would determine when the operation actually executes.
Rl
- Rl + R2 R3-R3 + I R4 - shrR4
R5
-0
Add contents of R2 to Rl
(RI gets Rl + R2) Increment R3 by I (count upwards )
Shift right R4 Clear R5
1.
Transfer
operations. which transfer data from one register to another.
2.
Arithmetic
operations. which perform arithmetic on data in registers.
3.
Logic
operations. which perform bit manipulation (e.g., logical OR) of nonnumeric
data in registers.
4.
Shift
operations. which shift data between registers.
The transfer operation does not
change the information content of the data being moved from the source register
to the destination register. The other three operations change the n content
during the transfer. The register transfer notation and the symbols used to
represent the various register transfer operations are not standardized. In
this text, we employ two types of notation. The notation introduced in this
section will be used infonn ally to specify and explain digital systems at the register
transfer level. The next section introduces the RTL symbols used in the Verilog
HDL.
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