1. Outline
Registers
Counters
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2. Registers Definition:
Array of flip-flops (and possibly added combinational gates) to store binary data
Each flip-flop stores one bit
Register with n flip-flop stores n bits
There are many types of registers, each providing a certain form of storing data
In many registers, a clock (or a combination of clock and data) controls the timing of register; i.e. clock says when to update the data stored at register
Registers are heavily used in computers
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3. Registers (continued)
A simple register:
What data to be stored: D1, D2, D3 and D4
Where to store data: Q1, Q2, Q3 and Q4
When to load data: Clock
How to reset (clear) the register: Clear
Symbol:
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4. Registers (continued)
Disadvantage:
We may not want to update data on each clock, i.e. we want loading at some particular times and not all clock pulses
Solution 1: Clock gating
Clock is gated with an input called “load”
that tells the circuit when we want
to load data
If Load = 1, C input = Clock (updating happens on each positive edge of Clock)
If Load = 0, C input = 1
(no edge of Clock will
be observed and therefore
no updating)
Disadvantage: Clock skew
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5. Registers (continued)
Solution 2: Control loading via flip-flop inputs
Load input is applied
in forming data lines
If Load = 1, input data
reach the flip-flip inputs
If Load = 0, the present
data at the output (Q)
will be fed back and
reloaded., i.e. new
input data is ignored
Clock skew is avoided
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6. Registers (continued)
Shift registers:
A simple shift register:
Circuit
Serial Input
Serial Output
Clock: connected
to all flip-flops
Symbol:
Clock can be gated (similar to other registers)
Gating clock creates clock skew
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7. Registers (continued)
Serial transfer between shift registers:
Clock is gated
Registers with more bits can be formed similarly
Timing diagram
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8. Registers (continued)
Table for serial transfer:
Serial addition:
Using shift registers to perform addition
Suitable for cases where bits of numbers to be added are received via a serial line
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9. Registers (continued)
Shift register with
parallel load:
Feature:
shift register is
load in parallel
Circuit
If Load = 1,
register is loaded
in parallel
If Shift = 1,
register is shifted
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10. Registers (continued)
Table:
Symbol:
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11. Counters
Definition:
A register that goes through a prescribed sequence of states upon the application of input
There are many types of counters, each providing a certain form of counting
Two general types of counters:
Ripple counter: the control input of some flip-flops are not derived by the circuit clock
Synchronous counter: a synchronous sequential circuit, i.e. each flip-flop receives the clock in its control input
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12. Counters (continued) Ripple counter: Circuit Control inputs of three
flip-flops are derived by the
output of other flip-flops
J and K inputs of all flip-flops
are connected to 1, as a result
for each rising edge on the
control input, the outputs
change
Since two changes of output
makes a pulse, the period of
pulse is divided by two in the next flip-flop
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13. Counters (continued)
Table:
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14. Counters (continued) Synchronous binary counter:
A sequential circuit: design is similar to other sequential circuits
Table
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