Download - MSI Shift Registers
MSI Shift Registers• 74LS194 4-Bit
Bidirectional Universal Shift Register
• may be used in the following data register transfers– serial-serial, – shift left, – shift right, – serial-parallel, – parallel-serial, – and parallel-parallel
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MSI Shift Registers
• 74LS194 4-Bit Bidirectional Universal Shift Register
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MSI Shift Registers
• 74LS194 control inputs S1 and S0
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MSI Shift Registers
• 74LS194 4-Bit Bidirectional Universal Shift Register
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01 11 10 00 01 11 10 0001 11 10 0001 11 10 00
MSI Shift Registers
• 74LS194 4-Bit Bidirectional Universal Shift Register
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“Universal” shift register
74x194
• Shift left• Shift right• Load• Hold
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MSI Shift Registers
• One stage of the 74x194
MSI Shift Register 74195
MSI Shift Register 74195
MSI Shift Register 74195
or D0–D3or D0–D3
MSI Shift Register 74195
MSI Shift Register 74195• 74195 logic diagram
/P3/P3/P0/P0 /P1/P1 /P2/P2
Ring Counter
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• A ring counter is a loop of flip-flops interconnected in such a manner that only one of the devices may be in a specified state at one time
• If the specified state is HIGH, then only one device may be HIGH at one time.
• As the clock, or input, signal is received, the specified state will shift to the next device at a rate of 1 shift per clock, or input, pulse.
MSI Shift Registers
• 74LS194 control inputs S1 and S0
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Shift-Register Counters
• Ring counter
• For Shift right
S0Vcc, S1 for Load and Reset, AVcc, BCD Gnd,
QD RIN, LIN is not connected
Ring counter (Self correcting)• 4 bit, 4 state with a single circulating 1
00010001
00100010
01000100
10001000
10101010
01010101 11011101
01100110
00110011 10111011
11101110
01110111 11111111
1100110010011001
00000000
State diagram for a self correcting ring counter
Ring counter (Self correcting)• 4 bit, 4 state with a single circulating 0
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Johnson Counter (“Twisted ring” counter)
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Timing diagram for a 4-bit Johnson counter
State Name
Q3 Q2 Q1 Q0 Decoding
S1 0 0 0 0 Q3’•Q0’
S2 0 0 0 1 Q1’•Q0
S3 0 0 1 1 Q2’•Q0
S4 0 1 1 1 Q3’•Q2
S5 1 1 1 1 Q3•Q0
S6 1 1 1 0 Q1•Q0’
S7 1 1 0 0 Q2•Q1’
S8 1 0 0 0 Q3•Q2’
States of an 4-bit Johnson counter
* Can be decoded with 2-input gates
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Self correcting Johnson Counter• n-bit counter • 2n - 2n unused states• 0x…x0 → 00…01• 2 input NOR gate
performs correction
Linear Feedback Shift Register Counter• n-bit shift register counters have far less than the
maximum number of 2n normal states • n states for ring counter, 2n states for Johnson counter• An n-bit LFSR counter can have 2n – 1 states
also called as maximum length sequence generator• Design is based on the theory of finite fields• Developed by French mathematician Evariste Galois• Serial input is connected to the sum modulo 2 of a
certain set of output bits• These feedback connections determine the state
sequence of the counter• By convention, outputs are always numbered and
shifted in the direction shown in figure on next slide
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• There exists at least one equation which makes the counter go through all the 2n - 1 states before repeating
• It can never cycle through all 2n states
• Regardless of the connection pattern 0…0 → 0…0
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Feedback equations for LFSR counters
3-bit LFSR counter
X0X0
X1X1X2X2
X3X3
X2 X1 X0
1 0 0
0 1 0
1 0 1
1 1 0
1 1 1
0 1 1
0 0 1
1 0 0
Sequence
Modified LFSR Counter• An LFSR can be modified to have 2n states including
the all 0’s state• In an n-bit LFSR counter, an extra EXOR gate and an
n – 1 input NOR gate connected to all the shift register outputs except X0 accomplishes the task
• The states are not visited in binary order • Usually used where this characteristic is an advantage
- Generating test inputs for logic circuits- Encoding and decoding circuits for certain error-
detecting and error-correcting codes including CRC codes
- Scrambling and descrambling data patterns in data communications- Pseudo random binary sequence generator
Modified 3-bit LFSR counter to include all 0’s
X2 X1 X0
1 0 0
0 1 0
1 0 1
1 1 0
1 1 1
0 1 1
0 0 1
0 0 0
1 0 0
Sequence
X0X0
X1X1
X2X2
X3X3