EKT 221 : Digital 2 Serial Transfers & Microoperations
Serial Transfers & Microoperations Used for “narrow” transfer paths Example : Telephone or cable line Parallel – to – Serial : at source Serial – to – Parallel : at destination Serial micro-operations Example 1 : Addition Example 2 : Error – Correction for CDs Parallel Serial Serial Parallel Source Destination
Serial Transfers & Microoperations Serial mode in digital system: information in the system is transferred or manipulated one bit a time. Information is transferred one bit at a time by shifting the bits out of one register and into a second register. This transfer method is in contrast to parallel transfer, in which all the bits of the register are transferred simultaneously at the same time (during one clock pulse).
Serial Transfers Serial mode info is transferred / manipulated one bit at a time Serial transfer of information from Reg A to Reg B is done with shift registers
Serial Transfers Serial output (SO) of Reg A connected with serial input (SI) of Reg B SI of Reg A receives 0’s while data from Reg A are transferred to Reg B Initial content of Reg B shifted out to its SO and lost
Serial Transfers To maintain the data in Reg A, connect SO of Reg A to its SI Information is circulated back into Reg A
Serial Transfers Shift determines when & how many times the registers are shifted Clock pulse (Clock) can pass to shift register clock inputs C only when Shift is HIGH (1)
Serial Transfers Each shift register has four stages. Shift signal must be designed for a fixed time of four clock pulses. Shift in HIGH : Clock inputs C T1, T2, T3 and T4 Shift in LOW : Clock inputs C HIGH Shift Reg disabled, shifts stop Shift Reg enabled, Shift occurs in both registers
Serial Transfers SI (A) = 0 Reg B = Previous value of Reg A
Serial Micro-operations Serial addition is a low cost way to add large numbers of operands, since a “tree” of full adder cells can be made to any depth. Other operations can be performed serially as well, such as parity generation / checking or more complex error – check codes. Shifting a binary number left = multiplying by 2 E.g sl 0100 1000 Shifting a binary number right = dividing by 2 E.g sr 0100 0010
Serial Adder – demonstrate the serial mode operation The circuit shown uses 2 shift registers for operands A (3:0) and B (3:0) A full adder (FA), and one more FF (for carry) is used to compute the sum Result (sum) stored in Reg A and final carry stored in FF
Serial Adder – demonstrate the serial mode operation SHIFT = 1 Result (sum) stored in Reg A Final carry stored in FF Both registers are shifted once to right Carry FF = 0
Serial Adder SI of Reg B can receive new inputs In each clock pulse/cycle : New sum bit is transferred to Reg A New carry transferred to FF Both registers shifted once to the right Process continues until Shift = 0
Analyzing the circuit : Serial Adder Example 1: Find value in Reg A after 4 shifts. Reg A : 1000 Reg B : 0101 A3 A2 A1 A0 B3 B2 B1 B0 SUM (A+B) + Cin Cout Cin T0 1 T1 T2 T3 T4
Analyzing the circuit : Serial Adder B3 B2 B1 B0 SUM (A+B) + Cin Cout Cin T0 1 T1 T2 T3 T4 ANSWER : 1101, after 4 clock cycle
Analyzing the circuit : Serial Adder Example 1: Find value in Reg A after 4 shifts. Reg A : 1011 Reg B : 0101 A3 A2 A1 A0 B3 B2 B1 B0 SUM (A+B) + Cin Cout Cin T0 1 T1 T2 T3 T4
Analyzing the circuit : Serial Adder B3 B2 B1 B0 SUM (A+B) + Cin Cout Cin T0 1 T1 T2 T3 T4 1 is indicated in Cout ANSWER : 10000, after 4 clock cycle
Parallel Adder Reg A Sin Sout A0 A1 A2 A3 FA A0 B0 Cin A1 B1 A2 B2 A3 Cout S0 S1 S2 S3 Can be the input for Reg A Reg B Sin Sout B0 B1 B2 B3
Serial Vs Parallel Transfers Space Vs Time Trade-off Serial adder is a sequential circuit because it includes the carry from FF. But need n clock cycle to complete the addition (Less Space, more Time) Parallel adder is a combinational circuit because it needs n FA for n bit operation. Need only one clock cycle to complete the addition. (More Space, less Time) Gives the designer choice. More Space – More cost More Time – More delay (not fast)
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