EKT 221 : Digital 2 Serial Transfers & Microoperations Date : 4 September 2007

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 Serial mode  info is transferred / manipulated one bit at a time Serial transfer from RA to RB is done with shift registers

Serial Transfers Serial output (SO) of A connected with serial input (SI) of B SI of A receives 0’s Data from A transferred to B Initial content of B shifted out to SO of B and lost

Serial Transfers To maintain the data in A, connect SO of A to its SI

Serial Transfers Shift  determine when & how many times the registers are shifted Clock pulse (Clock) can pass to C only when Shift is HIGH (1)

Serial Transfers

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 The circuit shown uses 2 shift registers for operands B (3:0) A full adder, and one more FF (for carry) is used to compute the sum Result stored in A register and final carry in FF

Serial Adder SI of B can receive new inputs In each clock pulse / cycle : New sum bit is transferred to A New carry transferred to FF Both registers shifted once to the right Process cont. until Shift = 0

Analyzing the circuit : Serial Adder B3 B2 B1 B0 SUM (A+B) + Cin Cout Cin T0 1 T1 T2 T3 T4 Example 1: Reg A : 1000 Reg B : 0101

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 B3 B2 B1 B0 SUM (A+B) + Cin Cout Cin T0 1 T1 T2 T3 T4 Example 2: Reg A : 1011 Reg B : 0101

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 seq. cct because it includes the carry from FF. but need n clock cycle to complete the addition (Less Space, more Time) Parallel adder is a comb. cct 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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