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

2. 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

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

4. 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

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

6. 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)

7. Serial Transfers

8. 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

9. 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

10. 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

11. Analyzing the circuit : Serial AdderB3 B2 B1 B0
SUM (A+B) + Cin
Cout
Cin T0 1 T1 T2 T3 T4
Example 1:
Reg A : 1000
Reg B : 0101

12. Analyzing the circuit : Serial AdderB3 B2 B1 B0
SUM (A+B) + Cin
Cout
Cin T0 1 T1 T2 T3 T4
ANSWER : 1101, after 4 clock cycle

13. Analyzing the circuit : Serial AdderB3 B2 B1 B0
SUM (A+B) + Cin
Cout
Cin T0 1 T1 T2 T3 T4
Example 2:
Reg A : 1011
Reg B : 0101

14. Analyzing the circuit : Serial AdderB3 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

15. 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

16. 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)

17. Thank You