1. EKT 221 : Digital 2 Serial Transfers & Microoperations

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 & 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).

4. 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

5. 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

6. Serial Transfers To maintain the data in Reg A,
connect SO of Reg A to its SI
Information is circulated back into Reg A

7. 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)

8. 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

9. Serial Transfers
SI (A) = 0
Reg B = Previous value of Reg A

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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)

20. Thank You