1. Speical purpose Encoders/Comparators
ECE 3110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Examples
Barrel Shifter
Speical purpose Encoders/Comparators

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3. HW solutions
Ex. 5.46
Ex. 5.19

4. Design examples using combinational building blocks (decoders, encoders, multiplexers, comparators, three-state devices, adders).
Barrel shifter
Dual-priority encoder
Cascading comparators, mode-dependent comparator

5. Barrel shifter design example
n data inputs, n data outputs
Control inputs specify number of positions to rotate or shift data inputs
Example: n = 16
DIN[15:0], DOUT[15:0], S[3:0] (shift amount)
Many possible solutions, all based on multiplexers

6. Multiplexers (mux)
Eg. Put between Processor’s registers and ALU
A 16-bit processor where 3-bit field specifies on of 8 registers.
The 3-bit field is connected to the select inputs of an 8-input, 16-bit mux.
Select one of n sources of data to transmit on a bus.

7. MSI: 74x151 8-input 1-bit multiplexer

8. 16-to-1 barrel shifter
16-to-1 mux = 2 x 74x to-1 mux + NAND gate

9. 4 16-bit 2-to-1 muxes
16-bit 2-to-1 mux = 4 x 74x157 4-bit 2-to-1 mux

10. Properties of different approaches

11. Dual-Priority Encoder
A priority encoder identifies not only the highest but also the second-highest-priority asserted signal among a set of 8 request inputs.

12. Single-priority encoder1

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14. Parallel Cascading Comparators
74x85: serial cascading scheme, delay of propagating the cascading signals through a cascade of comparators.
74x682: Parallel approach, used to build very wide comparators.
Build 24-bit comparator, using 3 74x682
PEQQ=EQ2.EQ1.EQ0
PGTQ=GT2+EQ2.GT1+EQ2.EQ1.GT0

15. 8 bit Comparator +5V 74x85 74x85 A<B A<BIN A<BOUT A<BIN
Least Significant bits
Most Significant bits

17. Mode-dependent Comparator
Design a c.c. whose inputs are two 8-bit unsigned binary integers(X,Y), and a control signal(MIN/MAX). The output is an 8-bit unsigned binary integer Z, such that
Z=min(X,Y) if MIN/MAX=1;
Z=max(X,Y) otherwise

19. Next…
Sequential Logic Introduction