ECE 2110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices Encoders.

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ECE 2110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices Encoders

74x138/139 decoders

Encoders Multiple-input/multiple-output device. Performs the inverse function of a Decoder. Outputs ( m ) are less than inputs ( n ). Converts input code words into output code words. input code output code ENCODER

Encoders vs. Decoders Binary decoders/encoders Decoder Encoder n-to-2^n Input code : Binary Code Output code :1-out-of-2^n. 2^n-to-n encoder Input code : 1-out-of-2^n. Output code : Binary Code

Binary Encoder 2^n-to-n encoder : 2^n inputs and n outputs. Input code : 1-out-of-2^n. Output code : Binary Code Example : n=3, 8-to-3 encoder Inputs Outputs I0 I1 I2 I3 I4 I5 I6 I7 Y2 Y1 Y0 1 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 1 0 0 0 0 1 0 0 0 0 0 1 1 0 0 0 0 1 0 0 0 1 0 0 0 0 0 0 0 1 0 0 1 0 1 0 0 0 0 0 0 1 0 1 1 0 0 0 0 0 0 0 0 1 1 1 1 I4 Y2 I5 I6 I7

8-to-3 encoder Implementation Simplified implementation: - From the truth table Y0 = I1 + I3 + I5 + I7 Y1 = I2 + I3 + I6 + I7 Y2 = I4 + I5 + I6 + I7 Limitations : - I0 has no effect on the output - Only one input can be activated Application: Handling multiple devices requests But, no simultaneous requests Establishing priorities solve the problem of multiple requests Y2 I1 I2 Y1 I3 I4 I5 Y0 I6 I7

Need priority in most applications

Priority Encoder Assign priorities to the inputs When more than one input are asserted, the output generates the code of the input with the highest priority Priority Encoder : H7=I7 (Highest Priority) H6=I6.I7’ H5=I5.I6’.I7’ H4=I4.I5’.I6’.I7’ H3=I3.I4’.I5’.I6’.I7’ H2=I2.I3’.I4’.I5’.I6’.I7’ H1=I1. I2’.I3’.I4’.I5’.I6’.I7’ H0=I0.I1’. I2’.I3’.I4’.I5’.I6’.I7’ IDLE= I0’.I1’. I2’.I3’.I4’.I5’.I6’.I7’ - Encoder A0=Y0 = H1 + H3 + H5 + H7 A1=Y1 = H2 + H3 + H6 + H7 A2=Y2 = H4 + H5 + H6 + H7 I6 I5 I4 Y1 Y0 I3 I2 I1 I7 Y2 I0 Binary encoder Priority Circuit H6 H5 H4 H3 H2 H1 H7 H0 IDLE A1 A0 A2 Priority encoder

8-input priority encoder I7 has the highest priority,I0 least A2-A0 contain the number of the highest-priority asserted input if any. IDLE is asserted if no inputs are asserted.

74x148 8-input priority encoder

74x148 8-input priority encoder Active-low I/O Enable Input “Got Something": Group Select Enable Output

74x148 Truth Table

74x148 circuit

Cascading priority encoders 32-input priority encoder

Next… Three state devices Multiplexers Reading Wakerly CH-6.6-6.7