Saturday, 11 February 2012

Encoders


Encoder is a combinational circuit which converts one of its active input lines into corresponding binary number.  Encoder performs reverse operation of decoder. It consist of 2n input lines and produces n-bit output.
8 to 3 encoder:
Boolean equations for encoder is
A=D1+D3+D5+D7
B=D2+D3+D6+D7
C=D4+D5+D6+D7
8 to 3 priority encoder:
In a normal encoder, at a time only one input should be in active state. Otherwise the output of encoder is not correct. For example if D3 and D4 both are active then output of encoder is 111(i.e. 7), which is not equal to either 3 or 4. To avoid this problem priority encoder is used.
 
In above truth table x indicates don’t care. D7 has highest priority and priority decreases from D6 to D0. If D7 is 1, then irrespective of other inputs output is 111(i.e. 7). If D5 is 1 and D6, D7 both are 0’s then output is 101(i.e. 5). GS output means ‘got something’. If any of the input to encoder is active then GS becomes 1. If all the inputs to encoder are 0’s then GS is 0.
The Boolean equations for above priority encoder are
B2=D4+D5+D6+D7
B1=D7+D6+D5’ D4’ (D3+D2)
B0=D7+D6’D5+D6’D4’D3+D6’D4’D2’D1
GS=D0+D1+D2+D3+D4+D5+D6+D7

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