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Binary-to Thermometer Encoder model

This is a behavioral binary-to-thermometer encoder that converts an N-bit binary input into a one-hot thermometer output bus, a key building block in current-steering DAC decoders, pipelined ADC segment selectors, and programmable-gain amplifier stimulus blocks. With binary_bits = N, the model drives 2^N output lines; Start_Bit selects whether decimal 0 maps to the all-zero word (Start_Bit = 0) or to the LSB-high word (Start_Bit = 1), accommodating different DAC unit-element conventions. Output logic levels are set by vdd/vss and the input threshold by threshold. A typical use-case is driving the unit-element switches of a segmented current-steering DAC array from a binary counter output in a mixed-signal simulation, bypassing the need for a transistor-level decoder during early behavioral verification.

This page contains Verilog-A model of the binary-to-thermometer encoder. This block can be used for behavioral simulation of the pipelined ADCs, programmable gains/BW etc. This model will automatically select number of outputs based on selected number of input binary bits.

Usage:

Create a new cell in Library Manager named bin2therm and select cell type Verilog A;
Copy and paste the code provided;
Specify binary_bits variable to be the desired binary bits number;
Specify Start_Bit to be 0 if you want thermometer code to start from 0 or 1 if you want thermometer code to start from 1;
Perform Check and Save;
A cell symbol will be created;
Instantiate bin2term cell into your design;
Perform Check and Save and run the simulation.

Example: binary_bits = 2, Start_Bit = 0

Binary input	Thermometer code
00	000
01	001
10	010
11	100

Example: binary_bits = 2, Start_Bit = 1

Binary input	Thermometer code
00	0001
01	0010
10	0100
11	1000

Cell name: bin2term

Model type: Verilog-A

verilog

// Binary to Thermometer decoder
// Implements two options: 
// Start_Bit = 0: Decimal 0 equals thermometer 0
// Start_Bit = 1: Decimal 0 equals thermometer 1
// Change binary_bits variable for your needs!
// Source: AnalogHub.ie

`include "constants.vams"
`include "disciplines.vams"
`define binary_bits 4							// define number of binary bits here

module bin2therm(in,out);
input [`binary_bits-1:0] in;
output [2**`binary_bits-1:0] out;

voltage [`binary_bits-1:0] in;
voltage [2**`binary_bits-1:0] out;

parameter real vdd = 1;					// voltage level of logic 1 (V)
parameter real vss = 0;					// voltage level of logic 0 (V)
parameter real threshold = 0.5;			// logic threshold level (V)
parameter integer Start_Bit = 0;    	// defines if thermometer starts from 0 or 1

real dout[2**`binary_bits-1:0];			// internal result variable
integer code;
genvar i;

analog begin
// convert binary input to code
    code = 0;
    for (i = 0; i < `binary_bits; i = i + 1) begin
        @(cross(V(in[i]) - threshold))
            ;
        if (V(in[i]) > threshold)
            code = code + (1 << i);
            end
//$display("Code = %d", code);

case (Start_Bit)
    0: begin	// Decimal 0 equals thermometer 0
		for(i=1;i<2**`binary_bits+1;i=i+1) begin
          		if(code!=i) begin
              		dout[i-1]=vss;
          		end
      		else begin
          		dout[i-1]=vdd;
      		end
		end
	end 

    1: begin	// Decimal 0 equals thermometer 1
		for(i=0;i<2**`binary_bits;i=i+1) begin
          		if(code!=i) begin
              		dout[i]=vss;
          		end
      		else begin
          		dout[i]=vdd;
      		end
		end
	end
endcase

// Plotting outputs
for (i=0; i<2**`binary_bits; i=i+1)
	    V(out[i]) <+ transition(dout[i],0,0);
end
endmodule