Thermometer code

What is thermometer code: Thermometer code resembles the output produced by a thermometer. In thermometer code, a value representing number ‘N’ has the lowermost ‘N’ bits as ‘1’; others as 0. So, to move from N to ‘N+1’, just change the rightmost ‘0’ to ‘1’. Figure 1 below shows graphically the thermometer codes for values from ‘0’ to ‘7’. As is evident, each value resembles a reading in thermometer. This is how, thermometer code got its name. Flash ADCs, time-to-digital converters (TDC) are some of the circuits that utilize thermometer code.

A thermometer code is a series of zeroes followed by a series of ones. A 8-symbol thermometer code will have 7 bits that need to represent all symbols.
Thermometer code with 7 symbols







Characteristics of thermometer code:
  • Each symbol in thermometer code is a sequence of 0s followed by a sequence of 1s
  • There cannot be 0s in-between two 1s. For example, a symbol 01011 is invalid in thermometer code
  • For an n-bit binary code, the corresponding thermometer code will have 2n – 1 symbols; hence, as many bits will be needed to represent thermometer code for the same.

How to convert from binary to thermometer code: Given below is the VHDL code for a 3-bit binary to thermometer converter. A simple case statement can be utilized for the same.
                                  
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use ieee.numeric_std.all;
entity bin2therm2bit is
                port (
                                binary_input : in std_logic_vector (1 downto 0);
                                therm_output : out std_logic_vector (6 downto 0)
                );
end bin2therm6bit;

architecture Behavioral of bin2therm6bit is
begin
                process (binary_input)
                begin
                                label1 : case binary_input is
                                                when "000" => therm_output <= "0000000";
                                                when "001" => therm_output <= "0000001";
                                                when "010" => therm_output <= "0000011";
                                                when "011" => therm_output <= "0000111";
                                                when "100" => therm_output <= "0001111";
                                                when "101" => therm_output <= "0011111";
                                                when "110" => therm_output <= "0111111";
                                                when "111" => therm_output <= "1111111";
                                                when others => therm_output <= “xxxxxxx”;
                                end case;
                end process;
end Behavioral;

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