LAB 5: A Decoder
Synopsis
The camera has a different button for each exposure time, i.e. the data signals from the keypad are transferred via a 10-bit data bus. If a button is pressed, the corresponding wire is set to 1. The exposure times are calculated according to the following formula:
time(button) = 2^(-button), for button=0..9 (time in seconds).
The result (1/1 s, 1/2 s, 1/4 s, …, 1/512 s) shall be shown on the 7-segment display. Thus, another decoder is needed. We will not introduce any new VHDL concepts in this exercise but suggest to try out different implementations and to observe their impact on the synthesis result. The block diagram of the new module is rather simple.
Behaviour
As already mentioned, the functionality of this module is fairly simple. The denominator of the following calculation shall be coded in a suitable form for the 7-segment display.
2^(-button) = 1 / (2^(button) ) = 1/1, 1/2, 1/4, 1/8, ..., 1/512 (button = 0..9)
It is up to you to decide about the behaviour when more than one button is pressed. There are basically two options: Either the buttons have a certain priority, or the user input is ignored.
Data types
The input data is generated by an external keypad and consists of 10 wires, one per button. Thus a std_ulogic_vector is used for the KEYPAD signal. The decoded values shall be processed by the display logic, i.e. T_DIGITS is the most suitable data type.
To do
- Create new files for the decoder entity and its architectures if you want to try out different implementations. Remember that synthesis tools ignore the VHDL configuration and use the most recent architecture.
- Generate a testbench for the new design and add stimuli that cover “illegal” user input (more than one button pressed simultaneously).
- Compile the VHDL code (package→entity→architectures→configurations).
- Simulate the configurations and look out for differences in the behaviour.
- Synthesize the architectures and find out about their area and maximum delay from input to output.
Implementation
- DECODER_CASE.VHD
architecture RTL_CASE of DECODER is begin process(KEYPAD) begin case KEYPAD is when "1000000000" => KEY <= …; when "0100000000" => KEY <= … ; when "0010000000" => KEY <= … ; when "0001000000" => KEY <= … ; when "0000110000" => KEY <= … ; … when "0000000001" => KEY <= … ; end case; end process; end RTL_CASE;
- DECODER_IF.VHD
architecture RTL_IF of DECODER is begin architecture process(KEYPAD) begin process if KEYPAD(0) = '1' then KEY <= … ; elsif KEYPAD(1) = '1' then KEY <= … ; elsif KEYPAD(2) = '1' then KEY <= … ; elsif KEYPAD(3) = '1' then KEY <= … ; elsif KEYPAD(4) = '1' then KEY <= … ; … elsif KEYPAD(9) = '1' then KEY <= … ; else KEY <= … ; end if; end process; end RTL_IF;
- DECODER.VHD
use work.P_DISPLAY.all; entity DECODER is port (KEYPAD : in std_ulogic_vector(9 downto 0); KEY : out T_DIGITS); end DECODER;
Testbench
- TB_DECODER.VHD
library ieee; use ieee.std_logic_1164.all; use work.P_DISPLAY.all; entity TB_DECODER is end TB_DECODER; architecture TEST of TB_DECODER is component DECODER port (KEYPAD : in std_ulogic_vector (9 downto 0); KEY : out T_DIGITS); end component; signal W_KEYPAD : std_ulogic_vector (9 downto 0); signal W_KEY : T_DIGITS; begin DUT : DECODER port map (KEYPAD => W_KEYPAD, KEY => W_KEY); STIMULI : process begin W_KEYPAD <= "1000000000"; -- KEY = (5,1,2) (this works in both architectures!!) wait for 20 ns; W_KEYPAD <= "1111100000"; -- KEY = (0,3,2) (if) -- (0,0,0) (case) wait for 20 ns; W_KEYPAD <= "0000100000"; -- KEY = (0,3,2) (both architectures) wait for 20 ns; for I in 9 downto 0 loop W_KEYPAD <= (others => '0'); W_KEYPAD(I) <= '1'; -- KEY = (5,1,2),(2,5,6),(1,2,8),(0,6,4),(0,3,2), -- (0,1,6),(0,0,8),(0,0,4),(0,0,2),(0,0,1) -- (both architectures) wait for 20 ns; end loop; wait; end process STIMULI; end TEST; configuration CFG_TB_DECODER of TB_DECODER is for TEST end for; end CFG_TB_DECODER;
Package
LAB 4, 5, 6, 7, 8, 9, 10, 11
- P_DISPLAY.VHD
library ieee; use ieee.std_logic_1164.all; package P_DISPLAY is type T_DIGITS is array (2 downto 0) of integer range 0 to 10; type T_DISPLAY is array (2 downto 0) of std_ulogic_vector(6 downto 0); constant SEG_0 : std_ulogic_vector(6 downto 0):= "0111111"; constant SEG_1 : std_ulogic_vector(6 downto 0):= "0000011"; constant SEG_2 : std_ulogic_vector(6 downto 0):= "1101101"; constant SEG_3 : std_ulogic_vector(6 downto 0):= "1100111"; constant SEG_4 : std_ulogic_vector(6 downto 0):= "1010011"; constant SEG_5 : std_ulogic_vector(6 downto 0):= "1110110"; constant SEG_6 : std_ulogic_vector(6 downto 0):= "1111110"; constant SEG_7 : std_ulogic_vector(6 downto 0):= "0100011"; constant SEG_8 : std_ulogic_vector(6 downto 0):= "1111111"; constant SEG_9 : std_ulogic_vector(6 downto 0):= "1110111"; constant SEG_E : std_ulogic_vector(6 downto 0):= "1111100"; -- pragma translate_off procedure DISPLAY_DIGIT(signal DISPLAY:T_DISPLAY); -- pragma translate_on end P_DISPLAY; -- pragma translate_off use std.textio.all; package body P_DISPLAY is ------------------------------ -- -- 111111111112222 -- 123456789012345678901234 -- 1 #5## #### #### -- 2 # # # # # # -- 3 4 0 # # # # -- 4 # # # # # # -- 5 #6## #### #### -- 6 # # # # # # -- 7 3 1 # # # # -- 8 # # # # # # -- 9 #2## #### #### -- ------------------------------ constant ACTIVE_SEG: character := '#'; constant EMPTY_SEG: character := ' '; -- Width and height of a digit constant WIDTH: integer := 8; constant HEIGHT: integer := 9; -- Datatypes to store the complete display in a matrix subtype T_MATRIX_ROW is bit_vector (1 to 3*WIDTH); type T_DISP_MATRIX is array (1 to HEIGHT) of T_MATRIX_ROW; -- Definition of the appearance of the 6 segments subtype T_DIGIT_ROW is bit_vector(1 to WIDTH); type T_SEG_DEF is array (1 to HEIGHT) of T_DIGIT_ROW; type T_DIGIT_DEF is array (0 to 6) of T_SEG_DEF; constant DIGIT_DEF: T_DIGIT_DEF :=(("00000000", -- .... "00000010", -- . # "00000010", -- . 0 "00000010", -- . # "00000000", -- .... "00000000", -- . . "00000000", -- . . "00000000", -- . . "00000000"),-- .... ("00000000", -- .... "00000000", -- . . "00000000", -- . . "00000000", -- . . "00000000", -- .... "00000010", -- . # "00000010", -- . 1 "00000010", -- . # "00000000"),-- .... ("00000000", -- .... "00000000", -- . . "00000000", -- . . "00000000", -- . . "00000000", -- .... "00000000", -- . . "00000000", -- . . "00000000", -- . . "00111100"),-- #2## ("00000000", -- .... "00000000", -- . . "00000000", -- . . "00000000", -- . . "00000000", -- .... "01000000", -- # . "01000000", -- 3 . "01000000", -- # . "00000000"),-- .... ("00000000", -- .... "01000000", -- # . "01000000", -- 4 . "01000000", -- # . "00000000", -- .... "00000000", -- . . "00000000", -- . . "00000000", -- . . "00000000"),-- .... ("00111100", -- #5## "00000000", -- . . "00000000", -- . . "00000000", -- . . "00000000", -- .... "00000000", -- . . "00000000", -- . . "00000000", -- . . "00000000"),-- .... ("00000000", -- .... "00000000", -- . . "00000000", -- . . "00000000", -- . . "00111100", -- #6## "00000000", -- . . "00000000", -- . . "00000000", -- . . "00000000"));-- .... procedure DISPLAY_DIGIT(signal DISPLAY:T_DISPLAY) is file OUTFILE : TEXT is out "display.txt"; variable L : line; variable DISP_TEXT: string(T_MATRIX_ROW'range); variable DISP_MATRIX: T_DISP_MATRIX; variable COL_L: integer; variable COL_R: integer; begin -- Clear the display DISP_MATRIX := (others => (others => '0')); -- Loop over all digits for DIGIT in T_DISPLAY'range loop -- Calculate the left- and rightmost columns COL_L := DIGIT*WIDTH + 1; COL_R := DIGIT*WIDTH + WIDTH; -- Loop over all segments for SEGMENT in 0 to 6 loop if DISPLAY(DIGIT)(SEGMENT) = '1' then -- Copy the matrix to the final display -- Loop over all rows of the display for ROW in T_SEG_DEF'range loop DISP_MATRIX(ROW)(COL_L to COL_R) := DISP_MATRIX(ROW)(COL_L to COL_R) or DIGIT_DEF(SEGMENT)(ROW); end loop; end if; end loop; end loop; -- Write the matrix to a textfile for I in T_DISP_MATRIX'range loop -- Start with an empty line DISP_TEXT := (others => EMPTY_SEG); for J in T_MATRIX_ROW'range loop if DISP_MATRIX(I)(J) = '1' then DISP_TEXT(J) := ACTIVE_SEG; end if; end loop; -- Write the line write(L, DISP_TEXT); writeline(OUTFILE, L); end loop; end DISPLAY_DIGIT; end P_DISPLAY; -- pragma translate_on