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Debugging VHDL (or almost any computer input)
1) Expect errors. Nobody's perfect.
2) Automate to make it easy to re-run, e.g. Makefile
In Makefile, keep adding more tests
all: tadd32.out pmul8_test.out part1.out
# these are the final result files
# <tab> not spaces, precede ncvhdl, ncelab, ncsim
tadd32.out: add32.vhdl tadd32.vhdl tadd32.run
ncvhdl -v93 add32.vhdl
ncvhdl -v93 tadd32.vhdl
ncelab -v93 tadd32:circuits
ncsim -batch -logfile tadd32.out -input tadd32.run tadd32
# blank line ends commands
pmul8_test.out: pmul8.vhdl pmul8_test.vhdl pmul8_test.run
ncvhdl -v93 pmul8.vhdl
ncvhdl -v93 pmul8_test.vhdl
ncelab -v93 pmul8_test:circuits
ncsim -batch -logfile pmul8_test.out \
-input pmul8_test.run pmul8_test
part1.out: part1.vhdl add32.vhdl bshift.vhdl part1.chk \
part1.abs part1.run
ncvhdl -v93 add32.vhdl
ncvhdl -v93 bshift.vhdl
ncvhdl -v93 part1.vhdl
ncelab -v93 part1:schematic
ncsim -batch -logfile part1.out -input part1.run part1
diff -w part1.run part1.chk
3) tcsh
source vhdl_cshrc
gmake # results come to screen gmake on Solaris, make on Linux
# "don't know how to make xxx.yyy" file missing or typo
# much output flying by on screen, use:
gmake |& more # hit space for next page, enter for next line
gmake >& part1.prt # results, including error go to a file
# use editor to read file, you can search
4) FIX THE FIRST ERROR !!!!
Yes, you can fix other errors also, but one error can cause
a cascading effect and produce many errors.
Don't panic when there was only one error, you fixed that,
then the next run you get 37 errors. The compiler has stages,
it stops on a stage if there is an error. Fixing that error
lets the compiler move to the next stage and check for other
types of errors. Go to step 3)
5) Don't give up. Don't make wild guesses. Do experiment with
one change at a time. You may actually have to read some
of the handouts :)
6) Your circuit compiles and simulates but the output is not
correct. Solution: add debug print.
Most circuits in this course have a print process. You can
easily add printout of more signals. Look for the existing
code that has 'write' and 'writeline' statements.
To print out some signal, xxx, after a 'writeline' statement add
write(my_line, string'(" xxx=")); -- label printout
hwrite(my_line, xxx); -- hex for long signals
write(my_line, string'(" enb="));
write(my_line, enb); -- bit for single values
writeline(output, my_line); -- outputs line
7) You have a signal that seems to be wrong and you can not find
when it gets the wrong value. OK, create a new process to
print every change and when it occurs.
prtxxx: process (xxx)
variable my_line : LINE; -- my_line needs to be defined
begin
write(my_line, string'("xxx="));
write(my_line, xxx); -- or hwrite for long signals
write(my_line, string'(" at="));
write(my_line, now); -- "now" is simulation time
writeline(output, my_line); -- outputs line
end process prtxxx;
When adding 'write' statements, you may need to add the
context clause in front of the enclosing design unit. e.g.
library STD;
use STD.textio.all; -- defines LINE, writeline, etc.
library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_textio.all; -- defines write on std_logic (_vector)
8) Read your code.
Every identifier must be declared before it is used.
Every signal MUST be set exactly once, e.g.
xxx <= a;
xxx <= b; -- somewhere else, BAD !
a0: fadd port map(a(0), b(0), cin , sum(0), c(0));
a1: fadd port map(a(1), b(1), c(0), sum(1), c(0));
#### BAD !
Signals must match in type and size. An error having
"shape mismatch" means incompatible size. You can not put
one bit into a 32 bit signal nor 32 bits into a one bit signal.
"...type... error" Are you putting an integer into a std_logic?
You can not put an identifier of type std_logic into
std_logic_vector. a(31 downto 28) is of type std_logic_vector,
a(31) is of type std_logic.
Everywhere a specific signal name is used, these points are
wired together. For VHDL simulation purposes, all points on a
wire always have exactly the same value. Zero propagation delay
through a wire. Be careful what you wire together. Use the VHDL
reserved word 'open' for open circuits rather than NC for
no connection.
ncsim: 05.40-s011: (c) Copyright 1995-2005 Cadence Design Systems, Inc.
ncsim> run 7 ns
A= 1 B= 1 C= U D= U CNC= U DNC= U NC= U at time 0 ns
A= 1 B= 1 C= U D= U CNC= U DNC= U NC= U at time 1 ns
A= 1 B= 1 C= 1 D= U CNC= U DNC= U NC= U at time 2 ns
A= 1 B= 1 C= 1 D= U CNC= U DNC= U NC= U at time 3 ns
A= 1 B= 1 C= 1 D= 1 CNC= U DNC= U NC= U at time 4 ns
A= 1 B= 1 C= 1 D= 1 CNC= U DNC= U NC= U at time 5 ns
A= 1 B= 1 C= 1 D= 1 CNC= U DNC= U NC= U at time 6 ns
Ran until 7 NS + 0
ncsim> exit
!!! !!! never set due to connection
-- use_open.vhdl
library IEEE;
use IEEE.std_logic_1164.all;
entity AN is
port(IN1 : in std_logic;
IN2 : in std_logic;
OUTB : inout std_logic; -- because used internally, bad design
OUTT : out std_logic);
end entity AN;
architecture circuits of AN is
begin -- circuits
OUTB <= IN1 nand IN2 after 1 ns;
OUTT <= not OUTB after 1 ns;
end architecture circuits; -- of AN
library IEEE;
use IEEE.std_logic_1164.all;
use STD.textio.all;
use IEEE.std_logic_textio.all;
entity use_open is
end entity use_open;
architecture circuits of use_open is
signal A : std_logic := '1';
signal B : std_logic := '1';
signal C, CNC : std_logic;
signal D, DNC : std_logic;
signal NC : std_logic := '1'; -- for no connection or tied off
begin
my_print : process is
variable my_line : line;
begin
write(my_line, string'("A= "));
write(my_line, A);
write(my_line, string'(" B= "));
write(my_line, B);
write(my_line, string'(" C= "));
write(my_line, C);
write(my_line, string'(" D= "));
write(my_line, D);
write(my_line, string'(" CNC= "));
write(my_line, CNC);
write(my_line, string'(" DNC= "));
write(my_line, DNC);
write(my_line, string'(" NC= "));
write(my_line, NC);
write(my_line, string'(" at time "));
write(my_line, now);
writeline(output, my_line);
wait for 1 ns;
end process my_print;
n01: entity WORK.AN port map(A, B, open, C);
n02: entity WORK.AN port map('1', C, open, D);
n03: entity WORK.AN port map(A, B, NC, CNC);
n04: entity WORK.AN port map('1', CNC, NC, DNC);
end architecture circuits; -- of use_open
Truth tables using type std_logic
t_table.vhdl
Now, some Cadence VHDL error messages.
-- error.vhdl demonstrate VHDL compiler error messages
library IEEE;
use IEEE.std_logic_1164.all;
entity AN is
port(IN1 : in std_logic;
IN2 : in std_logic;
OUTB : inout std_logic; -- because used internally
OUTT : out std_logic;);
end entity AN;
architecture circuits of AN is
signal aaa : std_logic;
begin -- circuits
OUTB <= aa and IN1 and IN2 after 1 ns;
OUTT <= not OUTB after 1 ns;
end architecture circuits; -- of AN
tcsh
source vhdl_cshrc
ncsim -v93 error.vhdl >& error.out # note "|" and line numbers
ncvhdl: 05.40-s011: (c) Copyright 1995-2005 Cadence Design Systems, Inc.
OUTT : out std_logic;);
|
ncvhdl_p: *E,PORNKW (error.vhdl,10|28): identifier expected.
OUTT : out std_logic;);
|
ncvhdl_p: *E,MISCOL (error.vhdl,10|28): expecting a colon (':') 87[4.3.3] 93[4.3.2].
OUTT : out std_logic;);
|
ncvhdl_p: *E,PORNKW (error.vhdl,10|31): identifier expected.
OUTT : out std_logic;);
|
ncvhdl_p: *E,MISCOL (error.vhdl,10|31): expecting a colon (':') 87[4.3.3] 93[4.3.2].
end entity AN;
|
ncvhdl_p: *E,EXPRIS (error.vhdl,11|13): expecting the reserved word 'IS' [1.1].
OUTB <= aa and IN1 and IN2 after 1 ns;
|
ncvhdl_p: *E,IDENTU (error.vhdl,16|11): identifier (AA) is not declared [10.3].
Now you are ready to tackle Homework 6
sqrt examples of simplify
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