1. Dataflow Style Combinational Design with VHDL
IAY 0600 Digital Systems Design
Alexander Sudnitson
Tallinn University of Technology

2. Combinational systems
Combinational systems have no memory. A combinational system's outputs are a function of only its present input values.
No latches/FFs or closed feedback loop
Combinational system description can be in dataflow, behavioral, or structioral styles.
Concurrent signal assignment signal is basic for dataflow style combinational design.

3. More about architecture body. Coding styles
An architecture body defines either the behavior or structure of a design entity—how it accomplishes its function or how it is constructed. As such, it provides the internal view of a design entity.
We can have different architectures associated with the same entity declaration, creating different design entities.
An architecture can be written in one of three basic coding styles: dataflow, behavioral, or structural. The distinction between these styles is based on the type of concurrent statements used:
• A dataflow architecture uses only concurrent signal assignment statements.
• A behavioral architecture uses only process statements.
• A structural architecture uses only component instantiation statements.

4. BIT versus STD_LOGIC BIT type can only have a value of ‘0’ or ‘1’
STD_LOGIC can have nine values
'U',‘0’,’1’,’X’,’Z’,’W’,’L’,’H’,’-’
Useful mainly for simulation
‘0’,’1’, and ‘Z’ are synthesizable

5. Dataflow style half-adder description
library ieee;
use ieee.std_logic_1164. all;
entity half_adder is
port (a, b : in std_logic; sum, carry_out : out std_logic);
end half_adder;
architecture data_flow of half_adder is
begin
sum <= a xor b ; -- concurrent signal assignment
carry_out <= a and b ; -- concurrent signal assignment
end data_flow;
Signal assignment statement placed in the statement part of an architecture is a concurrent statement.

6. Behavioral style half-adder coding
entity half_adder is
port (a, b : in std_logic; sum, carry: out std_logic);
end half_adder;
architecture behavior of half_adder is
begin
ha: process (a, b)
if a = ‘1’ then
sum <= not b ; carry <= b ; --sequential signal assignments
else
sum <= b ; carry <= ‘0’ ;
end if;
end process ha ;
end behavior ;
We can have different architectures associated with the same entity declaration, creating different design entities.

7. Signal assignment statement
Signal assignment statement placed in the statement part of an architecture is a concurrent statement.
Concurrent signal assignment statements assigns a new value to the signal on the left-hand side of the signal assignment symbol (<=) whenever there is an event on one of the signals on the right-hand side of the assignment symbol. The signal assignment statement is said to be sensitive to the signals on the right-hand side of the statement. The symbol <= is read as “gets.”
We consider three kinds of concurrent signal assignment:
Concurrent signal assignment statement using a Boolean expression
Selected signal assignment statement
Conditional signal assignment statement

8. Concurrent signal assignment with a closed feedback loop
Signal assignment statement with a closed feedback loop:
a signal appears in both sides of a concurrent assignment statement
For example, q <= ((not q) and (not en)) or (d and en);
Syntactically correct
But form a closed feedback loop
Should be avoided

9. Concurrent signal assignment using a Boolean expression (4 bit equality comparator example)

10. Concurrent signal assignment using a Boolean expression (4 bit equality comparator example)

11. Concurrent signal assignment using a Boolean expression (4 bit equality comparator example)

12. type boolean is (false, true) ;
The result from evaluating a condition is type boolean.
type boolean is (false, true) ;
The leftmost literal in an enumeration listing is in position 0. Each enumeration type definition defines an ascending range of position numbers.
Example:
type std_ulogic is ('U', 'X', '0', '1', 'Z', 'W', 'L', 'H', '-') ;
' Z ' (with position number 4) iz greater than ' 1 ' (with position number 3

13. Conditional signal assignment statement (4 bit equality comparator example)

14. 4-to-1 multiplexer using a Boolean expression
library ieee;
use ieee.std_logic_1164.all
entity mux4to1 is
port (c3, c2, c1, co: in std_logic;
g_bar, b, a: in std_logic;
y: out std_logic);
end mux4to1;
architecture dataflow of mux4to1 is
begin
y <= not g_bar and (
(not b and a and c0) or
(not b and a and c1) or
(b and not a and c2) or
(b and a and c3));
end dataflow;
What is STD_LOGIC?

15. Selected signal assignment statement
with select_expression select
signal_name <=
value_expr_1 when choice_1,
value_expr_2 when choice_2,
value_expr_3 when choice_3,
. . .
value_expr_n when choice_n;
The value of one and only one choice must equal the value of the select expression.

16. Selected signal assignment statement. Example mux4to1
library ieee;
use ieee.std_logic_1164.all
entity mux4to1 is
port (c3, c2, c1, c0: in std_logic;
g_bar, b, a: in std_logic;
y: out std_logic);
end mux4to1;
architecture selected of mux4to1 is
begin
with std_logic_vector' (g_bar, b, a) select
y <= c0 when "000", c1 when "001",
c2 when "010", c3 when "011"
'0' when others; -- default
end selected;
Here we use an aggregate (g_bar, b, a) in a select expression

17. Type qualification. Choices’ completeness
We must explicitly specify the aggregat's (g_bar, b, a) type. This is accomplished using a type qualification ('):
std_logic_vector' (g_bar, b, a).
If we use this aggregate alone, the compiler cannot tell the aggregate’s type from the context. The compiler does not simply assume that an aggregate of std_logic elements is type std_logic_vector, since there are other array types, such as unsigned, that have std_logic elements (lect. 7)

18. Type qualification. Choices’ completeness
To describe combinational logic, the choices listed in a selected signal assignment must be all inclusive. That is, they must include every possible value of the select expression.
The last clause (default assignment) uses the keyword others to assign the value of 0 to y for the 725 values of the select expression not explicitly listed.
9**3=729, 729-4=725
y <= c0 when "000",
c1 when "001",
c2 when "010",
c3 when "011"
'0' when others;

19. Example mux4to1 library ieee ; use ieee.std_logic_1164.all ;
entity mux4to1 is
port (c3, c2, c1, co: in std_logic;
g_bar, b, a: in std_logic; y: out std_logic);
end mux4to1;
architecture conditional of mux4to1 is
signal tmp : std_logic_vector (2 downto o);
begin
tmp <= (g_bar, b, a);
y <= c0 when tmp = "000" else
c1 when tmp = "001" else
c2 when tmp = "010" else
c3 when tmp = "011" else
'0' ; -- default assignment
end conditional;

20. Avoiding implied latches
The last value_expression must not have an associated when condition clause (default assignment).
For example, let’s take previous description.
If the conditional signal is written as:
y <= c0 when tmp = "000" else
c1 when tmp = "001" else
c2 when tmp = "010" else
c3 when tmp = "011" ;
the VHDL interpretation of this construct is that, for the 4 specified values of tmp, a new value should be assigned to y. This implies that for all other values of tmp the y should retain its previous value.
As a result, the synthesizer generates a latch at the output of the combinational logic to store the value of y.

21. Synthesised mux logic with undesired latch