I have problems on communicating between the processes. I used to use flag and clearFlag to tackle this, but it's kind of annoying and not looking good. What is the best practice to handle this? Here is a sample code on how I did it before:
Proc_A : process (clk, reset, clrFlag)
begin
if clrFlag = '1' then
flag <='0';
elsif reset = '0' then
A <= (others => '0');
elsif rising_edge (clk) then
A <= in;
flag <= '1';
end if;
end process;
Proc_B : process (clk, reset)
begin
if reset = '0' then
B <= (others => '0');
elsif rising_edge (clk) then
if flag = '1' then
B <= data;
clrFlag <= '1';
else
clrFlag <= '0';
end if;
end if;
end process;
This way works but I don't think it is nice method. I have to write a flag and clrFlag couple to do this task. All I want to do is when something happened (e.g. A <= in;), it triggers another proc, Proc_B for example, to run once or a number of times. What is the best practice to this problem? Thanks!
For simulation, you can make a process wait on a signal:
Proc_B : process
begin
wait until flag'event;
B <= data;
end process;
and just write the flag with its inverse every time you need something to happen.
In synthesisable logic, you either have to exchange flag signals, as you do, or use some other higher-level communication (like a FIFO, messagebox, or similar).
However, if all your proc_b logic takes place in a single cycle - so you can guarantee not to miss a flag, and to be able to keep up even if flag is asserted all the time (as it looks like you do) - you can do this (and combine the two processes):
Proc : process (clk, reset, clrFlag)
begin
flag <='0';
if reset = '0' then
A <= (others => '0');
B <= (others => '0');
elsif rising_edge (clk) then
if some_trigger_event = '1' then
A <= in;
flag <= '1';
end if;
-- recall that due to VHDL's scheduling rules, this "if" will take place
-- one clock cycle after the flag is written to, just as if it were in a
-- separate process
if flag = '1' then
B <= data;
end if;
end if;
end process;
Side note - your code is not ideal for synthesis... you really only want the reset part outside the clocked part:
Proc_A : process (clk, reset)
begin
if reset = '0' then
A <= (others => '0');
elsif rising_edge (clk) then
if clrFlag = '1' then
flag <='0';
else
A <= in;
flag <= '1';
end if;
end process;
Related
I've a module that have a 8bit input and a serial output, I want to serialize input data and synchronize it with a clock.
I want to set my data when falling edge then wait when clock rise, when clock fall again I set another data. I don't want to connect the directly reference clock to the output because when I don't use this module I want a 1 state on clock output.
I've tried this code:
process(Clock, ModuleReset)
begin
if ModuleReset = '0' then
OutData <= '0';
OutCK <= '0';
counter <= 7;
elsif falling_edge(Clock) then
OutCK <= '0';
OutData <= Data(counter);
elsif rising_edge(Clock) then
OutCK <= '1';
end if;
end process;
The synthesizer gives me this error:
"Asynchronous load of non-constant data for SCK is not supported"
When I separate the code in two blocks like this:
process(Clock, ModuleReset)
begin
if ModuleReset = '0' then
OutData <= '0';
OutCK <= '0';
counter <= 7;
elsif falling_edge(Clock) then
OutCK <= '0';
OutData <= Data(counter);
end process;
process(Clock)
if rising_edge(Clock) then
OutCK <= '1';
end if;
end process;
I have these two errors:
"Multiple non tristate drivers for net SCK"
"Unresolved tristate drivers for net SCK"
Another code I've tried is:
process(Clock, ModuleReset)
if ModuleEN = '1' then
OutCK <= Clock;
end if;
begin
if ModuleReset = '0' then
OutData <= '0';
OutCK <= '0';
counter <= 7;
elsif falling_edge(Clock) then
OutCK <= '0';
OutData <= Data(counter);
end if;
end process;
But the output clock looks strange with a different frequency.
Your last idea, which I understand ended up working for you, is sub-optimal. If your clock is slow, it should be fine, but I suggest you fix it nevertheless.
if ModuleEN = '1' then
OutCK <= Clock;
else
OutCK <= '1';
end if;
Yields combinational logic with the clock signals for the output. Having the clock used as a logic signal is never recommended, since clocks use clock paths which doesn't route well to general routing resources. The output signal will have potential glitches (very bad for an output interface!) and large delay/skew.
Your first approach, to use a DDR register to forward the clock, is indeed the correct and best approach. With this scheme, your clock only use clock paths, and if both the registers outputing the clock and data are situated in IO blocks, they will have the same output delay with very little skew.
You didn't specify the technology you're using, but I suggest you lookup how to write code that maps to DDR register for your synthesizer. Alternatively, you can manually instantiate the DDR output register primitive, likely ODDR for Xilinx or ALTDDIO_OUT for altera.
The problem with your attempts is indeed that you have multiple drivers for the same signal, or that you assign a signal on both rising and falling edge of a clock. This is not synthesizable.
Try this:
process(Clock, ModuleReset, ModuleEN)
begin
if ModuleEN = '1' then
OutCK <= Clock;
else
OutCK <= '1';
end if;
if ModuleReset = '0' then
OutData <= '0';
counter <= 7;
elsif falling_edge(Clock) then
OutData <= Data(counter);
end if;
end process;
We are using OR1200 for our project and we would like to assign an interrupt to the 8th button of FPGA Board. Here is the code to generate interrupt:
inrpt: process(CLK_I, RST_I)
begin
if RST_I = '1' then
butt_int_pul <= '0';
butt_int_tmp <= '0';
elsif rising_edge(CLK_I) then
if(DATA_I(8) = '1' and butt_int_tmp = '0') then
butt_int_pul <= '1';
else
butt_int_pul <= '0';
end if;
butt_int_tmp <= DATA_I(8);
end if;
end process inrpt;
process(CLK_I, RST_I)
begin
if RST_I = '1' then
butt_int <= '0';
elsif butt_int_pul = '1' then
butt_int <= '1';
elsif clear_int = '1' then
butt_int <= '0';
end if;
end process;
We only want this interrupt to be handled only once (holding the button should not call the interrupt again), that's why we included a flag to check this (butt_int_tmp).
The problem is that the interrupt call is not stable. It does not call each time we press the button. When we remove the flag, it works, but in this case, it is handled as many as we hold the button.
What are we doing wrong?
To start with, that second process is not properly written. It should have a structure equivalent to the first process (i.e., if(rising_edge(CLK_I)) surrounding all but the reset logic). You are currently describing a latch with multiple enable signals and wrong sensitivity list.
Moving on, there's no real reason you need that second process at all. You just need one register to act as interrupt (butt_int), and one to keep track of the previous state of the button (butt_prev). The interrupt is triggered for one cycle when DATA_I(8) is '1' while butt_prev is '0' (i.e., the button changed from not-pressed to pressed).
process(CLK_I, RST_I) begin
if(RST_I='1') then
butt_prev <= '0';
butt_int <= '0';
elsif(rising_edge(CLK_I)) then
if(DATA_I(8)='1' and butt_prev='0') then
butt_int <= '1';
else
butt_int <= '0';
end if;
butt_prev <= DATA_I(8);
end if;
end process;
Note that this will only work if your button is properly debounced, otherwise you are likely to get multiple interrupts triggered when you press (or even release) the button.
Its best not to think about interrupts. As you're targetting an FPGA, you're describing digital logic, not a software processor.
There a numerous way to build a circuit with the behaviour you want.
The simplest is probably a re-timed latch
signal latched_button : std_logic;
signal meta_chain : std_logic_vector(2 downto 0);
p_async_latch: process(rst_i,data(8))
begin
if rst_i = '1' then
latched_button <= '0';
elsif data(8) = '1' then
latched_button <= '1';
end if;
end process;
p_meta_chain: process(rst_i,clk_i)
begin
if rst_i = '1' then
meta_chain <= (others => '0');
elsif rising_edge(clk_i) then
meta_chain <= meta_chain(1 downto 0) & latched_button;
end if;
end process;
button_int <= '1' when meta_chain(2 downto 1) = "01" else '0';
This causes the button press to be latched asynchronously. The latched signal is then clocked along a shift register, and the interrupt is only valid for one cycle, which is the first clock cycle that the latch is seen on the clock domain.
Let's say I have two registers named reg_operand1 and reg_operand2. For both of them I have an appropriate write-enable signal. Somewhere I read I should have separate process for each register assignment, something like this:
process(CLK, RESET)
begin
if (RESET = '1') then
reg_operand1 <= (others => '0');
elsif (CLK'event and CLK = '1') then
if reg_operand1_we='1' then
reg_operand1 <= DI;
end if;
end if;
end process;
process(CLK, RESET)
begin
if (RESET = '1') then
reg_operand2 <= (others => '0');
elsif (CLK'event and CLK = '1') then
if reg_operand2_we='1' then
reg_operand2 <= DI;
end if;
end if;
end process;
But what happen if I merge the processes into this? Will the synthesized circuit be different? Also, what if I put "elsif" between the if-statements in merged process? Will the synthesizer insert a multiplexor into the circuit? Thanks!
process(CLK, RESET)
begin
if (RESET = '1') then
reg_operand1 <= (others => '0');
reg_operand2 <= (others => '0');
elsif (CLK'event and CLK = '1') then
if reg_operand1_we='1' then
reg_operand1 <= DI;
end if;
if reg_operand2_we='1' then
reg_operand2 <= DI;
end if;
end if;
end process;
The second one will produce exactly the same hardware as the first, and as already been said, contains less boilerplate.
If I understand your question about elsif correctly, you're proposing:
process(CLK, RESET)
begin
if (RESET = '1') then
reg_operand1 <= (others => '0');
reg_operand2 <= (others => '0');
elsif (CLK'event and CLK = '1') then
if reg_operand1_we='1' then
reg_operand1 <= DI;
elsif reg_operand2_we='1' then
reg_operand2 <= DI;
end if;
end if;
end process;
This generates different hardware, and also gives different behaviour.
In the example without the elsif, reg_operand2 is assigned DI when reg_operand2_we is high, irrespective of the state of reg_operand1_we.
When elsif is used, the assignment to of reg_operand2 only occurs when reg_operand2_we is high and reg_operand1_we is low
In general, if the two assignments do not depend on each other, use a seperate if construct.
The second one is shorter and simpler, and will generate the same hardware. (from a brief inspection : i.e. assuming there are no accidental typos in one or the other)
An elsif in the second version, combining the two register writes, will just prioritise the registers; i.e. if you attempt to write to both registers by asserting both we signals in the same cycle, only reg_operand1 will actually be written. It will have no other effect on the design.
So...
Unless you have specific corporate style guides that prohibit it, use the second style as a general rule.
There may be a FEW cases where you want to COMPLETELY separate some functionality to make it clear that it IS separate; in that case it's better not to be dogmatic about this style; but USUALLY fewer lines of code means less to go wrong, especially where (as here) it's easier to read and understand.
"I read somewhere" ... it would be worth knowing where you read this. There are a LOT of excruciatingly bad books, teaching materials and example projects out there, waiting to ruin potential VHDL programmers, and it's worth publicising which ones to avoid...
I'm programming a N-bit non-restoring divider, but I faced a little problem.
I have an Operative Part (combinatorial) and a Control Part (Finite State Machine).
The Control Part has a 2 processes FSM, 1 for updating the next state and 1 for the "state sequence".
update: process(clk_in, next_state)
begin
if rising_edge(clk_in) then
current_state <= next_state;
end if;
end process;
And this is the second process:
control: process(current_state, start, S_in, counted)
variable sub_tmp : STD_LOGIC := '0';
begin
[...]
sub <= sub_tmp; -- sub is an output signal of my entity that goes in the Operative Part
case current_state is
when idle =>
if start='1' then
next_state <= init;
else
next_state <= idle;
end if;
when init =>
-- [...]
next_state <= subtract;
when subtract =>
en_A <= '1';
sub_tmp := '1';
next_state <= test;
when test => -- shift
en_Q <= '1';
if S_in='0' then
sub_tmp := '1';
else
sub_tmp := '0';
end if;
if counted=N/2-1 then
next_state <= finished;
else
next_state <= operation;
end if;
when operation =>
en_A <= '1';
next_state <= test;
when finished =>
stop <= '1';
next_state <= idle;
end case;
end process;
As you can see, I need to change the value of the sub ONLY in 2 cases (subtract and test), while I don't have to change in the other cases.
The problem is that when I try to synthesize this code it turns out that sub_tmp is a LATCH, but I don't want a latch.
I need to do something like this:
state 1 => set sub to '1' or '0' (depending on another input)
state 2 => do other operations (but sub must remain the value set before) and return to state 1
etc...
To clarify more: in certain states of my FSM (not all of them) I set the value of a variable (let's call it sub_tmp). In other states I don't change its value. Then let's say I have an output PIN called "sub_out". Now, independently of the variable value, I want to output its value to this pin (sub_out <= sub_tmp; or similar).
What am I missing?
What you are missing is the behavior you describe IS a latch. Anything with memory (ie: "in other states I don't change it's value") is either a latch or a register (flip-flop). If you don't want a latch or a register, you need to assign a specific value to the signal in each and every code path, and not let it 'remember' it's previous state.
Fellow SO users,
I'm programming my ADC (ADC0804 which is mounted on a breadboard connected to a Spartan-3 FPGA board). Now, I'm using this ADC to provide digital output for my humidity sensor. The ADC outputs an 8-bit value which I'm displaying on the LEDs on the FPGA board.
Now, I'm writing the state machine in such a way that the ADC would always continue to keep outputting values even when I vary the humidity level. But as for the current implementation I have, eventhough I'm looping back to the first state, I'm not getting a continuous stream of values. I'm only getting one 8-bit value at a time (I.E.; I have to keep pressing the reset button to update the value displayed on the LEDs). The following is my code.
FSM_NEXT_STATE_INIT : PROCESS (CLK, RST)
BEGIN
IF (RST = '1') THEN
CURR_STATE <= STARTUP;
ELSIF (CLK'EVENT AND CLK = '1') THEN
CURR_STATE <= NEXT_STATE;
END IF;
END PROCESS;
START_FSM : PROCESS (CURR_STATE, INTR)
BEGIN
CASE CURR_STATE IS
WHEN STARTUP =>
NEXT_STATE <= CONVERT;
WR <= '0';
READ_DATA <= '0';
WHEN CONVERT =>
IF (INTR = '0') THEN
NEXT_STATE <= READ1;
ELSE
NEXT_STATE <= CONVERT;
END IF;
WR <= '1';
READ_DATA <= '0';
WHEN READ1 =>
NEXT_STATE <= READ2;
WR <= '1';
READ_DATA <= '1';
WHEN READ2 =>
NEXT_STATE <= STARTUP;
WR <= '1';
READ_DATA <= '0';
WHEN OTHERS =>
NEXT_STATE <= STARTUP;
END CASE;
END PROCESS;
PROCESS (CLK, RST)
BEGIN
IF (RST = '1') THEN
Y <= (OTHERS => '0');
ELSIF (CLK'EVENT AND CLK = '1') THEN
IF (READ_DATA = '1') THEN
Y <= D7&D6&D5&D4&D3&D2&D1&D0; --Concatenate the 8-bit ADC output
END IF;
END IF;
END PROCESS;
You'll notice that in state 'READ2', I'm looping back to the beginning (so that I can keep reading values continuously as the states transition) but somehow I don't think this is working. Could anyone please provide some assistance on how to go about solving this?
After a look on the data sheet for the ADC0804 I found following which could be the/a possible reason:
Note: Read strobe must occur 8 clock periods (8/fCLK) after assertion of interrupt to guarantee reset of INTR.
Inserting a WAIT state between CONVERT and READ1 might fix the problem.