Is this Code synthesizable and any suggestions

[Rama]

I have a question for the synthesis experts out there. I am a novice in
this area and am more of a functional verification person.

However, here is a piece pf VHDL code - that i have and works fine in
the simulation world.

However, when I program this same code onto thr FPGA - Xilinix ISE - I
don't see what I should be seeing ( was expecting the clock to turn off
)
The point of this piece of code was --
at time 0 --- have a default setup
wait for certain clock cycles -- reset the PLL
wait for some more clock cycles -- Reset the DSP
wait for some more clock cycles -- Then Shut the CLOCK off - the DSP
clock here.

I tried doing the above -- in the code below.

This works in the simulation -- I do see the expected result i.e

I want to see the FPGA_MIZER_CLK - shut down after certain clock cycles
and after the DSP is reset. So i tried doing the sequence I need to
like this..

The question is ----- Is this synthesizable ?

What are the pitfalls ?

(if you see I also have a counter and it is just counting
continuously....)

I would like suggestions if there's different ways of doing the same
here.

Thanks,
RC

=============================

I/O's declaration...

FPGA_PLL_RESETN : out std_logic;
FPGA_PLL_SCLK : out std_logic;
FPGA_PLL_SCLK_SEL : out std_logic;
FPGA_MIZER_CLK : out std_logic;
FPGA_PLL_SIN : out std_logic;
FPGA_PLL_SEN : out std_logic;
FPGA_PLL_LD : out std_logic;

FPGA_TRSTN : out std_logic;
FPGA_SCAN_EN : out std_logic;
FPGA_DSP_N : out std_logic;

......
......CLK26MHZ : in std_logic;


architecture behavioral ...
begin
signal MISER_PLL_RESETN : std_logic := '0' ;
signal MIZER_CLK : std_logic := '0' ;
signal MISER_DSP_N : std_logic := '0';
signal MISER_CLKOFF : std_logic :='0';
signal COUNT1 : integer :=0;

....
....
...

miser_control : process ( CLK26MHZ )
begin

MIZER_CLK <= CLK26MHZ;
if rising_edge ( CLK26MHZ ) then
COUNT1 <= COUNT1 + 1 ; --- internal counter counting clk cycles
end if;

end process;

process ( COUNT1 )
begin
if ( COUNT1 >= 50 ) THEN -- e_pll_resetn - goes high after 50 clk
cycles
MISER_PLL_RESETN<='1';
end if;
if ( COUNT1 >= 100 ) THEN
MISER_PLL_RESETN <='0';
end if;
if ( COUNT1 >= 150 ) THEN
MISER_DSP_N <= '1';
end if;
end process;

process ( MISER_DSP_N, COUNT1)
begin
-- enabling Clock Off
if ( MISER_DSP_N = '1' ) THEN
if ( COUNT1 >= 200 ) THEN
MISER_CLKOFF <= '1';
end if;
if ( COUNT1 <= 250 ) THEN
MISER_CLKOFF <= '0';
end if;

end if;

end process;

FPGA_MIZER_CLK <= MIZER_CLK when ( MISER_CLKOFF = '0' ) else
'0' when ( MISER_CLKOFF = '1' ) else
'0';

FPGA_PLL_RESETN<= MISER_PLL_RESETN ;
FPGA_DSP_N <= MISER_DSP_N ;

end behavioral;

 

[kennheinrich]

I doubt this is synthesizable. Although there's no hard definition of
synthesizable short of what your tool vendor tells you,
IEEE-1076.6-2004 talks about a normal set of coding styles and
structures for synthesis. If you want a synchronous process (and what
good designer doesn't , you need to stick to a pretty conservative
definition of clock (i.e. using rising_edge() or clk'event and clk='1'
style). Further, if you want to use a process with multiple wait
statements, the spec I mentioned (in clause 6.1.3.4 requires that "each
wait statement shall specify the same clock edge of a single clock").
If you don't want a synchronous process, all wait conditions must use
the same, identical async_condition.

It's unfortunate, but once you start enjoying the ease of designing
simulation-only testbench code, you get kind of locked into staying in
simulation.

HTH,

- Kenn

 

[Thomas Stanka]

Hi,

The question is ----- Is this synthesizable ?

Not really.

What are the pitfalls ?

See below

signal MISER_PLL_RESETN : std_logic := '0' ;
signal MIZER_CLK : std_logic := '0' ;
signal MISER_DSP_N : std_logic := '0';
signal MISER_CLKOFF : std_logic :='0';
signal COUNT1 : integer :=0;

A default value for a signal is not synthesisable. In general you need
a reset to set all FF in a default value. But this is OK for Xilinx
fpgas and should synthesis perfect.

miser_control : process ( CLK26MHZ )
MIZER_CLK <= CLK26MHZ;
if rising_edge ( CLK26MHZ ) then
COUNT1 <= COUNT1 + 1 ; --- internal counter counting clk cycles
end if;

This is impossible. Mizer_Clk would need a FF reacting on both clock
edges which is not available.
But it should be ok to move this line out of the process as a
concurrent statement.

process ( COUNT1 )

This process inferrs two latches.

process ( MISER_DSP_N, COUNT1)

This process inferrs one latch.

I don't think your really want latches, so you should write out all
else cases in a combinatorial process in order to avoid a latch.

bye Thomas

 

[Rama]

Thank you so much for the good feedback Thomans and Kenn.
Yes I did think that was the case -- that it was not synthesizable -
and I was guessing about the inference of latches too - however was not
sure -

Okay - thinking about this whole - sequence How about if I just put the
delays in there...as I know for sure I will be getting a 38.46 ns -
Clock ( 26 Mhz )

How about like this -- for experimental purposes to see if the Clock is
really turning off.

-- miser : 01/15/07 - rc

MIZER_CLK <= CLK26MHZ;

MISER_PLL_RESETN <= '0' ,
'1' after 1923 ns, -- 50 clks of 38.46 ns period
'0' after 3846 ns; -- 100 clks of 38.46 ns period

MISER_DSP_N <= '0',
'1' after 5769 ns; -- 150 clks of 38.46 ns

MISER_CLKOFF <= '0',
'1' after 1923 ns when (MISER_DSP_N ='1') else -- 50 clks from
time DSP-reset is high
'0' after 3846 ns when (MISER_DSP_N ='1') ; -- 100 clks from
DSP_RESET is high

FPGA_MIZER_CLK <= MIZER_CLK when ( MISER_CLKOFF = '0' ) else
'0' when ( MISER_CLKOFF = '1' ) else -- do I need else here ? can
I end here?
'0';



Thanks a lot for your time !

Regards,
Rama

 

[Mike Treseler]

Okay - thinking about this whole - sequence How about if I just put the
delays in there...as I know for sure I will be getting a 38.46 ns -
Clock ( 26 Mhz )

How about following some sort of synchronous template:

one : process(reset, clock) is
-- <Process declarations here>
begin
if reset = '1' then
init_regs_here;
elsif rising_edge(clock) then
update_regs_here;
end if;
update_ports_here;
end process one;


-- Mike Treseler

 

[Andy]

Your code will generate a synchronous counter (good).
But it will generate combinatorially decoded outputs (potentially bad;
they will be glitchy).

I would suggest you decode your outputs in the same process as the
counter, so they will be synchronous (registered) too, and not have
glitches which could cause problems elsewhere (especially if the resets
are used asynchronously).

Also, by coding everything in clocked process(es), you avoid the
possibility of latches which are "a bad thing".

Andy

 

[Rama]

All,

thank you so much. You have given me good insight about synthesis.
Yes you get locked into simulation mode TB's -- once you begin writing
sim TB's.

I had one more question on how to write this equvalent synthesizable
code for this one statement -

I had written a synchronous counter ( with synchronous reset internal
to the counter )

count_test : count1 ( clk => CLK26MHZ,
reset => reset,
cntout => CNT_OUT );

If I were to do this statement's equivalent code now --- How would I do
this in synthesis code of VHDL ? Any ideas/suggestions ?

MISER_PLL_RESETN <= '1' when (CNT_OUT >= 50 and CNT_OUT <= 100) else
'0' ;

Thanks much.

 

[kennheinrich]

You have to handle two things here:

(1) your code as-is is written outside of a process, using the
when-else style assignment.
(2) you want to convert a combinatorial decode into a synchronous one

The usual solution will put your decoder into a process, and force you
to use a sequential-style signal assignment, so no "when" condition
(issue 1). Issue 2 means you have to decode the condition one clock
early, so roll back your comparisons one value. Also, always remeber
the reset, so if the combinatorial decode wold be false one clock after
the counter is reset, so should your synchronous decode.

The benefit is that your reset output will be clean, so no accidental
glitches to spuriously reset the pll.

Roughly, the basic process is:

process (clk26mhz) is
begin
if rising_edge(clk26mhz) then
if ((cnt_out >= 49) and (cnt_out <= 99)) and (reset /= '1') then
MISER_PLL_RESETN <= '1';
else
MISER_PLL_RESETN <= '0';
end if; -- selection
end if; -- clk
end process;

- Kenn