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Forum: FPGA, VHDL & Verilog Clock domain crossing

von Stefania M. (val_1010)

2020-11-17 16:08

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Hi! I have a doubt regarding clock domain crossing from a fast clock to 
a slower one. Is it possible to use the two flip flops scheme? If yes, 
would my code be okay?

sinc_in indicates if data in is valid, sinc_out if data out is valid in 
the secod clock domain and clk1 is faster than clk2, so clk2 is the new 
domain.

entity clock_sync is
     clk1 : in std_logic;
     clk2 : in std_logic;
     sync_in : in std_logic;
     Data_in : in std_logic_vector(7 downto 0);
     data_out : out std_logic_vector(7 downto 0);
     sync_out : out std_logic
architecture behavioral of clock_sync is
constant in: std_logic_vector (7 downto0):=(others=>0);
type ff is array(0 to 1) of std_logic_vector(7 downto 0);
signal sig_reg : ff:=(others=>in);
signal i: integer:=0;
  process(Clock2)
    begin
      if rising_edge(Clock2) and sync_in=1 then
        if i < 2 then
         i<=i+1;
         sig_reg(i)<= Data_in;
         sync_out<=0;
        else data_out<=sig_reg(i-1);
             sync_out<=1;
        end if;
      end if;
   end process;
end behavioral;


Thank you!!

2020-11-17 16:32: Edited by User

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Re: Clock domain crossing

von -gb- (Guest)

2020-11-17 16:23

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Stefania M. wrote:
> if i < 2 then
>          i<=i+1;

Uiuiuiuiui, looks like a programming language ...

Stefania M. wrote:
> type ff is array(0 to 1) of std_logic_vector(7 downto 0);
>
> signal buffer : ff:=(others=>in);

Why not:

signal buffer: std_logic_vector(15 downto 0):=(others => '0');

Stefania M. wrote:
> Is it possible to use the two flip flops scheme? If yes,
> would my code be okay?

if Data_in is set for only one cycle of the fast clock, you can miss it 
with the slow clock. So no, this will not work reliable.

But Lothar has written a component which takes a short puls and converts 
it to an full clock cycle of another clock:
http://www.lothar-miller.de/s9y/archives/19-Kurzer-Spike-in-Puls-umgewandelt.html

Vivado from xilinx also has CDC XPMs which you could use. They are in 
the language templates XPM_CDC.

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Re: Clock domain crossing

von Duke Scarring (Guest)

2020-11-17 16:28

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-gb- wrote:
> buffer
Naming a signal 'buffer' is not the best idea, because it is a reserved 
word in VHDL:
https://www.xilinx.com/support/documentation/sw_manuals/xilinx11/ite_r_vhdl_reserved_words.htm

Duke

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Re: Clock domain crossing

von Lothar M. (Company: Titel) (lkmiller) (Moderator)

2020-11-17 16:37

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Stefania M. wrote:
> sinc_in indicates if data in is valid, sinc_out if data out is valid in
> the secod clock domain and clk1 is faster than clk2, so clk2 is the new
> domain.
Can you sketch this down in a timing diagram?

> Is it possible to use the two flip flops scheme?
Only when the data rate on the fast clock domain is slower than the clk 
on the slow clock domain.

> If yes, would my code be okay?
> if rising_edge(Clock2) and sync_in=1 then
Forget this, because sync_in is asynchronus to clk2 and then the usual 
happens: sync_in acts as an enable for the counter i which consists ofd 
several flipflops.
And so sync_in is routet to each of the flipflops. Finally you will get 
this:
http://www.lothar-miller.de/s9y/archives/64-State-Machine-mit-asynchronem-Eingang.html
Or, lets simply say the counter i is a FSM (as each counter is).
And never ever(!!) use an async input in a FSM

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Re: Clock domain crossing

von -gb- (Guest)

2020-11-17 16:59

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Duke Scarring wrote:
> Naming a signal 'buffer' is not the best idea, because it is a reserved
> word in VHDL:

You are right, sorry ...

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Re: Clock domain crossing

von Christophz (Guest)

2020-11-18 09:27

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Stefania M. wrote:
> Is it possible to use the two flip flops scheme?

Yes. But as Lothar pointed out, Sync_in is asynchronous to Clk2 so you 
have to apply the two flip flops to this signal.

And you have to validate that Sync_in is stable for enough time with 
regard to the slow clock. Either this is true due to your application or 
you make it true with a pulse-stretcher.

> If yes, would my code be okay?

No.

You applied the two flip flops to the parallel data. This can randomly 
fail due to latency differences in the separate parallel paths.
Have a look at the paper "Fourteen ways to fool your synchronizer", you 
hit number 12 on page 6 :-)

Example to do CDR in this case:
You synchronize Sync_in and then use this (let's call it Sync_clk2) 
signal to enable your single output register. Add another flip flop to 
Sync_clk2 to generate Sync_out so it will be high at the same time the 
data is valid at the register output.

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Re: Clock domain crossing

von Jan M. (jamo)

2020-12-19 22:13

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Hey,

the general approach is:

- create a 1-bit(!) toggle event in the fast clock domain (*_event) when 
your data changes
  - make sure that it is stable long enough so that in can be captured 
safely in the slow clock domain
- bring that into the slow clock domain with at least two FFs (*_meta 
and *_rt)
- create another delay FF of *_rt (*_rt_z) and compare that with xor or 
neq
  - if this comparison is true capture your bus from fast into slow 
clock domain

- you need to tell the synthesis tool your (known good) cdc paths (for 
Xilinx Vivado I always use set_max_delay constraints, better then just a 
tig/set_false_path)
- also use async_reg constraint on your *_meta/*_rt FF pairs
- for Altera/Intel they must be similiar constraints available

Good luck!

BR,
Jan :)

2020-12-19 22:23: Edited by User

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Re: Clock domain crossing

von SparkyT (Guest)

2021-01-06 08:19

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Hi,
does this technique for domain crossing from slow-to-fast apply when 
both CLK signals come from the same PLL and are in phase?
I have been using the 2 FF synchronization from fast-to-slow, bet never 
the other way around.

Thanks
T

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