clk event vs rising edge

Question

I had always used this for detecting a rising edge   if  clk event and clk  1   then  but this can also be used   if rising edge clk  then  Reading this post  rising edge clk  is recommended  but there is also a comment indicating that rising edge clk  could lead to wrong behaviour   I can t decide which one to choose for the future  going on with  clk event and clk  1   or adopting rising edge clk    Any real-world expereince on these two  Any preferences   Thanks

User · Answer

Practical example   Imagine that you are modelling something like an I2C bus  signals called SCL for clock and SDA for data   where the bus is tri-state and both nets have a weak pull-up   Your testbench should model the pull-up resistor on the PCB with a value of  H    scl  lt    H   -- Testbench resistor pullup   Your I2C master or slave devices can drive the bus to  1  or  0  or leave it alone by assigning a  Z   Assigning a  1  to the SCL net will cause an event to happen  because the value of SCL changed    If you have a line of code that relies on  scl event and scl    1    then you ll get a false trigger  If you have a line of code that relies on  rising edge scl   then you won t get a false trigger    Continuing the example  you assign a  0  to SCL  then assign a  Z    The SCL net goes to  0   then back to  H    Here  going from  1  to  0  isn t triggering either case  but going from  0  to  H  will trigger a rising edge scl  condition  correct   but the  scl event and scl    1   case will miss it  incorrect    General Recommenation   Use rising edge clk  and falling edge clk  instead of clk event for all code

User · Answer

The linked comment is incorrect    L  to  1  will produce a rising edge   In addition  if your clock signal transitions from  H  to  1   rising edge clk  will  correctly  not trigger while  clk event and clk    1    incorrectly  will    Granted  that may look like a contrived example  but I have seen clock waveforms do that in real hardware  due to failures elsewhere

User · Answer

rising edge is defined as   FUNCTION rising edge   SIGNAL s   std ulogic  RETURN BOOLEAN IS BEGIN     RETURN  s EVENT AND  To X01 s     1   AND                          To X01 s LAST VALUE     0     END   FUNCTION To X01    s   std ulogic   RETURN  X01 IS BEGIN     RETURN  cvt to x01 s    END   CONSTANT cvt to x01   logic x01 table                            X    --  U                        X    --  X                        0    --  0                        1    --  1                        X    --  Z                        X    --  W                        0    --  L                        1    --  H                        X    --  -                           If your clock only goes from 0 to 1  and from 1 to 0  then rising edge will produce identical code  Otherwise  you can interpret the difference   Personally  my clocks only go from 0 to 1 and vice versa  I find rising edge clk  to be more descriptive than the  clk event and clk    1   variant

[vhdl] clk'event vs rising_edge()

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