VHDL Help Requested

[Altera_Forum]

Hello. I'm a VHDL newbie and request help on coding a counter. I'm trying to synchronize a BTRIG signal to the system clock and then toggle a short_long_sample_q signal every 4 synchronized BTRIG pulses. Here's the applicable section of code: 

  process ( SYS_CLK )  begin  if rising_edge ( SYS_CLK ) then  temp_pulse <= BTRIG ;  btrig_synched <= temp_pulse ;  end if ;  end process ;    process ( btrig_synched )  begin  if rising_edge ( btrig_synched ) then  btrig_counter <= btrig_counter + 1 ;  end if ;  end process ;     process ( btrig_counter , short_long_sample_q )  begin  if btrig_counter = "11" then   short_long_sample_q <= not short_long_sample_q ;  end if ;  end process ;   I don't get any compile errors but the tested behavior shows that short_long_sample_q does not consistently toggle every 4 BTRIGs. Is there a better way of coding this? Thanks.

[Altera_Forum]

Hello 

 In your code the process controlling short_long_sample_q is asynchronous, which means that it will keep toggling this signal until btrig_counter is different than "11". This is possibly why you are experiencing strange behaviors.  Make sure that SYSCLK > 2*BTRIG or else you will not be able to detect a rising edge on this signal  I didn't test this code, but you can give it a try:  process ( SYS_CLK ) begin if rising_edge ( SYS_CLK ) then if (btrig_previous = '0') and (BTRIG = '1') then -- Rising edge detected if (btrig_counter = "11") then -- Fourth rising edge detected short_long_sample_q <= not short_long_sample_q; btrig_counter <= "00"; else btrig_counter <= btrig_counter + 1; end if end if btrig_previous <= BTRIG; end if end process

[Altera_Forum]

For the next time, please post your code in code tags. This makes it easier to read.

[Altera_Forum]

process ( btrig_counter , short_long_sample_q ) begin if btrig_counter = "11" then short_long_sample_q <= not short_long_sample_q ; end if ; end process ; 

you have combinatorial loop. Avoid it. add Sys_clk and rewrite equation, after clock willbe added ,for short_long_sample_q you should assign when btrig_counter = 3 also define btrig_counter as integer range 0 to 3; if it applicable.  process ( btrig_synched ) begin if rising_edge ( btrig_synched ) then btrig_counter <= btrig_counter + 1 ; end if ; end process ;  it is better to have synchronus counter with sys_clk and btrig_synch as synchonous load signal  process ( SYS_CLK ) begin if rising_edge ( SYS_CLK ) then temp_pulse <= BTRIG ; btrig_synched <= temp_pulse ; end if ; end process ; process ( btrig_synched ) begin if rising_edge ( btrig_synched ) then btrig_counter <= btrig_counter + 1 ; end if ; end process ;  and in the pair you can try better to make Pulse-Out generator . You simply get it. But yes do some arrangment in frequency until you get "0".