Error in clock divider from FAQ

[yolanda3000]

Hello,

the clock divider from the FAQ is:

architecture Behavior of ClockDivider is
begin
process (ClkIn, Reset)
variable Count: Natural range 0 to Modulus-1;
begin
if Reset = '1' then
Count := 0;
ClkOut <= '0';
elsif ClkIn = '1' and ClkIn'event then
if Count = Modulus-1 then
Count := 0;
else
Count := Count + 1;
end if;
if Count >= Modulus/2 then
ClkOut <= '0';
else
ClkOut <= '1';
end if;
end if;
end process;
end Behavior;

In my simulation (ModelSim) the first '1' cycle of ClkOut is too short
by one ClkIn cycle.
Fixes for that:
- Initialize count with Modulus-1 in reset OR
- Count up _after_ ClkOut is set

I wonder why nobody came accross this problem. It's there quite
a long time.
I don't have information who is maintaining the FAQ and who could
fix this.

Bye,
Y

 

[charles.elias]

Hello,

the clock divider from the FAQ is:

architecture Behavior of ClockDivider is
begin
process (ClkIn, Reset)
variable Count: Natural range 0 to Modulus-1;
begin
if Reset = '1' then
Count := 0;
ClkOut <= '0';
elsif ClkIn = '1' and ClkIn'event then
if Count = Modulus-1 then
Count := 0;
else
Count := Count + 1;
end if;
if Count >= Modulus/2 then
ClkOut <= '0';
else
ClkOut <= '1';
end if;
end if;
end process;
end Behavior;

In my simulation (ModelSim) the first '1' cycle of ClkOut is too short
by one ClkIn cycle.
Fixes for that:
- Initialize count with Modulus-1 in reset OR
- Count up _after_ ClkOut is set

I wonder why nobody came accross this problem. It's there quite
a long time.
I don't have information who is maintaining the FAQ and who could
fix this.

Bye,
Y

I just simulated the divider using Active-HDL and got the same result
that you did. You are very observant! I have used this divider a
number of time and never noticed the problem.

Charles

 

[Zara]

I just simulated the divider using Active-HDL and got the same result
that you did. You are very observant! I have used this divider a
number of time and never noticed the problem.

Charles

The reason is pretty simple: In normal cycles, ClkOut changes from '0'
to '1' when Count changes from Modulus/2 to Modulus/2+1, and the other
way round when Count changes from '0' to '1'. So, simlutaneously with
Count=1, ClkOut='1'.

But on reset, Count is given the value '0' and ClkOut too, not
corresponding to the rest of the time.
Solutions:

a) Either initialize ClkOut on reset with the value '1'
b) Or change ths sign of both ClkOut transitions