variable timing signal

[Amit]

Hello group,

I will appreciate it if sombody give me only hint on this question. I
just don't get it!

Write a complete vhdl program to generate a variable timing signal
that has a period of 10ms if the control signal
C is '0' and has a period of 20ms if the control signal is '1'. Both
signals have a duty cycle of 20%. Assume that a one KHz clock signal
Clock 1ms is avaiable.

Your help is appreciated.

Amit

 

[KJ]

Hello group,

I will appreciate it if sombody give me only hint on this question. I
just don't get it!

Write a complete vhdl program to generate a variable timing signal
that has a period of 10ms if the control signal
C is '0' and has a period of 20ms if the control signal is '1'. Both
signals have a duty cycle of 20%. Assume that a one KHz clock signal
Clock 1ms is avaiable.

Your help is appreciated.

Amit

The problem statement you provided doesn't require the vhdl program to be
synthesizable so below is a sketch of how one could accomplish most of what
you asked for (left out is the 20% duty cycle). If this does need to be
synthesizable, I'd suggest a simple counter and use the 1 ms clock period
input (i.e. the 1KHz that I didn't need).

signal Clock_Period: time := 10 ms;
signal Clock: std_ulogic := '0';
....

Clock_Period <= 20 ms when (C = '1') else 10 ms;
Clock <= not(Clock) after Clock_Period;

KJ

 

[Amit]

The problem statement you provided doesn't require the vhdl program to be
synthesizable so below is a sketch of how one could accomplish most of what
you asked for (left out is the 20% duty cycle). If this does need to be
synthesizable, I'd suggest a simple counter and use the 1 ms clock period
input (i.e. the 1KHz that I didn't need).

signal Clock_Period: time := 10 ms;
signal Clock: std_ulogic := '0';
...

Clock_Period <= 20 ms when (C = '1') else 10 ms;
Clock <= not(Clock) after Clock_Period;

KJ- Hide quoted text -

- Show quoted text -

Hi KJ,

Thank you for your response. I have two questions:

1) why did you declare clock as std_ulogic not std_logic?

2)
Please correct me if I'm wrong:

I'm new to VHDL but I think since there is a sequence going on here so
I will need to put this in a process block. Right?

something like:

entity ent is

port(

clk : in std_logic;
q : out std_logic;
);

end ent;

architecture beh of ent is
begin

process(clk)
signal Clock_Period : time := 10 ms;
signal Clock : std_ulogic := '0';
begin

-- Your code

Clock_Period <= 20 ms when (C = '1') else 10 ms;
Clock <= not(Clock) after Clock_Period;

end process;

q <= clock;

end beh;


Any advice?

Thanks again.

 

[KJ]

Hi KJ,

Thank you for your response. I have two questions:

1) why did you declare clock as std_ulogic not std_logic?

Habit. I prefer std_ulogic over std_logic because then the compiler
immediately flags when I have two drivers on the same net. I don't
have to simulate and debug to find it.

2)
Please correct me if I'm wrong:

I'm new to VHDL but I think since there is a sequence going on here so
I will need to put this in a process block. Right?

No. The two signal assignments are simply concurrent signal
assignments there is no explicit process. The full blown thing is
shown below. Also, in the original post I had
Clock <= not(Clock) after Clock_Period;
which should have been
Clock <= not(Clock) after Clock_Period / 2;


library ieee;
use ieee.std_logic_1164.all;
entity ent is port(
C: in std_ulogic;
Q: out std_ulogic);
end ent;
architecture beh of ent is
signal Clock_Period : time := 10 ms;
signal Clock : std_ulogic := '0';
begin
Clock_Period <= 20 ms when (C = '1') else 10 ms;
Clock <= not(Clock) after Clock_Period / 2;
Q <= Clock;
end beh;


KJ

 

[beckjer]

Habit. I prefer std_ulogic over std_logic because then the compiler
immediately flags when I have two drivers on the same net. I don't
have to simulate and debug to find it.



No. The two signal assignments are simply concurrent signal
assignments there is no explicit process. The full blown thing is
shown below. Also, in the original post I had
Clock <= not(Clock) after Clock_Period;
which should have been
Clock <= not(Clock) after Clock_Period / 2;

library ieee;
use ieee.std_logic_1164.all;
entity ent is port(
C: in std_ulogic;
Q: out std_ulogic);
end ent;
architecture beh of ent is
signal Clock_Period : time := 10 ms;
signal Clock : std_ulogic := '0';
begin
Clock_Period <= 20 ms when (C = '1') else 10 ms;
Clock <= not(Clock) after Clock_Period / 2;
Q <= Clock;
end beh;

KJ- Hide quoted text -

- Show quoted text -

That's almost correct. You forgot that Amit was looking for a duty
cycle of 20%. The /2 makes it a 50% duty cycle.

Another fun way would be to create 2 clock sources, 1 run at 1mS the
other at 2mS. Feed both these clocks into a 10 decimal counter that
flips the bit on count 2 and 10. Then use the input 'C' to select
between the 2 clock sources.

-jerry

 

[KJ]

That's almost correct. You forgot that Amit was looking for a duty
cycle of 20%. The /2 makes it a 50% duty cycle.

As I mentioned in my first post, I intentionally left out the duty
cycle requirement...you have to leave at least some of the homework
for the student to accomplish.

KJ

 

[Mike Treseler]

I prefer std_ulogic over std_logic because then the compiler
immediately flags when I have two drivers on the same net. I don't
have to simulate and debug to find it.

Indeed, with the rare exception
of a real tri-state port pin, there is no
good reason *not* to use std_ulogic bits in my code.

No are conversions needed.

std_ulogic is 100% compatible with the
std_logic type I often find in code by others,
to which I must port map.

-- Mike Treseler

 

[Amit]

Indeed, with the rare exception
of a real tri-state port pin, there is no
good reason *not* to use std_ulogic bits in my code.

No are conversions needed.

std_ulogic is 100% compatible with the
std_logic type I often find in code by others,
to which I must port map.

-- Mike Treseler

So as conclusion using a signal of type std_ulogic is better. Right?