help on coding pls~

[leon86]

A device reads in a 4 bits input. Example '0001'. Then for '0001' it will
activate the timer for 30 minutes and start to count down until it reaches
0 min. After that is will output a '00'. I have a crystal oscillator with
1Mhz frequency.


I having problem on how to write the code for the timer to be set at 30
min in VHDL and then start to count down with the 1Mhz frequency.


Someone pls help me~~

Thanks

 

[backhus]

Hi Leon,
what's so difficult in dividing 30 min by 1 µs (Period time of 1MHz)?
Then you have a number to count to.

Of course you can not use time units to build a counter.

time <= time + 1 us; -- does not work for synthesis!


have a nice synthesis

Eilert

 

[Jeremy Ralph]

Perhaps something like this...

LIBRARY ieee;
USE ieee.std_logic_1164.ALL;
USE ieee.numeric_std.ALL;

ENTITY countdown IS
PORT(
clk : IN STD_LOGIC;
rstb : IN STD_LOGIC; -- active low async reset
activation : IN STD_LOGIC_VECTOR(3 DOWNTO 0);
the_output : OUT STD_LOGIC_VECTOR(1 DOWNTO 0)
);
END countdown;

ARCHITECTURE rtl OF countdown IS
CONSTANT ACTIVATION_CODE : STD_LOGIC_VECTOR(activation'RANGE) :=
"0001";
CONSTANT CYCLES_PER_SEC : natural := 10**6;
CONSTANT COUNTDOWN_LOAD : natural := CYCLES_PER_SEC * 60 * 30 - 1;
SIGNAL the_output_r : std_logic_vector(1 DOWNTO 0);
SIGNAL countdown_r : natural RANGE 0 TO COUNTDOWN_LOAD-1;
SIGNAL counting_r : boolean;
BEGIN

the_output <= the_output_r;

countdown_proc: PROCESS(clk,rstb)
BEGIN
IF rstb = '0' THEN
the_output_r <= (OTHERS => '1');
countdown_r <= COUNTDOWN_LOAD-1;
counting_r <= FALSE;
ELSIF clk'event AND clk = '1' THEN
IF (counting_r) THEN
IF (countdown_r = 0) THEN
-- countdown hit zero
counting_r <= FALSE;
countdown_r <= COUNTDOWN_LOAD-1;
the_output_r <= (OTHERS => '0');
ELSE
countdown_r <= countdown_r - 1;
the_output_r <= (OTHERS => '1');
END IF;
ELSE
IF activation = ACTIVATION_CODE THEN
counting_r <= TRUE;
END IF;
END IF;
END IF;
END PROCESS countdown_proc;

END rtl;