To generate a periodic time-gate

[vizziee]

Hello,

I want to generate a signal which is periodically toggling for a
specified time of the order of, say 2 seconds i.e. it is like a clock
with 4 sec period.

What is available to me is a clock of 4MHz frequency.

I used the following approach, rather unsuccessfully: I used a counter
(a 20-bit counter rather ) to count the clock periods till it amounted
to 2 seconds. Using counter values I am toggling my output signal in a
concurrent statement. The counter should reset after 4 seconds but
since this counter doesn't count all the 20-bit values, it doesn't
reset automatically after 4 sec. So what can be the workaround? Can I
avoid a counter with 20-bits and do with a smaller one?

Sincerely,

 

[jens]

It sounds like you need a 23-bit counter... when 2s expires toggle your
output signal and reset the counter so it can count 2s again.

An LFSR is a small and fast counter that would be well-suited for this
application.

However, if you're using this for an internal signal, you're better off
using an enable signal instead of another clock signal (i.e. keep the
design synchronous).

 

[OutputLogic]

Online LFSR counter generator

There is an online tool that generates a Verilog code for LFSR Counters of any value up to 31-bit wide. It's on "http OutputLogic dot com" [sorry, this site doesn't let me post a link in a regular way]
LFSR counters are much smaller than regular ones. Looks like that's what you need.