09-02-2009 08:22 PM
I have simulated a verilog design that used some propagation delays like:
and #20 (out, in1, in2); (using #20 as 20 nanoseconds)
and the simulation works well. Now I want to implement these delays on a Spartan 3 board, but after going through the documentation I still can't get a handle on how to do that.
btw I'm using ISE 11 Web Pack
09-02-2009 08:27 PM
I'm thinking of just using a timer from the on board 50 MHz clock, but now I'm wondering how can I access that clock in verilog? And I'm assuming 1/50MHz = 20ns is the shortest delay I can get?
09-03-2009 11:12 AM
sorry dont know verilog, we speak the other language over here,
You can not synthesise a delay into silicon. If you think of what the silicon would have to do, it would be a string of gates with a fixed delay.
You need to think syncronously, things happen on or because of a clock.
The Xilinx FPGA's have various DLL and PLL options, dependent upon the device , usefull amongst other things for making a faster internal clock.
You can also, if you are careful use both the rising and falling edge of a clcok, or even pahse shifts of a clock to get aparantly faster circuits.
As for how to acces the timer.
When you make the timer, it will have a counting register, you access that.
Can I suggest you start with a few on line tutorials, they might break you in tho this sort of thinking.
09-03-2009 01:43 PM
Delay constructs in either VHDL or Verilog are not synthesizable into silicon they are intended for simulation purposes only.
If you have a clock that is 50 MHz (20ns) then you every register stage that is added will delay the signal by 1 clock period (20nS).
09-03-2009 05:08 PM
#20 is a simulation construct...you can only have it working when in simulation software like ModelSim...if you read the stuff that the project navigator console prints out while synthesizing...if you something like #20 in your code it says that it is ignoring those delays...
If you want actual delays and dont want to use interrupts from timers (like, lets say in a MicroBlaze design) you can use something like this:
This will toggle the bit every 1 second on a 20ns (50MHz) clock like that on the Spartan 3 board...