Hi Kenn,
I truly appreciate your help and time you spent to expalin it. Ok,
I'll go with more details:
I'm planning to make a FIFO using (Quratus II and Altera MegaFunction
dcfifo) on DE2 board. Goal is palying music from a SD card or any
other storage unit. One of the steps is using a dcfifo is embedded
into the entity which I called it fifo.
As you know better, the main clock is 50Mhz which is what DE2 is
giving me but I need to have two other internal clocks (to drive the
read_clock signal and also drive a shift-register ).
These two clocks are clk0 and clk1.
clk0 which should be 18.432Mhz then base on this clock I need to have
clk1 which its frequency is 1.536Mhz.
Question: What is the proper method? do I have to write code to create
a clock with 18.432Mhz frequency? or only I can define a signal and
set it as clock in Waveform?
I did search for "vhdl+clock+custom" or ... but didn't find anything
helpful if you know please pass it to me.
I understand these questions are easy for members of this group and I
try not to post them as much as possible but sometimes I cannot find
everyting in books.
Once again thank you.
It can be kind of hard to just jump in and understand how to do all
these things if you have no background in hardware. A common mistake
that software people make is that VHDL is somehow "just another
language". The reality is that, in order to design hardware, you
really ought to understand what you want to build *first*, and have
the idea of the building blocks, registers, overall data flow, etc
mapped out in your head (if not on paper). Then, the task of
implementing it is just mechanical work. But if you don't understand
hardware to begin with, you're going to have a rough time. I'd really
advise you to find a good college-level digital design textbook and
work through it from front to back. Also, read as many application
notes (Xilinx or Altera) as you can handle and try to understand how
the problems are being solved in hardware, at a circuit level. Then
you'll start to see how to achieve what you need, and can move on to
the mechanical implementation stuff, be it using VHDL or something
else.
Now, it's still not clear if you're talking simulation or
synthesizable code. If all you want is a clock for simulation, just
set your simulator to picosecond resolution and use
clk0 <= not clk0 after 54253 ns;
which isn't exact but is close. Make sure you initialize your signal.
But assuming you need a synthesizable solution:
clk0 is exactly 1/12th the frequency of clk1. The canonical way of
creating this (through logic) is with a counter (counting at clk1
rate) that goes from 0 through 5 (hence a divide by six), toggling a
flop every time it rolls over, giving you a 50% duty cycle square
wave, which you can use as clk0. I *really* hope you already knew
this.
To generate an 18.432 MHz clock using simple sequential and
combinatorial logic directly from 50 MHz is easy, if you can live with
one clock cycle (20 ns) of jitter. The ratio is 1152/3125, so you
have a 12 bit counter (holds 3125 < 4096), increment it by 1152 every
clock, and if it exceeds 3125, you subtract off 3125. This gives you
a modulo-3125 counter, and the MSB of this counter toggles at a 18.432
MHz rate. There are other methods, but I suspect this is all moot, as
I'll explain next.
Why? You probably can't live with 20 ns of jitter. For anything
related to audio, you usually want a "real" clock, (one which isn't
jittered by 20ns!). This typically means you'll need to instantiate a
PLL configuration inside the FPGA. I have no Altera experience, but I
suspect there's some kind of GUI software wizard that will let you
type in the two frequencies and creates a block of code that you can
instantiate inside the FPGA.
For Altera-speicic clock module help, you should look to Altera's
website or an fpga newsgroup, as there's nothing VHDL specific there
until you get the PLL code. There are many more neat ways to generate
clocks using counters (like a power-of-two based NCO) but again, you
have to understand what you want to build *first*, then you can ask
how to code it in VHDL.
Regards,
- Kenn
