Reply by Jonathan Bromley●March 16, 20092009-03-16
On Sun, 15 Mar 2009 12:31:01 -0700 (PDT), rickman wrote:
[...]
>If that thread has not been posted I may well have forgotten
>the technique [...]
Somehow I suspect that you would have worked it all
out for yourself, postings or no postings. But your
point is well made. I'm often astonished at the way
information picked up long ago turns out to be useful
in unexpected contexts (I guess that's called
"experience"); and a little memory-jogger is usually
needed to bring it all back (I guess that's called
"forgetfulness").
--
Jonathan Bromley, Consultant
DOULOS - Developing Design Know-how
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Reply by rickman●March 15, 20092009-03-15
I recently read the thread started by Jonathan Bromley, titled "Is
this phase accumulator trick well-known???". I am building a DPLL and
I will be using this technique. I am using a FIFO to delay data by a
fixed time where the data is clocked in and out on variable rate
clocks. The two clocks run at the same average rate, the input clock
is gapped and the output clock is generated by the PLL to match the
average of the input clock. By "clocks" I really mean clock enable
just so we don't get into a big discussion on metastability and such.
The requirement for a fixed time delay with a variable clock means I
need to adjust the amount of data held in the FIFO as the clock speed
changes. It seems to me like a proportional control of the PLL
feedback using the FIFO data count will do just what I am looking
for. I did some calcs and I know exactly how much data needs to be
held at each clock rate. So I am going to design the phase
accumulator so that the data count is used as the phase increment
value. I have not analyzed the dynamic characteristics of this
circuit yet, but that will be next.
I just thought it was fortuitous that this discussion had come up
recently to remind me of this circuit design. I worked with phase
accumulators and NCOs some time back, but had forgotten most of the
details. If that thread has not been posted I may well have forgotten
the technique of setting the accumulator modulus to control the scale
of the phase increment. Thanks!
Rick