I heard about a quite effective low power technique buzzword: "gated clock"
Actually gating clocks is relatively evil. Having a gated clock that
transitions less often does save power. Every signal edge is a power
waster in the face of capacitance. Gated clocks create other
problems. Your clocks are no longer exactly lined up. Many chips
have a global clock that goes everywhere. Breaking it into pieces
isn't a good thing. You want to enable data rather than gate clocks.
Call the Enable 'E'. This looks something like:
If CLK'event and CLK = '1' then
Q <= (not(E) and Q) or (D and Q) or (D and E);
end if;
I realize this doesn't tell you how to gate clocks. I'm not gonna
tell you, because I can't tell you how to fix the problems it creates.
Happy Designing,
Roadie Roger
Depends on your target technology. The above may hold for FPGA's,
but for standard-cell based ASIC technologies, gated clocks are
an elegant (in my opinion) and clean methodology for low-power
design.
In this case, you will most likely be using a clock tree generator
anyway, and all of the clock tree generators I know easily handle
clock gates. They will automatically align the clocks AFTER any
possible gates, i.e. at the leaf nodes (clock inputs of registers).
In this case, generating a low-skew clock tree is far easier for
gated clocks than for derived clocks (i.e. divided clocks).
Modern synthesis tools also can insert clock gates automatically
by replacing register enables (as shown in the post above) by
gated clocks. This can be handy, but it has the drawback of
reducing your control over clock gates (names, hierarchy).
Inserting clock gates in a VHDL design is easy. Just decide what
gating style you want/need (and/or based, with/without latch, with
test bypass, observability, ...).
My choice is to then explicitly instantiate a clock gating
element where needed. This has the disadvantage of requiring one
to "route" different clock signals through the design, but
it gives you full control over clock gating elements for use
in later stages of the design (clock tree generation, physical
optimization, ...).
Information about "clean" clock gating can be found in books
or also in synthesis tool manuals (where automatic clock
gate insertion is described). Probably you'll also find
resources on the web.
Robert
