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Old 05-05-2006, 03:36 AM
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Default OPB clocking question

Is there information on how to use a user IP that is attached to the
OPB with different clock speeds? I currently have the IP core wrapped
in the default Import Peripheral Wizard files. The user_logic is
Verilog. The custom IP is instantiated in the user_logic. Different
bits of several slave registers are conected to the core. MicroBlaze
code "talks" to the slave registers by using the customary addressing
is C code.

There is one trigger bit that starts the user IP. Once the
functionlaity of the IP is done, it asserts a flag that is routed back
to the user_logic level. I added an "else if" statement to the default
slave register model implementation in the user_logic. The default
implemetation has two states. The first is "if ( Bus2IP_Reset == 1
)", the other is the "else" and assigns the slave registers based on
the "slv_reg_write_select" signal. I inserted an "else if" before the
aforementioned "else". It goes: "else if (done_flag_i)", and assigns
to slave register 2. It "clears" the bit that initially triggered the
module. The embedded code waits in a while loop until the bit is

This has worked GREAT (no problems!) while the exact same clock is
connected to both the OPB clock and the user IP's clock. We are trying
some things with clocking. If we connect the OPB clock of this core to
the common clock that attaches to all other OPB clocks and the uBlaze,
and then we connect another DCM-derived clock to the user IP core
clock, the behavior is hit or miss. We have tried different clock
speeds. One experiment was to use the two different clocks, but at the
same speed. Again, this doesn't work consistently. There are
definitely the facts that the two clocks are not EXACTLY the same
speed, and there is no phase certainty between the two clocks.

I am thinking that we either need to implement clock domain crossing
sync logic on each signal that passes between the user_logic and the IP
core we instantiate or that we need to take advantage of the
handshaking signals of the OPB...like the ack signal. However, I can't
seem to find any examples of how to do this -- the handshaking that is.
I know how to implement the sync flops. The opb_ipif_pdf vs 3.04a
document has info, but appears to not have any figures (timing
diagrams) where it is supposed to. Maybe mine is screwed up!

Any advice would be appreciated!!

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