Subject: Resolving timer/cross scheduling ambiguity using post timestep
From: Srikanth Chandrasekaran (schandra@asc.corp.mot.com)
Date: Sat Jul 07 2001 - 04:43:19 PDT
Reply To: schandra@asc.corp.mot.com
Organization: Motorola Australia Software Center
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Hi Committee Members,
This is regarding the ambiguity in the LRM regarding scheduling mechanism
for cross and timer events.
Problem: LRM does not specifically clarify when events (cross/timer) should
be evaluated and scheduled.
The arguments start_time and period are dynamic expressions and it should be
clarified how the changes to these arguments affect the simulation and when
these values are evaluated and scheduled.
Example:
module blackbox(...);
...
...
real start_time;
real period;
analog begin
@(initial_step) begin
start_time = 0.0;
period = 3.0;
end
...
...
period = 4.0;
@(timer(start_time, period) begin
period = 6.0;
...
end
...
period = 5.0;
...
end
endmodule
In the above example it is not very clear what the period would be when the
timer statement is evaluated...
If the timer "fires", is the next event scheduled with period=3.0, 4.0, 5.0
or 6.0?
We are trying the address the above scheduling problem with the following
proposal:
Scheduling of Events as Post-TimeStep Operation:
-----------------------------------------------
Event statements inside an analog block would be evaluated post timestep to
avoid ambiguity in the values assigned to the dynamic expressions namely
start_time and period.
The event statement "@" would be evaluated after evaluating all the statements
inside the "begin-end" analog block and the latest value for the dynamic
expression would be used. In the case of the event "firing" and these dynamic
expressions changing inside the event block (period=6.0) in the above set of
statements, the values set inside the event block would be used to schedule the
future events.
The changes to the dynamic expressions would not affect the current time step
and would take effect only on the next time step.
<Include above example>
In the above example, the statement @timer(...) would be evaluated after all
the statements are evaluated in the analog block as a post timestep operation.
When the timer statement is evaluated the value of start_time will be 1.0,
and events would be scheduled with a period of 5.0 (last evaluated value of
variable "period"). In case the timer event fires on that time step, the timer
events would be scheduled with a period = 6.0.
cheers,
Sri
-- Srikanth Chandrasekaran EDA Solutions and Products SBU Phone: +61-8-8168 3592 Fax: x3501 email: schandra@asc.corp.mot.com
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