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IDT Switch Core
Notes
PES48H12G2 User Manual
The request metering implementation in the PES48H12G2 makes a number of simplifying assumptions
that may or may not be true in all systems. Therefore, it should be expected that some amount of parameter
tuning may be required to achieve optimum performance.
Note that tuning of the request metering mechanism should take into account the completion timeout
value of the associated requesters (i.e., request metering should be tuned such that a requester's comple-
tion timeout value is not violated).
Operation
The completion transfer timer is implemented using a counter. The counter is loaded with an estimate of
the number of DWords that will be transferred on the link in servicing the completion and is decremented at
a rate that corresponds to the number of DWords that will be transferred on the link in a 4ns period.
Request metering is enabled on an input port when the Enable (EN) bit is set in the Requester Metering
Control (RMCTL) register. An non-posted request TLP is allowed to be transferred into the switch core
when the request metering counter is zero.
When a request is transferred into the switch core, the request metering counter is loaded with a value
that estimates the number of DWords associated with the corresponding completion(s). The method for
determining this value is described in section Completion Size Estimation on page 3-12.
– The request metering counter is a 24-bit counter. The count represents a signed-magnitude fixed-
point 0:13:11 number (i.e., a positive number with 13 integer bits and 11 fractional bits) but is
treated by the logic as a 24-bit unsigned integer.
– The value loaded into the request metering counter for the last non-posted request is available in
the Count (COUNT) field of the Request Metering Counter (RMCOUNT) register.
The requester metering initial counter value computed as described in section Comple-
tion Size Estimation on page 3-12 is a sign-magnitude fixed point 0:13:3 number (i.e., a
positive number with 13 integer bits and 3 fractional bits).
The least significant eight fractional bits of the initial counter value are always implicitly
zero.
Figure 3.5 shows the request metering count and its initial value.
R e q u e s t M e te r in g C o u n t
(2 4 b its )
In itia l V a lu e L o a d e d in to
R e q u e s t M e te rin g C o u n te r
Figure 3.5 Request Metering Count and Initial Value Loaded
The request metering counter is decremented by a value that corresponds to the number of DWords
transferred on the link per 4ns period. The value is equal to the sum of the decrement value plus the value
of the Decrement Value Adjustment (DVADJ) field in the RMCTL register.
The decrement value is a sign-magnitude fixed-point 0:4:3 number (i.e., an positive number with 4
integer bits and 3 fractional bits), determined by the port's negotiated link width and speed as shown in
Table 3.6.
– The least significant eight fractional bits of the decrement value are always implicitly zero.
0
D W o r d s
S ig n
1 3 b its
0
D W o r d s
S ig n
1 3 b its
3 - 10
F r a c tio n a l P a r t
1 1 b its
N ib b le s
0 0 0 0 0 0 0 0
3 b its
8 b its
April 5, 2013
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