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MPC563XM Reference Manual, Rev. 1
794
Freescale Semiconductor
Preliminary—Subject to Change Without Notice
Figure 23-37. Time-Slot Variation
23.4.4
Parameter Sharing and Coherency
SPRAM can be concurrently accessed by Host and Microengines (two in a Dual eTPU Engine system). In
general, there is no guaranteed order by which a group of parameters is accessed, which may lead to a lack
of internal consistency if two or more related parameters are read when only part of them is updated.
eTPU provides mechanisms to guarantee parameter coherency. The most generic mechanisms for
Host-eTPU coherency, suitable for any number of parameters, are:
•
the use of Transfer Service Thread mechanism.
•
the mailbox (or “software semaphore”) mechanism.
These mechanisms, described in
Section 23.5.3, “Multiple Parameter Coherency Methods
,” use
microcode to transfer parameters from temporary buffers in SPRAM to their definitive locations (or
vice-versa). These methods have the disadvantage of wasting processing and code memory resources.
eTPU also provides a
Coherent Dual-parameter Controller - CDC
- mechanism. It is used by Host to
coherently transfer pairs of parameters from/to a parameter buffer located on SPRAM to/from the
locations on SPRAM where parameters are accessed directly by the channels. Coherency is guaranteed by
SPRAM access arbitration. Although limited to two parameters only, it has lower latency and wastes no
microengine resources
1
Section 23.4.4.3, “Coherent Dual-parameter
For parameters shared by both Engines, eTPU provides
Hardware Semaphores
. Coherency is assured
given the semaphores are used to prevent concurrent access to the changing parameters. Microengine can
request semaphores using specific microinstructions (see
Section 23.4.9.1.7, “Semaphore Operations
”).
Hardware Semaphores are described in detail in
Section 23.4.4.4, “Hardware Semaphores
.”
Neither Host nor CDC have access to the hardware semaphores, but they can be combined with microcode
transfer mechanisms if Host must coherently access parameters which are also shared by both Engines.
In order to ensure coherent access to a group of parameters by two or more contenders,
each contender
must have atomic access
to the shared parameters. Atomicity conditions are discussed in
Section 23.4.4.1, “Host Side Atomic Access
” and
Section 23.4.4.2, “Microengine Side Atomic Accesses
.”
1.
a microengine access to the SPRAM in the moment CDC is performing the transfer may suffer a maximum of two
wait-states.
Microcycles
Time Slot
Fixed Priority Level
1
2
3
4
5
H
M
H
L
H
6
M
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