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Enhanced Time Processing Unit (eTPU2)
MPC5644A Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor
977
SGRs are not cleared individually by channel, but rather as priority level groups. The clearing of a group
of SGRs begins a new cycle for that priority level. An SGR group is cleared on the condition that a channel
of that priority level has just been serviced, and no other channel of that priority level is requesting service
(has a set SRR) and has not been granted service (has a clear SGR).
For example, if a middle-priority channel has just been serviced (either in a middle-priority time slot or a
high or low-priority time slot gained by priority passing), the SRRs and SGRs of all middle-priority
channels are compared. If there is no middle-priority channel with its SRR set and SGR cleared, the
scheduler clears all middle-level SGRs. If there is a middle-level channel with its SRR set and SGR
cleared, the scheduler does not clear the SGR group, and the requesting middle-level channel is serviced
on the next middle-level time slot (or possibly sooner by priority passing).
24.6.5.3.4
SPRAM collision rate
Most function threads read or write to the eTPU SPRAM at least once. Because both the eTPU
Microengine and Host can access the SPRAM but not at the same time, the Microengine may suspend
execution during the SPRAM access while waiting for the Host to finish accessing the SPRAM. At other
times the Host may wait for the Microengine. Wait states can take up to two system clocks, when the Host
accesses the SPRAM directly, without using CDC. Microengine(s) wait-states must be added into the
worst-case latency calculation. The system designer should estimate the percentage of SPRAM accesses
in the system that will result in Microengine wait-states. This percentage is called the RAM collision rate
(RCR). In each collision with direct Host accesses to the SPRAM the Microengine(s) wait for two system
clocks.
In eTPU the Coherent Dual-parameter Controller (CDC) may also access the SPRAM for atomic transfers
of two parameters. eTPU Microengine may wait on this operation (if it is in service time) until the transfer
is complete. CDC always transfers two parameters, making four consecutive accesses (read, write, read,
write) of one system clock each. The system designer should estimate the percentage of SPRAM accesses
in the system that will result in a Microengine wait due to coherent transfer, and multiply it with the
average number of system clocks the Microengine waits for each transfer. This percentage is called
Coherent Parameter Collision Rate (CPCR).
In addition, Microengine to Microengine multiple parameter coherent communication, using the hardware
semaphores, may hold one Microengine which waits to lock the semaphore while the other Microengine
is holding it. This waiting is due to a software loop, not hardware wait-states. Note that single parameter
access of one Microengine does not affect the timing of the other Microengine due to SPRAM time
interlace. This implies that single parameter Microengine to Microengine communication does not affect
the performance. The Microengine which waits for the semaphore will loop until it is freed by the other
Microengine. This time depends on the eTPU application. The system designer should estimate the
percentage of Microengine to Microengine coherent parameter communication that will result in eTPU
semaphore loops, and multiply it with the average number of system clocks the Microengine loops for each
such transfer. This percentage is called CCR (Communication Collision Rate).
A 100% collision rate for a system is the theoretical worst case. In many systems, however, the RCR,
CPCR and CCR would be very low, sometimes even near 0%. This is because the eTPU is an independent
processor capable of servicing most function needs, so that the Host rarely needs to access the eTPU
parameter RAM. Also coherent Microengine to Microengine communication of more than one parameter
may be rare. To find a realistic RCR, CPCR the system designer should evaluate the Host code and find
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