GR740-UM-DS, Nov 2017, Version 1.7
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GR740
While the transitions shown in table 362 are simplified they give an accurate view of the core delay. If
the read operation receives wait states, these cycles must be added to the cycle count in the table.
Table 363 below lists the delays incurred for single operations that traverse the bridge while the bridge
is in its idle state. The second column shows the number of cycles it takes the master side to perform
the requested access, this column assumes that the master slave gets access to the bus immediately
and that each access is completed with zero wait states. The table only includes the delay incurred by
traversing the core. For instance, when the access initiating master reads the core’s prefetch buffer,
each additional read will consume one clock cycle. However, this delay would also have been present
if the master accessed any other slave.
Write accesses are accepted with zero wait states if the bridge is idle, this means that performing a
write to the idle core does not incur any extra latency. However, the core must complete the write
operation on the master side before it can handle a new access on the slave side. If the core has not
transitioned into its idle state, pending the completion of an earlier access, the delay suffered by an
access be longer than what is shown in the tables in this section. Locked accesses that abort on-going
read operations will also mean additional delays.
With read and write combining, the number of cycles required for the master will change depending
on the access size and length of the incoming burst access.
18.3
Registers
The bridge does not implement any registers.
Table 363.
Access latencies
Access
Master acc. cycles Slave cycles
Delay incurred by performing access over core
Single read
3
2
5* clk
Burst read with prefetch
2 + (burst length)
x
4
(6 + burst length)* clk
Single write
xx
(2)
0
0
Burst write
xx
(2 + (burst length)) 0
0
x
A prefetch operation ends at the address boundary defined by the prefetch buffer’s size
xx
The core implements posted writes, the number of cycles taken by the master side can only affect the next access.