UM0434 datasheet(281/390 Pages) STMICROELECTRONICS

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External core complex interfaces

UM0434

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9.5.3

Address retraction

Address retraction is the process of replacing a request with a new unrelated one. Although

the AMBA AHB protocol requires an access request to remain driven unchanged once

presented on the bus, higher system performance may be obtained if this aspect of the

protocol is modified to allow an access request to be changed before being taken.

Figure 57 shows an example of address retraction during wait state operation. Signal

p_[d,i]_hready for the first request (addr

x) is not asserted during C2, so a wait state is

inserted during C3 until

p_[d,i]_hready is recognized.

Meanwhile, a subsequent request has been generated by the CPU for addry which is not

taken in C2 since the previous transaction is still outstanding. The address and transfer

attributes are retracted in cycle C3, and a new access request to addrz is requested and are

taken at the end of C3 because the previous access is completing. Data for addrx and a

ready/OKAY response is driven back by the slave device. In cycle C4, a request for addrw is

made. The request for access to addrw is taken at the end of C4; during C5, the data and a

ready/OKAY response is provided by the slave device. In cycle C5, no further accesses are

requested.

Figure 51.

Read transfer with Wait-State, address retraction

Figure 52 shows functional timing for a burst read with wait-state transfer where the second

beat to addr x+8 is retracted and replaced with a new burst transfer.

nonseq

idle

addr x

addr y

addr w

single

data x

data z

data w

okay

Read with wait-state, address retraction

m_clk

p_htrans

p_addr,p_hprot

p_hsize,

p_hbstrb, etc

p_hburst

p_hunalign

p_hwrite

p_hrdata

p_hwdata

p_hready

p_hresp

addr z

nonseq

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