UM0434 datasheet(286/390 Pages) STMICROELECTRONICS

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UM0434

External core complex interfaces

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Figure 56 shows another example of error termination, this time on the initial portion of a

burst read. The aborted burst is followed by a burst write.

Figure 56.

Burst read with error termination, burst write

The first portion of the burst read request is terminated with error. The second portion is

aborted by the CPU during the second cycle of the two cycle error response, and a

subsequent burst write access to addry becomes pending instead.

9.5.4

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_hready for the first request (addr

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

during C3 until

p_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 to addrz is requested and made at the

end of C3 because the previous access is completing. Data for addrx and a ready/OKAY

response are 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 are provided by the slave device. In cycle C5, no further accesses are requested.

nonseq

seq

idle

nonseq

seq

idle

addr x

addr x+8

addr y

addr y+8

addr y+16

addr y+24

wrap4

data x

data y

y+8

y+16

y+24

okay

error

okay

567

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

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