External Bus Interface
MPC885 PowerQUICC Family Reference Manual, Rev. 2
13-30
Freescale Semiconductor
13.4.7.2
Address Bus
The 32-bit address bus, A[0:31], is byte addressable, so each address can address one or more bytes. A[0]
is the msb. The address and its attributes are driven on the bus with TS; they remain valid until the bus
master receives a transfer acknowledge from the slave. To distinguish an individual byte, the slave device
must observe the TSIZ signals.
13.4.7.3
Transfer Attributes
The transfer attributes signal group consists of RD/WR, BURST, TSIZ[0:1], AT[0:3], STS, and BDIP.
These signals (with the exception of the BDIP) are available at the same time as the address bus.
13.4.7.3.1
Read/Write (RD/WR)
RD/WR high indicates a read access and low indicates a write access. Driven at the beginning of a bus
cycle, RD/WR is valid at the rising edge of the clock in which TS is asserted. RD/WR changes levels only
when a write cycle is preceded by a read cycle or vice versa. It may remain low for consecutive write
cycles.
13.4.7.3.2
Burst Indicator (BURST)
BURST is driven by the bus master at the beginning of the bus cycle (along with the address) to indicate
that the transfer is a burst transfer.
13.4.7.3.3
Transfer Size (TSIZ)
TSIZ[0:1] indicates the size of the requested data transfer. The TSIZ signals may be used with BURST and
A[30:31] to determine which data byte lanes are used in the transfer. For nonburst transfers, TSIZ[0:1]
specifies the number of bytes starting from the byte location addressed by A[30:31]. In burst transfers, the
value of TSIZ[0:1] is always 00.
13.4.7.3.4
Address Types (AT)
The address type signals (AT[0:3]), PTR and RSV, are outputs that indicate one of 16 address types to
which the address applies. These types are designated as either a normal/alternate master cycle,
user/supervisor (problem/privilege), and instruction/data types. The address type signals are valid at the
rising edge of the clock in which the special transfer start (STS) signal is asserted.
Table 13-4. BURST/TSIZ Encoding
BURST
TSIZ[0:1]
Transfer Size
1
01
Byte
1
10
Half word
1
11
x
1
00
Word
0
00
Burst (16 bytes)
Содержание PowerQUICC MPC870
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