MOTOROLA
MC68341 USER’S MANUAL
2- 7
2.2.5 Data Strobe (
DS)
DS is an output timing signal for M68300 transfers that applies to the data bus. For a read
cycle, the MC68341 asserts DS and AS simultaneously to signal the external device to
place data on the bus. For a write cycle, DS signals to the external device that the data to
be written is valid. The MC68341 asserts DS approximately one clock cycle after the
assertion of AS during a write cycle.
2.2.6 Upper And Lower Data Strobes (
UDS, LDS)
These three-state signals and R/W control the flow of data on the data bus for M68000
transfers. Table 2-9 lists the combinations of these signals and the corresponding data on
the bus. UDS and LDS assert with AS68K for read cycles, and one clock after AS for write
cycles. The equations of the data strobes are as follows:
UDS = A0
LDS = A0
× SIZ1 × SIZ0
Table 2-9. Data Strobe Control of Data Bus
UDS
LDS
R/
W
D15–D8
D7–D0
Low
High
Valid Data Bits
15–8
Valid Data Bits
7–0
High
Low
High
No Valid Data
Valid Data Bits
7–0
Low
High
Valid Data Bits
15–8
No Valid Data
Low
Valid Data Bits
15–8
Valid Data Bits
7–0
2.2.7 Byte Write Enable (
UWE, LWE)
On a write cycle to a 16-bit port, these active-low output signals indicate when the upper
or lower eight bits of the data bus contain valid data. The upper write enable (UWE )
indicates that the upper eight bits of the data bus contain valid data during a write cycle.
The lower write enable (LWE ) indicates that the lower eight bits of the data bus contain
valid data during a write cycle. UWE and LWE assert with DS for an MC68300 write and
with UDS/LDS for a 68000 write cycle. The equations of the byte write enables are as
follows:
UWE = R/W + AS + A0
LWE = R/W + AS + (A0
× SIZ0)
2.2.8 Read/Write (R/
W)
This active-high output signal is driven by the bus master to indicate the direction of a data
transfer on the bus. A logic one indicates a read from a slave device; a logic zero indicates
a write to a slave device.
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
..
.