6- 12
MC68341 USER'S MANUAL
MOTOROLA
6.4.2 Dual-Address Mode
The dual-address DMA bus cycle transfers data between a device or memory and the DMA
internal data holding register (DHR). In this mode, any operand transfer takes place in two
DMA bus cycles, one where a device is addressed and one where memory is addressed.
The data transferred during a dual-address operation is either read from the data bus into
the DHR or written from the DHR to the data bus.
Each DMA channel can be programmed to operate in the dual-address transfer mode. In
this mode, the operand is read from the source address specified in the SAR and placed in
the DHR. The operand read may take up to four bus cycles to complete because of
differences in operand sizes of the source and destination. The operand is then written to
the address specified in the DAR. This transfer may also be up to four bus cycles long. In
this manner, various combinations of peripheral, memory, and operand sizes may be used.
See 6.7 Register Description for more information.
The dual-address transfers can be started by either the internal request mode or by an
external device using the DREQ≈ input signal. When the external device uses DREQ≈, the
channel can be programmed to operate in either burst transfer mode or cycle steal mode.
6.4.2.1 DUAL-ADDRESS READ. During the dual-address read cycle, the DMA reads data
from a device or memory into the internal DHR. The device or memory is selected by the
address specified in the SAR, the source function codes in the FCR, and the source size in
the CCR. Data is read from the memory or peripheral and placed in the DHR when the bus
cycle is terminated. When the complete operand has been read, the SAR is incremented by
0, 1, 2, or 4, according to the size and increment information specified by the SSIZE and
SAPI bits of the CCR. The DMA control signals (DACK≈ and DONE≈) are asserted in the
source (read) cycle when the source device makes a request. See Figures 6-9 and 6-10 for
timing diagrams of dual-address reads for external burst and cycle steal modes.
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
..
.