MC68336/376
SYSTEM INTEGRATION MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
5-25
5.6 Bus Operation
Internal microcontroller modules are typically accessed in two system clock cycles.
Regular external bus cycles use handshaking between the MCU and external periph-
erals to manage transfer size and data. These accesses take three system clock
cycles, with no wait states. During regular cycles, wait states can be inserted as
Fast termination cycles, which are two-cycle external accesses with no wait states,
use chip-select logic to generate handshaking signals internally. Chip-select logic can
also be used to insert wait states before internal generation of handshaking signals.
Bus control signal timing, as well as chip-select signal timing, are specified in APPEN- (SIMRM/AD) for more information about each type of bus cycle.
5.6.1 Synchronization to CLKOUT
External devices connected to the MCU bus can operate at a clock frequency different
from the frequencies of the MCU as long as the external devices satisfy the interface
signal timing constraints. Although bus cycles are classified as asynchronous, they are
interpreted relative to the MCU system clock output (CLKOUT).
Descriptions are made in terms of individual system clock states, labeled {S0, S1,
S2,..., SN}. The designation “state” refers to the logic level of the clock signal and does
not correspond to any implemented machine state. A clock cycle consists of two suc-
cessive states. Refer to Table A-4 for more information.
Bus cycles terminated by DSACK assertion normally require a minimum of three CLK-
OUT cycles. To support systems that use CLKOUT to generate DSACK and other
inputs, asynchronous input setup time and asynchronous input hold times are speci-
Table 5-12 Operand Alignment
Current
Cycle
Transfer Case1
NOTES:
1. All transfers are aligned. The CPU32 does not support misaligned word or long-word transfers.
SIZ1
SIZ0
ADDR0
DSACK1 DSACK0
DATA
[15:8]
DATA
[7:0]
Next
Cycle
1
Byte to 8-bit port (even)
0
1
0
1
0
OP0
(OP0)2
2. Operands in parentheses are ignored by the CPU32 during read cycles.
—
2
Byte to 8-bit port (odd)
0
1
0
OP0
(OP0)
—
3
Byte to 16-bit port (even)
0
1
0
1
OP0
(OP0)
—
4
Byte to 16-bit port (odd)
0
1
0
1
(OP0)
OP0
—
5
Word to 8-bit port
1
0
1
0
OP0
(OP1)
2
6
Word to 16-bit port
1
0
1
OP0
OP1
—
7
3-Byte to 8-bit port3
3. Three-Byte transfer cases occur only as a result of a long word to 8-bit port transfer.
1
0
OP0
(OP0)
5
8
Long word to 8-bit port
0
1
0
OP0
(OP0)
7
9
Long word to 16-bit port
0
1
OP0
OP1
6