MC68F375
SINGLE-CHIP INTEGRATION MODULE 2 (SCIM2E)
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
4-36
4.5.1.11 Autovector Signal
The autovector signal (AVEC) can be used to terminate interrupt acknowledgment
cycles for external interrupts only. Assertion of AVEC causes the CPU32 to generate
vector numbers to locate an interrupt handler routine. If AVEC is continuously
asserted, autovectors are generated for all external interrupt requests. AVEC is
ignored during all other bus cycles. Refer to 4.8 Interrupts for more information. AVEC
for external interrupt requests can also be supplied internally by chip-select logic.
Refer to 4.9 Chip Selects for more information. The autovector function is disabled
more information.
4.5.2 Dynamic Bus Sizing
The MCU dynamically interprets the port size of an addressed device during each bus
cycle, allowing operand transfers to or from 8-bit and 16-bit ports.
During a bus transfer cycle, an external device signals its port size and indicates com-
pletion of the bus cycle to the MCU through the use of the DSACK inputs, as shown in
Table 4-19. Chip-select logic can generate data size acknowledge signals for an exter-
If the CPU is executing an instruction that reads a long-word operand from a 16-bit
port, the MCU latches the first 16 bits of valid data and then runs another bus cycle to
obtain the other 16 bits. The operation for an 8-bit port is similar, but requires four read
cycles. The addressed device uses the DSACK signals to indicate the port width. For
instance, a 16-bit external device always returns DSACK for a 16-bit port (regardless
of whether the bus cycle is a byte or word operation).
Dynamic bus sizing requires that the portion of the data bus used for a transfer to or
from a particular port size be fixed. A 16-bit port must reside on data bus bits [15:0],
and an 8-bit port must reside on data bus bits [15:8]. This minimizes the number of bus
cycles needed to transfer data and ensures that the MCU transfers valid data.
The MCU always attempts to transfer the maximum amount of data on all bus cycles.
For a word operation, it is assumed that the port is 16 bits wide when the bus cycle
begins.
Operand bytes are designated as shown in Figure 4-10. OP[0:3] represent the order
of access. For instance, OP0 is the most significant byte of a long-word operand, and
is accessed first, while OP3, the least significant byte, is accessed last. The two bytes
Table 4-19 Effect of DSACK Signals
DSACK1
DSACK0
Result
1
Insert wait states in current bus cycle
1
0
Complete cycle — Data bus port size is 8 bits
0
1
Complete cycle — Data bus port size is 16 bits
00
Reserved
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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