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MOTOROLA
MC68349 USER'S MANUAL
3- 5
The internal bus monitor can be used to generate an internal bus error signal for internal
and internal-to-external transfers. If the bus cycles of an external bus master are to be
monitored, external BERR generation must be provided since the internal bus error
monitor has no information about transfers initiated by an external bus master.
3.1.7.3 AUTOVECTOR (
AVEC). This signal can be used to terminate interrupt
acknowledge cycles, indicating that the MC68349 should internally generate a vector
(autovector) number to locate an interrupt handler routine. AVEC can be generated either
externally or internally by the SIM49 (see Section 4 System Integration Module for
additional information). AVEC is ignored during all other bus cycles.
3.2 DATA TRANSFER MECHANISM
The MC68349 supports byte, word, and long-word operands, allowing access to 8-,16-,
and 32-bit data ports through the use of asynchronous cycles controlled by DSACK1 and
DSACK0. The MC68349 also supports byte, word, and long-word operands, allowing
access to 8-, 16, and 32-bit data ports through the use of synchronous cycles controlled
by the fast-termination capability of the SIM49.
3.2.1 Dynamic Bus Sizing
The MC68349 dynamically interprets the port size of the addressed device during each
bus cycle, allowing operand transfers to or from 8-, 16-, and 32-bit ports. During an
operand transfer cycle, the slave device signals its port size (byte, word, or long word) and
indicates completion of the bus cycle to the MC68349 through the use of the DSACK≈
inputs. Refer to Table 3-2 for DSACK≈ encoding.
Table 3-2.
DSACK Encoding
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
0
Complete Cycle—Data Bus Port Size is 32 Bits
For example, if the MC68349 is executing an instruction that reads a long-word operand
from a long-word aligned address, it attempts to read 32 bits during the first bus cycle.
(Refer to 3.2.2 Misaligned Operands for the case of a word or byte address.) If the port
responds that it is 32 bits wide, the MC68349 latches all 32 bits of data and continues with
the next operation. If the port responds that it is 16 bits wide, the MC68349 latches the 16
bits of valid data and 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 32-bit device always returns
DSACK≈ for a 32-bit port (regardless of whether the bus cycle is a byte, word, or long-
word operation).